     [file gdb-config.texi not included]

     START-INFO-DIR-ENTRY
     * Gdb: (gdb).                   The GNU debugger.
     END-INFO-DIR-ENTRY

This file documents the GNU debugger GDBN.

This is Edition 4.09, April 1993, of Debugging with GDBN: the GNU Source-Level Debugger for GDB Version GDBVN.

Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled ``GNU General Public License'' is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU General Public License'' may be included in a translation approved by the Free Software Foundation instead of in the original English.

Debugging with GDBN
*******************

This file describes GDBN, the GNU symbolic debugger.

This is Edition 4.09, April 1993, for GDB Version GDBVN.

Summary of GDBN
***************

The purpose of a debugger such as GDBN is to allow you to see what is going on ``inside'' another program while it executes---or what another program was doing at the moment it crashed.

GDBN can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

  * Start your program, specifying anything that might affect its behavior.

  * Make your program stop on specified conditions.

  * Examine what has happened, when your program has stopped.

  * Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

You can use GDBN to debug programs written in C, C++, and Modula-2. Fortran support will be added when a GNU Fortran compiler is ready.

Free software
=============

GDBN is free software, protected by the GNU General Public License (GPL).  The GPL gives you the freedom to copy or adapt a licensed program---but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.

Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.

For full details, see GNU GENERAL PUBLIC LICENSE.

Contributors to GDB
===================

Richard Stallman was the original author of GDB, and of many other GNU programs.  Many others have contributed to its development.  This section attempts to credit major contributors.  One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here.  The file ChangeLog in the GDB distribution approximates a blow-by-blow account.

Changes much prior to version 2.0 are lost in the mists of time.

     Plea: Additions to this section are particularly welcome.  If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names!

So that they may not regard their long labor as thankless, we particularly thank those who shepherded GDB through major releases: Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, 4.4), John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, 3.3); and Randy Smith (releases 3.2, 3.1, 3.0).  As major maintainer of GDB for some period, each contributed significantly to the structure, stability, and capabilities of the entire debugger.

Richard Stallman, assisted at various times by Pete TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.

Michael Tiemann is the author of most of the GNU C++ support in GDB, with significant additional contributions from Per Bothner.  James Clark wrote the GNU C++ demangler.  Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).

GDB 4 uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.

David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.

Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support.  Jean-Daniel Fekete contributed Sun 386i support.  Chris Hanson improved the HP9000 support.  Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support.  David Johnson contributed Encore Umax support.  Jyrki Kuoppala contributed Altos 3068 support. Keith Packard contributed NS32K support.  Doug Rabson contributed Acorn Risc Machine support.  Chris Smith contributed Convex support (and Fortran debugging).  Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support.  Tim Tucker contributed support for the Gould NP1 and Gould Powernode.  Pace Willison contributed Intel 386 support.  Jay Vosburgh contributed Symmetry support.

Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.

Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about several machine instruction sets.

Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging.  Intel Corporation and Wind River Systems contributed remote debugging modules for their products.

Brian Fox is the author of the readline libraries providing command-line editing and command history.

Andrew Beers of SUNY Buffalo wrote the language-switching code and the Modula-2 support, and contributed the Languages chapter of this manual.

Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.

Hitachi America, Ltd. sponsored the support for Hitachi microprocessors.

A Sample GDBN Session
*********************

You can use this manual at your leisure to read all about GDBN. However, a handful of commands are enough to get started using the debugger.  This chapter illustrates those commands.

One of the preliminary versions of GNU m4 (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working.  In the following short m4 session, we define a macro foo which expands to 0000; we then use the m4 built-in defn to define bar as the same thing.  However, when we change the open quote string to <QUOTE> and the close quote string to <UNQUOTE>, the same procedure fails to define a new synonym baz:

     $ cd gnu/m4
     $ ./m4
     define(foo,0000)

     foo
     0000
     define(bar,defn(`foo'))

     bar
     0000
     changequote(<QUOTE>,<UNQUOTE>)

     define(baz,defn(<QUOTE>foo<UNQUOTE>))
     baz
     C-d
     m4: End of input: 0: fatal error: EOF in string

Let us use GDBN to try to see what is going on.

     $ GDBP m4
     GDB is free software and you are welcome to distribute copies
      of it under certain conditions; type "show copying" to see
      the conditions.
     There is absolutely no warranty for GDB; type "show warranty"
      for details.
     GDB GDBVN, Copyright 1993 Free Software Foundation, Inc...
     (GDBP)

GDBN reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly.  We now tell GDBN to use a narrower display width than usual, so that examples will fit in this manual.

     (GDBP) set width 70

We need to see how the m4 built-in changequote works. Having looked at the source, we know the relevant subroutine is m4_changequote, so we set a breakpoint there with the GDBN break command.

     (GDBP) break m4_changequote
     Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the run command, we start m4 running under GDBN control; as long as control does not reach the m4_changequote subroutine, the program runs as usual:

     (GDBP) run
     Starting program: /work/Editorial/gdb/gnu/m4/m4
     define(foo,0000)

     foo
     0000

To trigger the breakpoint, we call changequote.  GDBN suspends execution of m4, displaying information about the context where it stops.

     changequote(<QUOTE>,<UNQUOTE>)

     Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
         at builtin.c:879
     879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))

Now we use the command n (next) to advance execution to the next line of the current function.

     (GDBP) n
     882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
      : nil,

set_quotes looks like a promising subroutine.  We can go into it by using the command s (step) instead of next. step goes to the next line to be executed in any subroutine, so it steps into set_quotes.

     (GDBP) s
     set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
         at input.c:530
     530         if (lquote != def_lquote)

The display that shows the subroutine where m4 is now suspended (and its arguments) is called a stack frame display.  It shows a summary of the stack.  We can use the backtrace command (which can also be spelled bt), to see where we are in the stack as a whole: the backtrace command displays a stack frame for each active subroutine.

     (GDBP) bt
     #0  set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
         at input.c:530
     #1  0x6344 in m4_changequote (argc=3, argv=0x33c70)
         at builtin.c:882
     #2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
     #3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
         at macro.c:71
     #4  0x79dc in expand_input () at macro.c:40
     #5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195

We will step through a few more lines to see what happens.  The first two times, we can use s; the next two times we use n to avoid falling into the xstrdup subroutine.

     (GDBP) s
     0x3b5c  532         if (rquote != def_rquote)
     (GDBP) s
     0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
     def_lquote : xstrdup(lq);
     (GDBP) n
     536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
      : xstrdup(rq);
     (GDBP) n
     538         len_lquote = strlen(rquote);

The last line displayed looks a little odd; we can examine the variables lquote and rquote to see if they are in fact the new left and right quotes we specified.  We use the command p (print) to see their values.

     (GDBP) p lquote
     $1 = 0x35d40 "<QUOTE>"
     (GDBP) p rquote
     $2 = 0x35d50 "<UNQUOTE>"

lquote and rquote are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the l (list) command.

     (GDBP) l
     533             xfree(rquote);
     534
     535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
      : xstrdup (lq);
     536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
      : xstrdup (rq);
     537
     538         len_lquote = strlen(rquote);
     539         len_rquote = strlen(lquote);
     540     }
     541
     542     void

Let us step past the two lines that set len_lquote and len_rquote, and then examine the values of those variables.

     (GDBP) n
     539         len_rquote = strlen(lquote);
     (GDBP) n
     540     }
     (GDBP) p len_lquote
     $3 = 9
     (GDBP) p len_rquote
     $4 = 7

That certainly looks wrong, assuming len_lquote and len_rquote are meant to be the lengths of lquote and rquote respectively.  We can set them to better values using the p command, since it can print the value of any expression---and that expression can include subroutine calls and assignments.

     (GDBP) p len_lquote=strlen(lquote)
     $5 = 7
     (GDBP) p len_rquote=strlen(rquote)
     $6 = 9

Is that enough to fix the problem of using the new quotes with the m4 built-in defn?  We can allow m4 to continue executing with the c (continue) command, and then try the example that caused trouble initially:

     (GDBP) c
     Continuing.

     define(baz,defn(<QUOTE>foo<UNQUOTE>))

     baz
     0000

Success!  The new quotes now work just as well as the default ones.  The problem seems to have been just the two typos defining the wrong lengths.  We allow m4 exit by giving it an EOF as input:

     C-d
     Program exited normally.

The message Program exited normally. is from GDBN; it indicates m4 has finished executing.  We can end our GDBN session with the GDBN quit command.

     (GDBP) quit

Getting In and Out of GDBN
**************************

This chapter discusses how to start GDBN, and how to get out of it. (The essentials: type GDBP to start GDB, and type quit or C-d to exit.)

Invoking GDBN
=============

Invoke GDBN by running the program GDBP.  Once started, GDBN reads commands from the terminal until you tell it to exit.

You can also run GDBP with a variety of arguments and options, to specify more of your debugging environment at the outset.

The most usual way to start GDBN is with one argument, specifying an executable program:

     GDBP PROGRAM

You can also start with both an executable program and a core file specified:

     GDBP PROGRAM CORE

You can, instead, specify a process ID as a second argument, if you want to debug a running process:

     GDBP PROGRAM 1234

would attach GDBN to process 1234 (unless you also have a file named 1234; GDBN does check for a core file first).

Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDBN as a remote debugger attached to a bare board, there may not be any notion of ``process'', and there is often no way to get a core dump.

You can further control how GDBN starts up by using command-line options.  GDBN itself can remind you of the options available.

Type

     GDBP -help

to display all available options and briefly describe their use (GDBP -h is a shorter equivalent).

All options and command line arguments you give are processed in sequential order.  The order makes a difference when the -x option is used.

     [file gdbinv-s.texi not included]

Choosing files
--------------

When GDBN starts, it reads any arguments other than options as specifying an executable file and core file (or process ID).  This is the same as if the arguments were specified by the -se and -c options respectively.  (GDBN reads the first argument that does not have an associated option flag as equivalent to the -se option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the -c option followed by that argument.)

Many options have both long and short forms; both are shown in the following list.  GDBN also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with -- rather than -, though we illustrate the more usual convention.)

-symbols=FILE
-s FILE
     Read symbol table from file FILE.

-exec=FILE
-e FILE
     Use file FILE as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump.

-se=FILE
     Read symbol table from file FILE and use it as the executable file.

-core=FILE
-c FILE
     Use file FILE as a core dump to examine.

-command=FILE
-x FILE
     Execute GDBN commands from file FILE.  See Command files.

-directory=DIRECTORY
-d DIRECTORY
     Add DIRECTORY to the path to search for source files.

-m
-mapped
     Warning: this option depends on operating system facilities that are not
     supported on all systems.

     If memory-mapped files are available on your system through the mmap system call, you can use this option to have GDBN write the symbols from your program into a reusable file in the current directory.  If the program you are debugging is called /tmp/fred, the mapped symbol file will be ./fred.syms. Future GDBN debugging sessions will notice the presence of this file, and will quickly map in symbol information from it, rather than reading the symbol table from the executable program.

     The .syms file is specific to the host machine where GDBN is run.  It holds an exact image of the internal GDBN symbol table.  It cannot be shared across multiple host platforms.

-r
-readnow
     Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster.

The -mapped and -readnow options are typically combined in order to build a .syms file that contains complete symbol information.  (see Commands to specify files, for information on .syms files.)  A simple GDB invocation to do nothing but build a .syms file for future use is:

     	gdb -batch -nx -mapped -readnow programname

Choosing modes
--------------

You can run GDBN in various alternative modes---for example, in batch mode or quiet mode.

-nx
-n
     Do not execute commands from any GDBINIT initialization files. Normally, the commands in these files are executed after all the command options and arguments have been processed. See Command files.

-quiet
-q
     ``Quiet''.  Do not print the introductory and copyright messages.  These messages are also suppressed in batch mode.

-batch
     Run in batch mode.  Exit with status 0 after processing all the command files specified with -x (and GDBINIT, if not inhibited). Exit with nonzero status if an error occurs in executing the GDBN commands in the command files.

     Batch mode may be useful for running GDBN as a filter, for example to download and run a program on another computer; in order to make this more useful, the message

          Program exited normally.

     (which is ordinarily issued whenever a program running under GDBN control terminates) is not issued when running in batch mode.

-cd=DIRECTORY
     Run GDBN using DIRECTORY as its working directory, instead of the current directory.

-fullname
-f
     Emacs sets this option when it runs GDBN as a subprocess.  It tells GDBN to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops).  This recognizable format looks like two \032 characters, followed by the file name, line number and character position separated by colons, and a newline.  The Emacs-to-GDBN interface program uses the two \032 characters as a signal to display the source code for the frame.

Quitting GDBN
=============

quit
     To exit GDBN, use the quit command (abbreviated q), or type an end-of-file character (usually C-d).

An interrupt (often C-c) will not exit from GDBN, but rather will terminate the action of any GDBN command that is in progress and return to GDBN command level.  It is safe to type the interrupt character at any time because GDBN does not allow it to take effect until a time when it is safe.

If you have been using GDBN to control an attached process or device, you can release it with the detach command (see Debugging an already-running process).

Shell commands
==============

If you need to execute occasional shell commands during your debugging session, there is no need to leave or suspend GDBN; you can just use the shell command.

shell COMMAND STRING
     Invoke a shell to execute COMMAND STRING.  If it exists, the environment variable SHELL is used for the name of the shell to run. Otherwise GDBN uses /bin/sh.

The utility make is often needed in development environments. You do not have to use the shell command for this purpose in GDBN:

make MAKE-ARGS
     Execute the make program with the specified arguments.  This is equivalent to shell make MAKE-ARGS.

GDBN Commands
*************

You can abbreviate a GDBN command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDBN commands by typing just RET.  You can also use the TAB key to get GDBN to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).

Command syntax
==============

A GDBN command is a single line of input.  There is no limit on how long it can be.  It starts with a command name, which is followed by arguments whose meaning depends on the command name.  For example, the command step accepts an argument which is the number of times to step, as in step 5.  You can also use the step command with no arguments.  Some command names do not allow any arguments.

GDBN command names may always be truncated if that abbreviation is unambiguous.  Other possible command abbreviations are listed in the documentation for individual commands.  In some cases, even ambiguous abbreviations are allowed; for example, s is specially defined as equivalent to step even though there are other commands whose names start with s.  You can test abbreviations by using them as arguments to the help command.

A blank line as input to GDBN (typing just RET) means to repeat the previous command. Certain commands (for example, run) will not repeat this way; these are commands for which unintentional repetition might cause trouble and which you are unlikely to want to repeat.

The list and x commands, when you repeat them with RET, construct new arguments rather than repeating exactly as typed.  This permits easy scanning of source or memory.

GDBN can also use RET in another way: to partition lengthy output, in a way similar to the common utility more (see Screen size).  Since it is easy to press one RET too many in this situation, GDBN disables command repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does nothing.  This is useful mainly in command files (see Command files).

Command completion
==================

GDBN can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time.  This works for GDBN commands, GDBN subcommands, and the names of symbols in your program.

Press the TAB key whenever you want GDBN to fill out the rest of a word.  If there is only one possibility, GDBN will fill in the word, and wait for you to finish the command (or press RET to enter it).  For example, if you type

     (GDBP) info bre TAB

GDBN fills in the rest of the word breakpoints, since that is the only info subcommand beginning with bre:

     (GDBP) info breakpoints

You can either press RET at this point, to run the info breakpoints command, or backspace and enter something else, if breakpoints does not look like the command you expected.  (If you were sure you wanted info breakpoints in the first place, you might as well just type RET immediately after info bre, to exploit command abbreviations rather than command completion).

If there is more than one possibility for the next word when you press TAB, GDBN will sound a bell.  You can either supply more characters and try again, or just press TAB a second time, and GDBN will display all the possible completions for that word.  For example, you might want to set a breakpoint on a subroutine whose name begins with make_, but when you type b make_TAB GDBN just sounds the bell.  Typing TAB again will display all the function names in your program that begin with those characters, for example:

     (GDBP) b make_ TAB
     GDBN sounds bell; press TAB again, to see:
     make_a_section_from_file     make_environ
     make_abs_section             make_function_type
     make_blockvector             make_pointer_type
     make_cleanup                 make_reference_type
     make_command                 make_symbol_completion_list
     (GDBP) b make_

After displaying the available possibilities, GDBN copies your partial input (b make_ in the example) so you can finish the command.

If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice. M-? means META ?.  You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.

Sometimes the string you need, while logically a ``word'', may contain parentheses or other characters that GDBN normally excludes from its notion of a word.  To permit word completion to work in this situation, you may enclose words in ' (single quote marks) in GDBN commands.

The most likely situation where you might need this is in typing the name of a C++ function.  This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type).  For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of name that takes an int parameter, name(int), or the version that takes a float parameter, name(float).  To use the word-completion facilities in this situation, type a single quote ' at the beginning of the function name.  This alerts GDBN that it may need to consider more information than usual when you press TAB or M-? to request word completion:

     (GDBP) b 'bubble( M-?
     bubble(double,double)    bubble(int,int)
     (GDBP) b 'bubble(

In some cases, GDBN can tell that completing a name will require quotes.  When this happens, GDBN will insert the quote for you (while completing as much as it can) if you do not type the quote in the first place:

     (GDBP) b bub TAB
     GDBN alters your input line to the following, and rings a bell:
     (GDBP) b 'bubble(

In general, GDBN can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.

Getting help
============

You can always ask GDBN itself for information on its commands, using the command help.

help
h
     You can use help (abbreviated h) with no arguments to display a short list of named classes of commands:

          (GDBP) help
          List of classes of commands:

          running -- Running the program
          stack -- Examining the stack
          data -- Examining data
          breakpoints -- Making program stop at certain points
          files -- Specifying and examining files
          status -- Status inquiries
          support -- Support facilities
          user-defined -- User-defined commands
          aliases -- Aliases of other commands
          obscure -- Obscure features

          Type "help" followed by a class name for a list of
          commands in that class.
          Type "help" followed by command name for full
          documentation.
          Command name abbreviations are allowed if unambiguous.
          (GDBP)

help CLASS
     Using one of the general help classes as an argument, you can get a list of the individual commands in that class.  For example, here is the help display for the class status:

          (GDBP) help status
          Status inquiries.

          List of commands:

          show -- Generic command for showing things set
           with "set"
          info -- Generic command for printing status

          Type "help" followed by command name for full
          documentation.
          Command name abbreviations are allowed if unambiguous.
          (GDBP)

help COMMAND
     With a command name as help argument, GDBN will display a short paragraph on how to use that command.

In addition to help, you can use the GDBN commands info and show to inquire about the state of your program, or the state of GDBN itself.  Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context.  The listings under info and under show in the Index point to all the sub-commands.  See Index.

info
     This command (abbreviated i) is for describing the state of your program.  For example, you can list the arguments given to your program with info args, list the registers currently in use with info registers, or list the breakpoints you have set with info breakpoints. You can get a complete list of the info sub-commands with help info.

show
     In contrast, show is for describing the state of GDBN itself. You can change most of the things you can show, by using the related command set; for example, you can control what number system is used for displays with set radix, or simply inquire which is currently in use with show radix.

     To display all the settable parameters and their current values, you can use show with no arguments; you may also use info set.  Both commands produce the same display.

Here are three miscellaneous show subcommands, all of which are exceptional in lacking corresponding set commands:

show version
     Show what version of GDBN is running.  You should include this information in GDBN bug-reports.  If multiple versions of GDBN are in use at your site, you may occasionally want to determine which version of GDBN you are running; as GDBN evolves, new commands are introduced, and old ones may wither away.  The version number is also announced when you start GDBN with no arguments.

show copying
     Display information about permission for copying GDBN.

show warranty
     Display the GNU ``NO WARRANTY'' statement.

Running Programs Under GDBN
***************************

When you run a program under GDBN, you must first generate debugging information when you compile it. You may start it with its arguments, if any, in an environment of your choice.  You may redirect your program's input and output, debug an already running process, or kill a child process.

Compiling for debugging
=======================

In order to debug a program effectively, you need to generate debugging information when you compile it.  This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.

To request debugging information, specify the -g option when you run the compiler.

Many C compilers are unable to handle the -g and -O options together.  Using those compilers, you cannot generate optimized executables containing debugging information.

NGCC, the GNU C compiler, supports -g with or without -O, making it possible to debug optimized code.  We recommend that you always use -g whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck.

When you debug a program compiled with -g -O, remember that the optimizer is rearranging your code; the debugger will show you what is really there.  Do not be too surprised when the execution path does not exactly match your source file!  An extreme example: if you define a variable, but never use it, GDBN will never see that variable---because the compiler optimizes it out of existence.

Some things do not work as well with -g -O as with just -g, particularly on machines with instruction scheduling.  If in doubt, recompile with -g alone, and if this fixes the problem, please report it as a bug (including a test case!).

Older versions of the GNU C compiler permitted a variant option -gg for debugging information.  GDBN no longer supports this format; if your GNU C compiler has this option, do not use it.

Starting your program
=====================

run
r
     Use the run command to start your program under GDBN.  You must first specify the program name with an argument to GDBN (see Getting In and Out of GDBN), or by using the file or exec-file command (see Commands to specify files).

If you are running your program in an execution environment that supports processes, run creates an inferior process and makes that process run your program.  (In environments without processes, run jumps to the start of your program.)

The execution of a program is affected by certain information it receives from its superior.  GDBN provides ways to specify this information, which you must do before starting your program.  (You can change it after starting your program, but such changes will only affect your program the next time you start it.)  This information may be divided into four categories:

The arguments.
     Specify the arguments to give your program as the arguments of the run command.  If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments.  In Unix systems, you can control which shell is used with the SHELL environment variable. See Your program's arguments.

The environment.
     Your program normally inherits its environment from GDBN, but you can use the GDBN commands set environment and unset
     environment to change parts of the environment that will be given to your program.  See Your program's environment.

The working directory.
     Your program inherits its working directory from GDBN.  You can set the GDBN working directory with the cd command in GDBN. See Your program's working directory.

The standard input and output.
     Your program normally uses the same device for standard input and standard output as GDBN is using.  You can redirect input and output in the run command line, or you can use the tty command to set a different device for your program. See Your program's input and output.

     Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDBN is likely to wind up debugging the wrong program.

When you issue the run command, your program begins to execute immediately.  See Stopping and continuing, for discussion of how to arrange for your program to stop.  Once your program has stopped, you may calls functions in your program, using the print or call commands.  See Examining Data.

If the modification time of your symbol file has changed since the last time GDBN read its symbols, GDBN will discard its symbol table and re-read it.  When it does this, GDBN tries to retain your current breakpoints.

Your program's arguments
========================

The arguments to your program can be specified by the arguments of the run command.  They are passed to a shell, which expands wildcard characters and performs redirection of I/O, and thence to your program. GDBN uses the shell indicated by your SHELL environment variable if it exists; otherwise, GDBN uses /bin/sh.

run with no arguments uses the same arguments used by the previous run, or those set by the set args command.

set args
     Specify the arguments to be used the next time your program is run.  If set args has no arguments, run will execute your program with no arguments.  Once you have run your program with arguments, using set args before the next run is the only way to run it again without arguments.

show args
     Show the arguments to give your program when it is started.

Your program's environment
==========================

The environment consists of a set of environment variables and their values.  Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run.  Usually you set up environment variables with the shell and they are inherited by all the other programs you run.  When debugging, it can be useful to try running your program with a modified environment without having to start GDBN over again.

path DIRECTORY
     Add DIRECTORY to the front of the PATH environment variable (the search path for executables), for both GDBN and your program. You may specify several directory names, separated by : or whitespace.  If DIRECTORY is already in the path, it is moved to the front, so it will be searched sooner.

     You can use the string $cwd to refer to whatever is the current working directory at the time GDBN searches the path.  If you use . instead, it refers to the directory where you executed the path command.  GDBN fills in the current path where needed in the DIRECTORY argument, before adding it to the search path.

show paths
     Display the list of search paths for executables (the PATH environment variable).

show environment [VARNAME]
     Print the value of environment variable VARNAME to be given to your program when it starts.  If you do not supply VARNAME, print the names and values of all environment variables to be given to your program.  You can abbreviate environment as env.

set environment VARNAME [=] VALUE
     Set environment variable VARNAME to VALUE.  The value changes for your program only, not for GDBN itself.  VALUE may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself.  The VALUE parameter is optional; if it is eliminated, the variable is set to a null value.

     For example, this command:

          set env USER = foo

     tells a Unix program, when subsequently run, that its user is named foo.  (The spaces around = are used for clarity here; they are not actually required.)

unset environment VARNAME
     Remove variable VARNAME from the environment to be passed to your program.  This is different from set env VARNAME =; unset environment removes the variable from the environment, rather than assigning it an empty value.

Your program's working directory
================================

Each time you start your program with run, it inherits its working directory from the current working directory of GDBN. The GDBN working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in GDBN with the cd command.

The GDBN working directory also serves as a default for the commands that specify files for GDBN to operate on.  See Commands to specify files.

cd DIRECTORY
     Set the GDBN working directory to DIRECTORY.

pwd
     Print the GDBN working directory.

Your program's input and output
===============================

By default, the program you run under GDBN does input and output to the same terminal that GDBN uses.  GDBN switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.

info terminal
     Displays information recorded by GDBN about the terminal modes your program is using.

You can redirect your program's input and/or output using shell redirection with the run command.  For example,

     run > outfile

starts your program, diverting its output to the file outfile.

Another way to specify where your program should do input and output is with the tty command.  This command accepts a file name as argument, and causes this file to be the default for future run commands.  It also resets the controlling terminal for the child process, for future run commands.  For example,

     tty /dev/ttyb

directs that processes started with subsequent run commands default to do input and output on the terminal /dev/ttyb and have that as their controlling terminal.

An explicit redirection in run overrides the tty command's effect on the input/output device, but not its effect on the controlling terminal.

When you use the tty command or redirect input in the run command, only the input for your program is affected.  The input for GDBN still comes from your terminal.

Debugging an already-running process
====================================

attach PROCESS-ID
     This command attaches to a running process---one that was started outside GDBN.  (info files will show your active targets.)  The command takes as argument a process ID.  The usual way to find out the process-id of a Unix process is with the ps utility, or with the jobs -l shell command.

     attach will not repeat if you press RET a second time after executing the command.

To use attach, you must be debugging in an environment which supports processes.  You must also have permission to send the process a signal, and it must have the same effective user ID as the GDBN process.

When using attach, you should first use the file command to specify the program running in the process and load its symbol table. See Commands to Specify Files.

The first thing GDBN does after arranging to debug the specified process is to stop it.  You can examine and modify an attached process with all the GDBN commands that are ordinarily available when you start processes with run.  You can insert breakpoints; you can step and continue; you can modify storage.  If you would rather the process continue running, you may use the continue command after attaching GDBN to the process.

detach
     When you have finished debugging the attached process, you can use the detach command to release it from GDBN control.  Detaching the process continues its execution.  After the detach command, that process and GDBN become completely independent once more, and you are ready to attach another process or start one with run. detach will not repeat if you press RET again after executing the command.

If you exit GDBN or use the run command while you have an attached process, you kill that process.  By default, you will be asked for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (see Optional warnings and messages).

Killing the child process
=========================

kill
     Kill the child process in which your program is running under GDBN.

This command is useful if you wish to debug a core dump instead of a running process.  GDBN ignores any core dump file while your program is running.

On some operating systems, a program cannot be executed outside GDBN while you have breakpoints set on it inside GDBN.  You can use the kill command in this situation to permit running your program outside the debugger.

The kill command is also useful if you wish to recompile and relink your program, since on many systems it is impossible to modify an executable file while it is running in a process.  In this case, when you next type run, GDBN will notice that the file has changed, and will re-read the symbol table (while trying to preserve your current breakpoint settings).

Additional process information
==============================

Some operating systems provide a facility called /proc that can be used to examine the image of a running process using file-system subroutines.  If GDBN is configured for an operating system with this facility, the command info proc is available to report on several kinds of information about the process running your program.

info proc
     Summarize available information about the process.

info proc mappings
     Report on the address ranges accessible in the program, with information on whether your program may read, write, or execute each range.

info proc times
     Starting time, user CPU time, and system CPU time for your program and its children.

info proc id
     Report on the process IDs related to your program: its own process ID, the ID of its parent, the process group ID, and the session ID.

info proc status
     General information on the state of the process.  If the process is stopped, this report includes the reason for stopping, and any signal received.

info proc all
     Show all the above information about the process.

Stopping and Continuing
***********************

The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.

Inside GDBN, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a GDBN command such as step.  You may then examine and change variables, set new breakpoints or remove old ones, and then continue execution.  Usually, the messages shown by GDBN provide ample explanation of the status of your program---but you can also explicitly request this information at any time.

info program
     Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.

Breakpoints, watchpoints, and exceptions
========================================

A breakpoint makes your program stop whenever a certain point in the program is reached.  For each breakpoint, you can add various conditions to control in finer detail whether your program will stop. You can set breakpoints with the break command and its variants (see Setting breakpoints), to specify the place where your program should stop by line number, function name or exact address in the program. In languages with exception handling (such as GNU C++), you can also set breakpoints where an exception is raised (see Breakpoints and exceptions).

A watchpoint is a special breakpoint that stops your program when the value of an expression changes.  You must use a different command to set watchpoints (see Setting watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically whenever GDBN stops at a breakpoint.  See Automatic display.

GDBN assigns a number to each breakpoint or watchpoint when you create it; these numbers are successive integers starting with one.  In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.

Setting breakpoints
-------------------

Breakpoints are set with the break command (abbreviated b).  The debugger convenience variable $bpnum records the number of the beakpoint you've set most recently; see Convenience variables, for a discussion of what you can do with convenience variables.

You have several ways to say where the breakpoint should go.

break FUNCTION
     Set a breakpoint at entry to function FUNCTION. When using source languages that permit overloading of symbols, such as C++, FUNCTION may refer to more than one possible place to break. See Breakpoint menus, for a discussion of that situation.

break +OFFSET
break -OFFSET
     Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected frame.

break LINENUM
     Set a breakpoint at line LINENUM in the current source file. That file is the last file whose source text was printed.  This breakpoint will stop your program just before it executes any of the code on that line.

break FILENAME:LINENUM
     Set a breakpoint at line LINENUM in source file FILENAME.

break FILENAME:FUNCTION
     Set a breakpoint at entry to function FUNCTION found in file FILENAME.  Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions.

break *ADDRESS
     Set a breakpoint at address ADDRESS.  You can use this to set breakpoints in parts of your program which do not have debugging information or source files.

break
     When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see Examining the Stack).  In any selected frame but the innermost, this will cause your program to stop as soon as control returns to that frame.  This is similar to the effect of a finish command in the frame inside the selected frame---except that finish does not leave an active breakpoint.  If you use break without an argument in the innermost frame, GDBN will stop the next time it reaches the current location; this may be useful inside loops.

     GDBN normally ignores breakpoints when it resumes execution, until at least one instruction has been executed.  If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint.  This rule applies whether or not the breakpoint already existed when your program stopped.

break ... if COND
     Set a breakpoint with condition COND; evaluate the expression COND each time the breakpoint is reached, and stop only if the value is nonzero---that is, if COND evaluates as true. ... stands for one of the possible arguments described above (or no argument) specifying where to break.  See Break conditions, for more information on breakpoint conditions.

tbreak ARGS
     Set a breakpoint enabled only for one stop.  ARGS are the same as for the break command, and the breakpoint is set in the same way, but the breakpoint is automatically disabled after the first time your program stops there.  See Disabling breakpoints.

rbreak REGEX
     Set breakpoints on all functions matching the regular expression REGEX.  This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set. Once these breakpoints are set, they are treated just like the breakpoints set with the break command.  They can be deleted, disabled, made conditional, etc., in the standard ways.

     When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

info breakpoints [N]
info break [N]
info watchpoints [N]
     Print a table of all breakpoints and watchpoints set and not deleted, with the following columns for each breakpoint:

     Breakpoint Numbers

     Type
          Breakpoint or watchpoint.

     Disposition
          Whether the breakpoint is marked to be disabled or deleted when hit.

     Enabled or Disabled
          Enabled breakpoints are marked with y.  n marks breakpoints that are not enabled.

     Address
          Where the breakpoint is in your program, as a memory address

     What
          Where the breakpoint is in the source for your program, as a file and line number.

     Breakpoint commands, if any, are listed after the line for the corresponding breakpoint.

     info break with a breakpoint number N as argument lists only that breakpoint.  The convenience variable $_ and the default examining-address for the x command are set to the address of the last breakpoint listed (see Examining memory).

GDBN allows you to set any number of breakpoints at the same place in your program.  There is nothing silly or meaningless about this.  When the breakpoints are conditional, this is even useful (see Break conditions).

GDBN itself sometimes sets breakpoints in your program for special purposes, such as proper handling of longjmp (in C programs). These internal breakpoints are assigned negative numbers, starting with -1; info breakpoints does not display them.

You can see these breakpoints with the GDBN maintenance command maint info breakpoints.

maint info breakpoints
     Using the same format as info breakpoints, display both the breakpoints you've set explicitly, and those GDBN is using for internal purposes.  Internal breakpoints are shown with negative breakpoint numbers.  The type column identifies what kind of breakpoint is shown:

     breakpoint
          Normal, explicitly set breakpoint.

     watchpoint
          Normal, explicitly set watchpoint.

     longjmp
          Internal breakpoint, used to handle correctly stepping through longjmp calls.

     longjmp resume
          Internal breakpoint at the target of a longjmp.

     until
          Temporary internal breakpoint used by the GDBN until command.

     finish
          Temporary internal breakpoint used by the GDBN finish command.

Setting watchpoints
-------------------

You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.

Watchpoints currently execute two orders of magnitude more slowly than other breakpoints, but this can well be worth it to catch errors where you have no clue what part of your program is the culprit.  Some processors provide special hardware to support watchpoint evaluation; future releases of GDBN will use such hardware if it is available.

watch EXPR
     Set a watchpoint for an expression.

info watchpoints
     This command prints a list of watchpoints and breakpoints; it is the same as info break.

Breakpoints and exceptions
--------------------------

Some languages, such as GNU C++, implement exception handling.  You can use GDBN to examine what caused your program to raise an exception, and to list the exceptions your program is prepared to handle at a given point in time.

catch EXCEPTIONS
     You can set breakpoints at active exception handlers by using the catch command.  EXCEPTIONS is a list of names of exceptions to catch.

You can use info catch to list active exception handlers. See Information about a frame.

There are currently some limitations to exception handling in GDBN. These will be corrected in a future release.

  * If you call a function interactively, GDBN normally returns control to you when the function has finished executing.  If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program to simply continue running until it hits a breakpoint, catches a signal that GDBN is listening for, or exits.

  * You cannot raise an exception interactively.

  * You cannot interactively install an exception handler.

Sometimes catch is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it is better to stop before the exception handler is called, since that way you can see the stack before any unwinding takes place.  If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised.

To stop just before an exception handler is called, you need some knowledge of the implementation.  In the case of GNU C++, exceptions are raised by calling a library function named __raise_exception which has the following ANSI C interface:

         /* ADDR is where the exception identifier is stored.
            ID is the exception identifier.  */
         void __raise_exception (void **ADDR, void *ID);

To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on __raise_exception (see Breakpoints; watchpoints; and exceptions).

With a conditional breakpoint (see Break conditions) that depends on the value of ID, you can stop your program when a specific exception is raised.  You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.

Deleting breakpoints
--------------------

It is often necessary to eliminate a breakpoint or watchpoint once it has done its job and you no longer want your program to stop there.  This is called deleting the breakpoint.  A breakpoint that has been deleted no longer exists; it is forgotten.

With the clear command you can delete breakpoints according to where they are in your program.  With the delete command you can delete individual breakpoints or watchpoints by specifying their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it.  GDBN automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.

clear
     Delete any breakpoints at the next instruction to be executed in the selected stack frame (see Selecting a frame).  When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.

clear FUNCTION
clear FILENAME:FUNCTION
     Delete any breakpoints set at entry to the function FUNCTION.

clear LINENUM
clear FILENAME:LINENUM
     Delete any breakpoints set at or within the code of the specified line.

delete [breakpoints] [BNUMS...]
     Delete the breakpoints or watchpoints of the numbers specified as arguments.  If no argument is specified, delete all breakpoints (GDBN asks confirmation, unless you have set confirm off).  You can abbreviate this command as d.

Disabling breakpoints
---------------------

Rather than deleting a breakpoint or watchpoint, you might prefer to disable it.  This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.

You disable and enable breakpoints and watchpoints with the enable and disable commands, optionally specifying one or more breakpoint numbers as arguments.  Use info break or info watch to print a list of breakpoints or watchpoints if you do not know which numbers to use.

A breakpoint or watchpoint can have any of four different states of enablement:

  * Enabled.  The breakpoint will stop your program.  A breakpoint set with the break command starts out in this state.

  * Disabled.  The breakpoint has no effect on your program.

  * Enabled once.  The breakpoint will stop your program, but when it does so it will become disabled.  A breakpoint set with the tbreak command starts out in this state.

  * Enabled for deletion.  The breakpoint will stop your program, but immediately after it does so it will be deleted permanently.

You can use the following commands to enable or disable breakpoints and watchpoints:

disable [breakpoints] [BNUMS...]
     Disable the specified breakpoints---or all breakpoints, if none are listed.  A disabled breakpoint has no effect but is not forgotten.  All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later.  You may abbreviate disable as dis.

enable [breakpoints] [BNUMS...]
     Enable the specified breakpoints (or all defined breakpoints).  They become effective once again in stopping your program.

enable [breakpoints] once BNUMS...
     Enable the specified breakpoints temporarily.  Each will be disabled again the next time it stops your program.

enable [breakpoints] delete BNUMS...
     Enable the specified breakpoints to work once and then die.  Each of the breakpoints will be deleted the next time it stops your program.

Save for a breakpoint set with tbreak (see Setting breakpoints), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above.  (The command until can set and delete a breakpoint of its own, but it will not change the state of your other breakpoints; see Continuing and stepping.)

Break conditions
----------------

The simplest sort of breakpoint breaks every time your program reaches a specified place.  You can also specify a condition for a breakpoint.  A condition is just a Boolean expression in your programming language (see Expressions).  A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated---that is, when the condition is false.  In C, if you want to test an assertion expressed by the condition ASSERT, you should set the condition ! ASSERT on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow---but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.

Break conditions can have side effects, and may even call functions in your program.  This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address.  (In that case, GDBN might see the other breakpoint first and stop your program without checking the condition of this one.)  Note that breakpoint commands are usually more convenient and flexible for the purpose of performing side effects when a breakpoint is reached (see Breakpoint command lists).

Break conditions can be specified when a breakpoint is set, by using if in the arguments to the break command.  See Setting breakpoints.  They can also be changed at any time with the condition command.  The watch command does not recognize the if keyword; condition is the only way to impose a further condition on a watchpoint.

condition BNUM EXPRESSION
     Specify EXPRESSION as the break condition for breakpoint or watchpoint number BNUM.  From now on, this breakpoint will stop your program only if the value of EXPRESSION is true (nonzero, in C).  When you use condition, GDBN checks EXPRESSION immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint. GDBN does not actually evaluate EXPRESSION at the time the condition command is given, however.  See Expressions.

condition BNUM
     Remove the condition from breakpoint number BNUM.  It becomes an ordinary unconditional breakpoint.

A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times.  This is so useful that there is a special way to do it, using the ignore count of the breakpoint.  Every breakpoint has an ignore count, which is an integer.  Most of the time, the ignore count is zero, and therefore has no effect.  But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues.  As a result, if the ignore count value is N, the breakpoint will not stop the next N times it is reached.

ignore BNUM COUNT
     Set the ignore count of breakpoint number BNUM to COUNT. The next COUNT times the breakpoint is reached, your program's execution will not stop; other than to decrement the ignore count, GDBN takes no action.

     To make the breakpoint stop the next time it is reached, specify a count of zero.

continue COUNT
c COUNT
fg COUNT
     Continue execution of your program, setting the ignore count of the breakpoint where your program stopped to COUNT minus one. Thus, your program will not stop at this breakpoint until the COUNT'th time it is reached.

     An argument to this command is meaningful only when your program stopped due to a breakpoint.  At other times, the argument to continue is ignored.

     The synonym fg is provided purely for convenience, and has exactly the same behavior as other forms of the command.

If a breakpoint has a positive ignore count and a condition, the condition is not checked.  Once the ignore count reaches zero, the condition will be checked.

You could achieve the effect of the ignore count with a condition such as $foo-- <= 0 using a debugger convenience variable that is decremented each time.  See Convenience variables.

Breakpoint command lists
------------------------

You can give any breakpoint (or watchpoint) a series of commands to execute when your program stops due to that breakpoint.  For example, you might want to print the values of certain expressions, or enable other breakpoints.

commands [BNUM]
... COMMAND-LIST ...
end
     Specify a list of commands for breakpoint number BNUM.  The commands themselves appear on the following lines.  Type a line containing just end to terminate the commands.

     To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.

     With no BNUM argument, commands refers to the last breakpoint or watchpoint set (not to the breakpoint most recently encountered).

Pressing RET as a means of repeating the last GDBN command is disabled within a COMMAND-LIST.

You can use breakpoint commands to start your program up again.  Simply use the continue command, or step, or any other command that resumes execution.

Any other commands in the command list, after a command that resumes execution, are ignored.  This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint---which could have its own command list, leading to ambiguities about which list to execute.

If the first command you specify in a command list is silent, the usual message about stopping at a breakpoint is not printed.  This may be desirable for breakpoints that are to print a specific message and then continue.  If none of the remaining commands print anything, you will see no sign that the breakpoint was reached.  silent is meaningful only at the beginning of a breakpoint command list.

The commands echo and output that allow you to print precisely controlled output are often useful in silent breakpoints. See Commands for controlled output.

For example, here is how you could use breakpoint commands to print the value of x at entry to foo whenever x is positive.

     break foo if x>0
     commands
     silent
     echo x is\040
     output x
     echo \n
     cont
     end

One application for breakpoint commands is to compensate for one bug so you can test for another.  Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them.  End with the continue command so that your program does not stop, and start with the silent command so that no output is produced.  Here is an example:

     break 403
     commands
     silent
     set x = y + 4
     cont
     end

One deficiency in the operation of automatically continuing breakpoints under Unix appears when your program uses raw mode for the terminal. GDBN switches back to its own terminal modes (not raw) before executing commands, and then must switch back to raw mode when your program is continued.  This causes any pending terminal input to be lost.

Under Unix, you can get around this problem by writing actions into the breakpoint condition rather than in commands.  For example,

     condition 5  (x = y + 4), 0

specifies a condition expression (see Expressions) that will change x as needed, then always have the value zero so your program will not stop.  No input is lost here, because GDBN evaluates break conditions without changing the terminal modes.  When you want to have nontrivial conditions for performing the side effects, the operators &&, || and ?...: may be useful.

Breakpoint menus
----------------

Some programming languages (notably C++) permit a single function name to be defined several times, for application in different contexts. This is called overloading.  When a function name is overloaded, break FUNCTION is not enough to tell GDBN where you want a breakpoint.  If you realize this will be a problem, you can use something like break FUNCTION(TYPES) to specify which particular version of the function you want.  Otherwise, GDBN offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the prompt >.  The first two options are always [0] cancel and [1] all.  Typing 1 sets a breakpoint at each definition of FUNCTION, and typing 0 aborts the break command without setting any new breakpoints.

For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol String::after. We choose three particular definitions of that function name:

     (GDBP) b String::after
     [0] cancel
     [1] all
     [2] file:String.cc; line number:867
     [3] file:String.cc; line number:860
     [4] file:String.cc; line number:875
     [5] file:String.cc; line number:853
     [6] file:String.cc; line number:846
     [7] file:String.cc; line number:735
     > 2 4 6
     Breakpoint 1 at 0xb26c: file String.cc, line 867.
     Breakpoint 2 at 0xb344: file String.cc, line 875.
     Breakpoint 3 at 0xafcc: file String.cc, line 846.
     Multiple breakpoints were set.
     Use the "delete" command to delete unwanted breakpoints.
     (GDBP)

``Cannot insert breakpoints''
-----------------------------

Under some operating systems, breakpoints cannot be used in a program if any other process is running that program.  In this situation, attempting to run or continue a program with a breakpoint causes GDBN to stop the other process.

When this happens, you have three ways to proceed:

 1. Remove or disable the breakpoints, then continue.

 2. Suspend GDBN, and copy the file containing your program to a new name. Resume GDBN and use the exec-file command to specify that GDBN should run your program under that name.  Then start your program again.

 3. Relink your program so that the text segment is nonsharable, using the linker option -N.  The operating system limitation may not apply to nonsharable executables.

Continuing and stepping
=======================

Continuing means resuming program execution until your program completes normally.  In contrast, stepping means executing just one more ``step'' of your program, where ``step'' may mean either one line of source code, or one machine instruction (depending on what particular command you use).  Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or to a signal.  (If due to a signal, you may want to use handle, or use signal 0 to resume execution. See Signals.)

continue [IGNORE-COUNT]
     Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed.  The optional argument IGNORE-COUNT allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore (see Break conditions).

     To resume execution at a different place, you can use return (see Returning from a function) to go back to the calling function; or jump (see Continuing at a different address) to go to an arbitrary location in your program.

A typical technique for using stepping is to set a breakpoint (see Breakpoints; watchpoints; and exceptions) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

step
     Continue running your program until control reaches a different source line, then stop it and return control to GDBN.  This command is abbreviated s.

          Warning: If you use the step command while control is within a function that was compiled without debugging information, execution will proceed until control reaches another function.

step COUNT
     Continue running as in step, but do so COUNT times.  If a breakpoint is reached, or a signal not related to stepping occurs before COUNT steps, stepping stops right away.

next [COUNT]
     Continue to the next source line in the current (innermost) stack frame. Similar to step, but any function calls appearing within the line of code are executed without stopping.  Execution stops when control reaches a different line of code at the stack level which was executing when the next command was given.  This command is abbreviated n.

     An argument COUNT is a repeat count, as for step.

     next within a function that lacks debugging information acts like step, but any function calls appearing within the code of the function are executed without stopping.

finish
     Continue running until just after function in the selected stack frame returns.  Print the returned value (if any).

     Contrast this with the return command (see Returning from a function).

until

u
     Continue running until a source line past the current line, in the current stack frame, is reached.  This command is used to avoid single stepping through a loop more than once.  It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump.

     This means that when you reach the end of a loop after single stepping though it, until will cause your program to continue execution until the loop is exited.  In contrast, a next command at the end of a loop will simply step back to the beginning of the loop, which would force you to step through the next iteration.

     until always stops your program if it attempts to exit the current stack frame.

     until may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines.  For example, in the following excerpt from a debugging session, the f (frame) command shows that execution is stopped at line 206; yet when we use until, we get to line 195:

          (GDBP) f
          #0  main (argc=4, argv=0xf7fffae8) at m4.c:206
          206                 expand_input();
          (GDBP) until
          195             for ( ; argc > 0; NEXTARG) {

     This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop---even though the test in a C for-loop is written before the body of the loop.  The until command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement---not in terms of the actual machine code.

     until with no argument works by means of single instruction stepping, and hence is slower than until with an argument.

until LOCATION

u LOCATION
     Continue running your program until either the specified location is reached, or the current stack frame returns.  LOCATION is any of the forms of argument acceptable to break (see Setting breakpoints).  This form of the command uses breakpoints, and hence is quicker than until without an argument.

stepi
si
     Execute one machine instruction, then stop and return to the debugger.

     It is often useful to do display/i $pc when stepping by machine instructions.  This will cause the next instruction to be executed to be displayed automatically at each stop.  See Automatic display.

     An argument is a repeat count, as in step.

nexti
ni
     Execute one machine instruction, but if it is a function call, proceed until the function returns.

     An argument is a repeat count, as in next.

Examining the Stack
*******************

When your program has stopped, the first thing you need to know is where it stopped and how it got there.

Each time your program performs a function call, the information about where in your program the call was made from is saved in a block of data called a stack frame.  The frame also contains the arguments of the call and the local variables of the function that was called.  All the stack frames are allocated in a region of memory called the call stack.

When your program stops, the GDBN commands for examining the stack allow you to see all of this information.

One of the stack frames is selected by GDBN and many GDBN commands refer implicitly to the selected frame.  In particular, whenever you ask GDBN for the value of a variable in your program, the value is found in the selected frame.  There are special GDBN commands to select whichever frame you are interested in.

When your program stops, GDBN automatically selects the currently executing frame and describes it briefly as the frame command does (see Information about a frame).

Stack frames
============

The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function.  The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.

When your program is started, the stack has only one frame, that of the function main.  This is called the initial frame or the outermost frame.  Each time a function is called, a new frame is made.  Each time a function returns, the frame for that function invocation is eliminated.  If a function is recursive, there can be many frames for the same function.  The frame for the function in which execution is actually occurring is called the innermost frame.  This is the most recently created of all the stack frames that still exist.

Inside your program, stack frames are identified by their addresses.  A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one of those bytes whose address serves as the address of the frame.  Usually this address is kept in a register called the frame pointer register while execution is going on in that frame.

GDBN assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward.  These numbers do not really exist in your program; they are assigned by GDBN to give you a way of designating stack frames in GDBN commands.

Some compilers provide a way to compile functions so that they operate without stack frames.  (For example, the GCC option -fomit-frame-pointer will generate functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time.  GDBN has limited facilities for dealing with these function invocations.  If the innermost function invocation has no stack frame, GDBN will nevertheless regard it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain.  However, GDBN has no provision for frameless functions elsewhere in the stack.

Backtraces
==========

A backtrace is a summary of how your program got where it is.  It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

backtrace
bt
     Print a backtrace of the entire stack: one line per frame for all frames in the stack.

     You can stop the backtrace at any time by typing the system interrupt character, normally C-c.

backtrace N
bt N
     Similar, but print only the innermost N frames.

backtrace -N
bt -N
     Similar, but print only the outermost N frames.

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown---unless you use set print address off.  The backtrace also shows the source file name and line number, as well as the arguments to the function.  The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace.  It was made with the command bt 3, so it shows the innermost three frames.

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
         at builtin.c:993
     #1  0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
     #2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
         at macro.c:71
     (More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

Selecting a frame
=================

Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment.  Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.

frame N
f N
     Select frame number N.  Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on.  The highest-numbered frame is the one for main.

frame ADDR
f ADDR
     Select the frame at address ADDR.  This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDBN to assign numbers properly to all frames.  In addition, this can be useful when your program has multiple stacks and switches between them.

up N
     Move N frames up the stack.  For positive numbers N, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer.  N defaults to one.

down N
     Move N frames down the stack.  For positive numbers N, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently.  N defaults to one.  You may abbreviate down as do.

All of these commands end by printing two lines of output describing the frame.  The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame.  The second line shows the text of that source line.

For example:

     (GDBP) up
     #1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
         at env.c:10
     10              read_input_file (argv[i]);

After such a printout, the list command with no arguments will print ten lines centered on the point of execution in the frame. See Printing source lines.

up-silently N
down-silently N
     These two commands are variants of up and down, respectively; they differ in that they do their work silently, without causing display of the new frame.  They are intended primarily for use in GDBN command scripts, where the output might be unnecessary and distracting.

Information about a frame
=========================

There are several other commands to print information about the selected stack frame.

frame
f
     When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame.  It can be abbreviated f.  With an argument, this command is used to select a stack frame. See Selecting a frame.

info frame
info f
     This command prints a verbose description of the selected stack frame, including the address of the frame, the addresses of the next frame down (called by this frame) and the next frame up (caller of this frame), the language that the source code corresponding to this frame was written in, the address of the frame's arguments, the program counter saved in it (the address of execution in the caller frame), and which registers were saved in the frame.  The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions.

info frame ADDR
info f ADDR
     Print a verbose description of the frame at address ADDR, without selecting that frame.  The selected frame remains unchanged by this command.

info args
     Print the arguments of the selected frame, each on a separate line.

info locals
     Print the local variables of the selected frame, each on a separate line.  These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame.

info catch
     Print a list of all the exception handlers that are active in the current stack frame at the current point of execution.  To see other exception handlers, visit the associated frame (using the up, down, or frame commands); then type info catch. See Breakpoints and exceptions.

Examining Source Files
**********************

GDBN can print parts of your program's source, since the debugging information recorded in the program tells GDBN what source files were used to build it.  When your program stops, GDBN spontaneously prints the line where it stopped.  Likewise, when you select a stack frame (see Selecting a frame), GDBN prints the line where execution in that frame has stopped.  You can print other portions of source files by explicit command.

If you use GDBN through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see Using GDBN under GNU Emacs.

Printing source lines
=====================

To print lines from a source file, use the list command (abbreviated l).  There are several ways to specify what part of the file you want to print.

Here are the forms of the list command most commonly used:

list LINENUM
     Print lines centered around line number LINENUM in the current source file.

list FUNCTION
     Print lines centered around the beginning of function FUNCTION.

list
     Print more lines.  If the last lines printed were printed with a list command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (see Examining the Stack), this prints lines centered around that line.

list -
     Print lines just before the lines last printed.

By default, GDBN prints ten source lines with any of these forms of the list command.  You can change this using set listsize:

set listsize COUNT
     Make the list command display COUNT source lines (unless the list argument explicitly specifies some other number).

show listsize
     Display the number of lines that list will currently display by default.

Repeating a list command with RET discards the argument, so it is equivalent to typing just list.  This is more useful than listing the same lines again.  An exception is made for an argument of -; that argument is preserved in repetition so that each repetition moves up in the source file.

In general, the list command expects you to supply zero, one or two linespecs.  Linespecs specify source lines; there are several ways of writing them but the effect is always to specify some source line. Here is a complete description of the possible arguments for list:

list LINESPEC
     Print lines centered around the line specified by LINESPEC.

list FIRST,LAST
     Print lines from FIRST to LAST.  Both arguments are linespecs.

list ,LAST
     Print lines ending with LAST.

list FIRST,
     Print lines starting with FIRST.

list +
     Print lines just after the lines last printed.

list -
     Print lines just before the lines last printed.

list
     As described in the preceding table.

Here are the ways of specifying a single source line---all the kinds of linespec.

NUMBER
     Specifies line NUMBER of the current source file. When a list command has two linespecs, this refers to the same source file as the first linespec.

+OFFSET
     Specifies the line OFFSET lines after the last line printed. When used as the second linespec in a list command that has two, this specifies the line OFFSET lines down from the first linespec.

-OFFSET
     Specifies the line OFFSET lines before the last line printed.

FILENAME:NUMBER
     Specifies line NUMBER in the source file FILENAME.

FUNCTION
     Specifies the line of the open-brace that begins the body of the function FUNCTION.

FILENAME:FUNCTION
     Specifies the line of the open-brace that begins the body of the function FUNCTION in the file FILENAME.  You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files.

*ADDRESS
     Specifies the line containing the program address ADDRESS. ADDRESS may be any expression.

Searching source files
======================

There are two commands for searching through the current source file for a regular expression.

forward-search REGEXP
search REGEXP
     The command forward-search REGEXP checks each line, starting with the one following the last line listed, for a match for REGEXP.  It lists the line that is found.  You can use synonym search REGEXP or abbreviate the command name as fo.

reverse-search REGEXP
     The command reverse-search REGEXP checks each line, starting with the one before the last line listed and going backward, for a match for REGEXP.  It lists the line that is found.  You can abbreviate this command as rev.

Specifying source directories
=============================

Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names.  Even when they do, the directories could be moved between the compilation and your debugging session.  GDBN has a list of directories to search for source files; this is called the source path.  Each time GDBN wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.  Note that the executable search path is not used for this purpose.  Neither is the current working directory, unless it happens to be in the source path.

If GDBN cannot find a source file in the source path, and the object program records a directory, GDBN tries that directory too.  If the source path is empty, and there is no record of the compilation directory, GDBN will, as a last resort, look in the current directory.

Whenever you reset or rearrange the source path, GDBN will clear out any information it has cached about where source files are found, where each line is in the file, etc.

When you start GDBN, its source path is empty. To add other directories, use the directory command.

directory DIRNAME ...
     Add directory DIRNAME to the front of the source path.  Several directory names may be given to this command, separated by : or whitespace.  You may specify a directory that is already in the source path; this moves it forward, so it will be searched sooner.

     You can use the string $cdir to refer to the compilation directory (if one is recorded), and $cwd to refer to the current working directory.  $cwd is not the same as .---the former tracks the current working directory as it changes during your GDBN session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.

directory
     Reset the source path to empty again.  This requires confirmation.

show directories
     Print the source path: show which directories it contains.

If your source path is cluttered with directories that are no longer of interest, GDBN may sometimes cause confusion by finding the wrong versions of source.  You can correct the situation as follows:

 1. Use directory with no argument to reset the source path to empty.

 2. Use directory with suitable arguments to reinstall the directories you want in the source path.  You can add all the directories in one command.

Source and machine code
=======================

You can use the command info line to map source lines to program addresses (and vice versa), and the command disassemble to display a range of addresses as machine instructions.

info line LINESPEC
     Print the starting and ending addresses of the compiled code for source line LINESPEC.  You can specify source lines in any of the ways understood by the list command (see Printing source lines).

For example, we can use info line to discover the location of the object code for the first line of function m4_changequote:

     (GDBP) info line m4_changecom
     Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.

We can also inquire (using *ADDR as the form for LINESPEC) what source line covers a particular address:

     (GDBP) info line *0x63ff
     Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.

After info line, the default address for the x command is changed to the starting address of the line, so that x/i is sufficient to begin examining the machine code (see Examining memory).  Also, this address is saved as the value of the convenience variable $_ (see Convenience variables).

disassemble
     This specialized command dumps a range of memory as machine instructions.  The default memory range is the function surrounding the program counter of the selected frame.  A single argument to this command is a program counter value; the function surrounding this value will be dumped.  Two arguments specify a range of addresses (first inclusive, second exclusive) to dump.

We can use disassemble to inspect the object code range shown in the last info line example (the example shows SPARC machine instructions):

     (GDBP) disas 0x63e4 0x6404
     Dump of assembler code from 0x63e4 to 0x6404:
     0x63e4 <builtin_init+5340>:     ble 0x63f8 <builtin_init+5360>
     0x63e8 <builtin_init+5344>:     sethi %hi(0x4c00), %o0
     0x63ec <builtin_init+5348>:     ld [%i1+4], %o0
     0x63f0 <builtin_init+5352>:     b 0x63fc <builtin_init+5364>
     0x63f4 <builtin_init+5356>:     ld [%o0+4], %o0
     0x63f8 <builtin_init+5360>:     or %o0, 0x1a4, %o0
     0x63fc <builtin_init+5364>:     call 0x9288 <path_search>
     0x6400 <builtin_init+5368>:     nop
     End of assembler dump.

Examining Data
**************

The usual way to examine data in your program is with the print command (abbreviated p), or its synonym inspect. It evaluates and prints the value of an expression of the language your program is written in (see Using GDBN with Different Languages).

print EXP
print /F EXP
     EXP is an expression (in the source language).  By default the value of EXP is printed in a format appropriate to its data type; you can choose a different format by specifying /F, where F is a letter specifying the format; see Output formats.

print
print /F
     If you omit EXP, GDBN displays the last value again (from the value history; see Value history).  This allows you to conveniently inspect the same value in an alternative format.

A more low-level way of examining data is with the x command. It examines data in memory at a specified address and prints it in a specified format.  See Examining memory.

If you are interested in information about types, or about how the fields of a struct or class are declared, use the ptype EXP command rather than print. See Examining the Symbol Table.

Expressions
===========

print and many other GDBN commands accept an expression and compute its value.  Any kind of constant, variable or operator defined by the programming language you are using is valid in an expression in GDBN.  This includes conditional expressions, function calls, casts and string constants.  It unfortunately does not include symbols defined by preprocessor #define commands.

Because C is so widespread, most of the expressions shown in examples in this manual are in C.  See Using GDBN with Different Languages, for information on how to use expressions in other languages.

In this section, we discuss operators that you can use in GDBN expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer so as to examine a structure at that address in memory.

GDBN supports these operators in addition to those of programming languages:

@
     @ is a binary operator for treating parts of memory as arrays. See Artificial arrays, for more information.

::
     :: allows you to specify a variable in terms of the file or function where it is defined.  See Program variables.

{TYPE} ADDR
     Refers to an object of type TYPE stored at address ADDR in memory.  ADDR may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast).  This construct is allowed regardless of what kind of data is normally supposed to reside at ADDR.

Program variables
=================

The most common kind of expression to use is the name of a variable in your program.

Variables in expressions are understood in the selected stack frame (see Selecting a frame); they must either be global (or static) or be visible according to the scope rules of the programming language from the point of execution in that frame.  This means that in the function

     foo (a)
          int a;
     {
       bar (a);
       {
         int b = test ();
         bar (b);
       }
     }

you can examine and use the variable a whenever your program is executing within the function foo, but you can only use or examine the variable b while your program is executing inside the block where b is declared.

There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file.  But it is possible to have more than one such variable or function with the same name (in different source files).  If that happens, referring to that name has unpredictable effects.  If you wish, you can specify a static variable in a particular function or file, using the colon-colon notation:

     FILE::VARIABLE
     FUNCTION::VARIABLE

Here FILE or FUNCTION is the name of the context for the static VARIABLE.  In the case of file names, you can use quotes to make sure GDBN parses the file name as a single word---for example, to print a global value of x defined in f2.c:

     (GDBP) p 'f2.c'::x

This use of :: is very rarely in conflict with the very similar use of the same notation in C++.  GDBN also supports use of the C++ scope resolution operator in GDBN expressions.

     Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function---just after entry to a new scope, and just before exit.

You may see this problem when you are stepping by machine instructions. This is because on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built.  On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

Artificial arrays
=================

It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.

You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator @.  The left operand of @ should be the first element of the desired array, as an individual object.  The right operand should be the desired length of the array.  The result is an array value whose elements are all of the type of the left argument.  The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on.  Here is an example.  If a program says

     int *array = (int *) malloc (len * sizeof (int));

you can print the contents of array with

     p *array@len

The left operand of @ must reside in memory.  Array values made with @ in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see Value history), after printing one out.)

Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent---for example, if you are interested in the values of pointers in an array.  One useful work-around in this situation is to use a convenience variable (see Convenience variables) as a counter in an expression that prints the first interesting value, and then repeat that expression via RET.  For instance, suppose you have an array dtab of pointers to structures, and you are interested in the values of a field fv in each structure.  Here is an example of what you might type:

     set $i = 0
     p dtab[$i++]->fv
     RET
     RET
     ...

Output formats
==============

By default, GDBN prints a value according to its data type.  Sometimes this is not what you want.  For example, you might want to print a number in hex, or a pointer in decimal.  Or you might want to view data in memory at a certain address as a character string or as an instruction.  To do these things, specify an output format when you print a value.

The simplest use of output formats is to say how to print a value already computed.  This is done by starting the arguments of the print command with a slash and a format letter.  The format letters supported are:

x
     Regard the bits of the value as an integer, and print the integer in hexadecimal.

d
     Print as integer in signed decimal.

u
     Print as integer in unsigned decimal.

o
     Print as integer in octal.

t
     Print as integer in binary.  The letter t stands for ``two''. (b cannot be used because these format letters are also used with the x command, where b stands for ``byte''; see Examining memory.)

a
     Print as an address, both absolute in hex and as an offset from the nearest preceding symbol.  This format can be used to discover where (in what function) an unknown address is located:

          (GDBP) p/a 0x54320
          $3 = 0x54320 <_initialize_vx+396>

c
     Regard as an integer and print it as a character constant.

f
     Regard the bits of the value as a floating point number and print using typical floating point syntax.

For example, to print the program counter in hex (see Registers), type

     p/x $pc

Note that no space is required before the slash; this is because command names in GDBN cannot contain a slash.

To reprint the last value in the value history with a different format, you can use the print command with just a format and no expression.  For example, p/x reprints the last value in hex.

Examining memory
================

You can use the command x (for ``examine'') to examine memory in any of several formats, independently of your program's data types.

x/NFU ADDR
x ADDR
x
     Use the x command to examine memory.

N, F, and U are all optional parameters that specify how much memory to display and how to format it; ADDR is an expression giving the address where you want to start displaying memory. If you use defaults for NFU, you need not type the slash /. Several commands set convenient defaults for ADDR.

N, the repeat count
     The repeat count is a decimal integer; the default is 1.  It specifies how much memory (counting by units U) to display.

F, the display format
     The display format is one of the formats used by print, or s (null-terminated string) or i (machine instruction). The default is x (hexadecimal) initially, or the format from the last time you used either x or print.

U, the unit size
     The unit size is any of

     b
          Bytes.

     h
          Halfwords (two bytes).

     w
          Words (four bytes).  This is the initial default.

     g
          Giant words (eight bytes).

     Each time you specify a unit size with x, that size becomes the default unit the next time you use x.  (For the s and i formats, the unit size is ignored and is normally not written.)

ADDR, starting display address
     ADDR is the address where you want GDBN to begin displaying memory.  The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. See Expressions, for more information on expressions.  The default for ADDR is usually just after the last address examined---but several other commands also set the default address: info breakpoints (to the address of the last breakpoint listed), info line (to the starting address of a line), and print (if you use it to display a value from memory).

For example, x/3uh 0x54320 is a request to display three halfwords (h) of memory, formatted as unsigned decimal integers (u), starting at address 0x54320.  x/4xw $sp prints the four words (w) of memory above the stack pointer (here, $sp; see Registers) in hexadecimal (x).

Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order will work.  The output specifications 4xw and 4wx mean exactly the same thing. (However, the count N must come first; wx4 will not work.)

Even though the unit size U is ignored for the formats s and i, you might still want to use a count N; for example, 3i specifies that you want to see three machine instructions, including any operands.  The command disassemble gives an alternative way of inspecting machine instructions; see Source and machine code.

All the defaults for the arguments to x are designed to make it easy to continue scanning memory with minimal specifications each time you use x.  For example, after you have inspected three machine instructions with x/3i ADDR, you can inspect the next seven with just x/7.  If you use RET to repeat the x command, the repeat count N is used again; the other arguments default as for successive uses of x.

The addresses and contents printed by the x command are not saved in the value history because there is often too much of them and they would get in the way.  Instead, GDBN makes these values available for subsequent use in expressions as values of the convenience variables $_ and $__.  After an x command, the last address examined is available for use in expressions in the convenience variable $_.  The contents of that address, as examined, are available in the convenience variable $__.

If the x command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.

Automatic display
=================

If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic
display list so that GDBN will print its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:

     2: foo = 38
     3: bar[5] = (struct hack *) 0x3804

This display shows item numbers, expressions and their current values.  As with displays you request manually using x or print, you can specify the output format you prefer; in fact, display decides whether to use print or x depending on how elaborate your format specification is---it uses x if you specify a unit size, or one of the two formats (i and s) that are only supported by x; otherwise it uses print.

display EXP
     Add the expression EXP to the list of expressions to display each time your program stops.  See Expressions.

     display will not repeat if you press RET again after using it.

display/FMT EXP
     For FMT specifying only a display format and not a size or count, add the expression EXP to the auto-display list but arranges to display it each time in the specified format FMT. See Output formats.

display/FMT ADDR
     For FMT i or s, or including a unit-size or a number of units, add the expression ADDR as a memory address to be examined each time your program stops.  Examining means in effect doing x/FMT ADDR.  See Examining memory.

For example, display/i $pc can be helpful, to see the machine instruction about to be executed each time execution stops ($pc is a common name for the program counter; see Registers).

undisplay DNUMS...
delete display DNUMS...
     Remove item numbers DNUMS from the list of expressions to display.

     undisplay will not repeat if you press RET after using it. (Otherwise you would just get the error No display number ....)

disable display DNUMS...
     Disable the display of item numbers DNUMS.  A disabled display item is not printed automatically, but is not forgotten.  It may be enabled again later.

enable display DNUMS...
     Enable display of item numbers DNUMS.  It becomes effective once again in auto display of its expression, until you specify otherwise.

display
     Display the current values of the expressions on the list, just as is done when your program stops.

info display
     Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values.  This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available.

If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up.  Such an expression is disabled when execution enters a context where one of its variables is not defined.  For example, if you give the command display last_char while inside a function with an argument last_char, then this argument will be displayed while your program continues to stop inside that function.  When it stops elsewhere---where there is no variable last_char---display is disabled.  The next time your program stops where last_char is meaningful, you can enable the display expression once again.

Print settings
==============

GDBN provides the following ways to control how arrays, structures, and symbols are printed.

These settings are useful for debugging programs in any language:

set print address

set print address on
     GDBN will print memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses.  The default is on.  For example, this is what a stack frame display looks like, with set print address on:

          (GDBP) f
          #0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
              at input.c:530
          530         if (lquote != def_lquote)

set print address off
     Do not print addresses when displaying their contents.  For example, this is the same stack frame displayed with set print address off:

          (GDBP) set print addr off
          (GDBP) f
          #0  set_quotes (lq="<<", rq=">>") at input.c:530
          530         if (lquote != def_lquote)

     You can use set print address off to eliminate all machine dependent displays from the GDBN interface.  For example, with print address off, you should get the same text for backtraces on all machines---whether or not they involve pointer arguments.

show print address
     Show whether or not addresses are to be printed.

set print array
set print array on
     GDBN will pretty-print arrays.  This format is more convenient to read, but uses more space.  The default is off.

set print array off
     Return to compressed format for arrays.

show print array
     Show whether compressed or pretty format is selected for displaying arrays.

set print elements NUMBER-OF-ELEMENTS
     If GDBN is printing a large array, it will stop printing after it has printed the number of elements set by the set print elements command. This limit also applies to the display of strings.

show print elements
     Display the number of elements of a large array that GDBN will print before losing patience.

set print pretty on
     Cause GDBN to print structures in an indented format with one member per line, like this:

          $1 = {
            next = 0x0,
            flags = {
              sweet = 1,
              sour = 1
            },
            meat = 0x54 "Pork"
          }

set print pretty off
     Cause GDBN to print structures in a compact format, like this:

          $1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
          meat = 0x54 "Pork"}

     This is the default format.

show print pretty
     Show which format GDBN will use to print structures.

set print sevenbit-strings on
     Print using only seven-bit characters; if this option is set, GDBN will display any eight-bit characters (in strings or character values) using the notation \NNN.  For example, M-a is displayed as \341.

set print sevenbit-strings off
     Print using either seven-bit or eight-bit characters, as required.  This is the default.

show print sevenbit-strings
     Show whether or not GDBN will print only seven-bit characters.

set print union on
     Tell GDBN to print unions which are contained in structures.  This is the default setting.

set print union off
     Tell GDBN not to print unions which are contained in structures.

show print union
     Ask GDBN whether or not it will print unions which are contained in structures.

     For example, given the declarations

          typedef enum {Tree, Bug} Species;
          typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
          typedef enum {Caterpillar, Cocoon, Butterfly}
                        Bug_forms;

          struct thing {
            Species it;
            union {
              Tree_forms tree;
              Bug_forms bug;
            } form;
          };

          struct thing foo = {Tree, {Acorn}};

     with set print union on in effect p foo would print

          $1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}

     and with set print union off in effect it would print

          $1 = {it = Tree, form = {...}}

set print max-symbolic-offset MAXOFF
     Tell GDBN to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than MAXOFF.  The default is 0, which means to always print the symbolic form of an address, if any symbol precedes it.

show print max-symbolic-offset
     Ask how large the maximum offset is that GDBN will print in a symbolic address.

These settings are of interest when debugging C++ programs:

set print demangle
set print demangle on
     Print C++ names in their source form rather than in the encoded (``mangled'') form passed to the assembler and linker for type-safe linkage.  The default is on.

show print demangle
     Show whether C++ names will be printed in mangled or demangled form.

set print asm-demangle
set print asm-demangle on
     Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off.

show print asm-demangle
     Show whether C++ names in assembly listings will be printed in mangled or demangled form.

set demangle-style STYLE
     Choose among several encoding schemes used by different compilers to represent C++ names.  The choices for STYLE are currently:

     auto
          Allow GDBN to choose a decoding style by inspecting your program.

     gnu
          Decode based on the GNU C++ compiler (g++) encoding algorithm.

     lucid
          Decode based on the Lucid C++ compiler (lcc) encoding algorithm.

     arm
          Decode using the algorithm in the C++ Annotated Reference Manual. Warning: this setting alone is not sufficient to allow debugging cfront-generated executables.  GDBN would require further enhancement to permit that.

show demangle-style
     Display the encoding style currently in use for decoding C++ symbols.

set print object
set print object on
     When displaying a pointer to an object, identify the actual (derived) type of the object rather than the declared type, using the virtual function table.

set print object off
     Display only the declared type of objects, without reference to the virtual function table.  This is the default setting.

show print object
     Show whether actual, or declared, object types will be displayed.

set print vtbl
set print vtbl on
     Pretty print C++ virtual function tables.  The default is off.

set print vtbl off
     Do not pretty print C++ virtual function tables.

show print vtbl
     Show whether C++ virtual function tables are pretty printed, or not.

Value history
=============

Values printed by the print command are saved in the GDBN value
history so that you can refer to them in other expressions.  Values are kept until the symbol table is re-read or discarded (for example with the file or symbol-file commands).  When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table.

The values printed are given history numbers for you to refer to them by.  These are successive integers starting with one.  print shows you the history number assigned to a value by printing $NUM = before the value; here NUM is the history number.

To refer to any previous value, use $ followed by the value's history number.  The way print labels its output is designed to remind you of this.  Just $ refers to the most recent value in the history, and $$ refers to the value before that. $$N refers to the Nth value from the end; $$2 is the value just prior to $$, $$1 is equivalent to $$, and $$0 is equivalent to $.

For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure.  It suffices to type

     p *$

If you have a chain of structures where the component next points to the next one, you can print the contents of the next one with this:

     p *$.next

You can print successive links in the chain by repeating this command---which you can do by just typing RET.

Note that the history records values, not expressions.  If the value of x is 4 and you type these commands:

     print x
     set x=5

then the value recorded in the value history by the print command remains 4 even though the value of x has changed.

show values
     Print the last ten values in the value history, with their item numbers. This is like p $$9 repeated ten times, except that show values does not change the history.

show values N
     Print ten history values centered on history item number N.

show values +
     Print ten history values just after the values last printed.  If no more values are available, produces no display.

Pressing RET to repeat show values N has exactly the same effect as show values +.

Convenience variables
=====================

GDBN provides convenience variables that you can use within GDBN to hold on to a value and refer to it later.  These variables exist entirely within GDBN; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program.  That is why you can use them freely.

Convenience variables are prefixed with $.  Any name preceded by $ can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by $.  See Value history.)

You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:

     set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by object_ptr.

Using a convenience variable for the first time creates it; but its value is void until you assign a new value.  You can alter the value with another assignment at any time.

Convenience variables have no fixed types.  You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type.  The convenience variable, when used as an expression, has the type of its current value.

show convenience
     Print a list of convenience variables used so far, and their values. Abbreviated show con.

One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced.  For example, to print a field from successive elements of an array of structures:

     set $i = 0
     print bar[$i++]->contents
     ... repeat that command by typing RET.

Some convenience variables are created automatically by GDBN and given values likely to be useful.

$_
     The variable $_ is automatically set by the x command to the last address examined (see Examining memory).  Other commands which provide a default address for x to examine also set $_ to that address; these commands include info line and info breakpoint.  The type of $_ is void * except when set by the x command, in which case it is a pointer to the type of $__.

$__
     The variable $__ is automatically set by the x command to the value found in the last address examined.  Its type is chosen to match the format in which the data was printed.

Registers
=========

You can refer to machine register contents, in expressions, as variables with names starting with $.  The names of registers are different for each machine; use info registers to see the names used on your machine.

info registers
     Print the names and values of all registers except floating-point registers (in the selected stack frame).

info all-registers
     Print the names and values of all registers, including floating-point registers.

info registers REGNAME ...
     Print the relativized value of each specified register REGNAME. REGNAME may be any register name valid on the machine you are using, with or without the initial $.

GDBN has four ``standard'' register names that are available (in expressions) on most machines---whenever they do not conflict with an architecture's canonical mnemonics for registers.  The register names $pc and $sp are used for the program counter register and the stack pointer.  $fp is used for a register that contains a pointer to the current stack frame, and $ps is used for a register that contains the processor status.  For example, you could print the program counter in hex with

     p/x $pc

or print the instruction to be executed next with

     x/i $pc

or add four to the stack pointer (This is a way of removing one word from the stack, on machines where stacks grow downward in memory (most machines, nowadays).  This assumes that the innermost stack frame is selected; setting $sp is not allowed when other stack frames are selected.  To pop entire frames off the stack, regardless of machine architecture, use return; see Returning from a function.) with

     set $sp += 4

Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict.  The info registers command shows the canonical names.  For example, on the SPARC, info registers displays the processor status register as $psr but you can also refer to it as $ps.

GDBN always considers the contents of an ordinary register as an integer when the register is examined in this way.  Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values.  There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with print/f $REGNAME).

Some registers have distinct ``raw'' and ``virtual'' data formats.  This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees.  For example, the registers of the 68881 floating point coprocessor are always saved in ``extended'' (raw) format, but all C programs expect to work with ``double'' (virtual) format.  In such cases, GDBN normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Normally, register values are relative to the selected stack frame (see Selecting a frame).  This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored.  In order to see the true contents of hardware registers, you must select the innermost frame (with frame 0).

However, GDBN must deduce where registers are saved, from the machine code generated by your compiler.  If some registers are not saved, or if GDBN is unable to locate the saved registers, the selected stack frame will make no difference.

Floating point hardware
=======================

Depending on the host machine architecture, GDBN may be able to give you more information about the status of the floating point hardware.

info float
     Display hardware-dependent information about the floating point unit.  The exact contents and layout vary depending on the floating point chip; on some platforms, info float is not available at all.

Using GDBN with Different Languages
***********************************

Although programming languages generally have common aspects, they are rarely expressed in the same manner.  For instance, in ANSI C, dereferencing a pointer p is accomplished by *p, but in Modula-2, it is accomplished by p^.  Values can also be represented (and displayed) differently.  Hex numbers in C are written like 0x1ae, while in Modula-2 they appear as 1AEH.

Language-specific information is built into GDBN for some languages, allowing you to express operations like the above in your program's native language, and allowing GDBN to output values in a manner consistent with the syntax of your program's native language.  The language you use to build expressions, called the working
language, can be selected manually, or GDBN can set it automatically.

Switching between source languages
==================================

There are two ways to control the working language---either have GDBN set it automatically, or select it manually yourself.  You can use the set language command for either purpose.  On startup, GDBN defaults to setting the language automatically.

Setting the working language
----------------------------

If you allow GDBN to set the language automatically, expressions are interpreted the same way in your debugging session and your program.

If you wish, you may set the language manually.  To do this, issue the command set language LANG, where LANG is the name of a language, such as c or modula-2.  For a list of the supported languages, type set language.

Setting the language manually prevents GDBN from updating the working language automatically.  This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages---but means different things.  For instance, if the current source file were written in C, and GDBN was parsing Modula-2, a command such as:

     print a = b + c

might not have the effect you intended.  In C, this means to add b and c and place the result in a.  The result printed would be the value of a.  In Modula-2, this means to compare a to the result of b+c, yielding a BOOLEAN value.

Having GDBN infer the source language
-------------------------------------

To have GDBN set the working language automatically, use set
language local or set language auto.  GDBN then infers the language that a program was written in by looking at the name of its source files, and examining their extensions:

*.mod
     Modula-2 source file

*.c
     C source file

*.C
*.cc
     C++ source file

This information is recorded for each function or procedure in a source file.  When your program stops in a frame (usually by encountering a breakpoint), GDBN sets the working language to the language recorded for the function in that frame.  If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDBN issues a warning.

This may not seem necessary for most programs, which are written entirely in one source language.  However, program modules and libraries written in one source language can be used by a main program written in a different source language.  Using set language auto in this case frees you from having to set the working language manually.

Displaying the language
=======================

The following commands will help you find out which language is the working language, and also what language source files were written in.

show language
     Display the current working language.  This is the language you can use with commands such as print to build and compute expressions that may involve variables in your program.

info frame
     Among the other information listed here (see Information about a frame) is the source language for this frame.  This is the language that will become the working language if you ever use an identifier that is in this frame.

info source
     Among the other information listed here (see Examining the Symbol Table) is the source language of this source file.

Type and range checking
=======================

     Warning: In this release, the GDBN commands for type and range checking are included, but they do not yet have any effect.  This section documents the intended facilities.

Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks.  These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time.  Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running.

GDBN can check for conditions like the above if you wish. Although GDBN will not check the statements in your program, it can check expressions entered directly into GDBN for evaluation via the print command, for example.  As with the working language, GDBN can also decide whether or not to check automatically based on your program's source language.  See Supported languages, for the default settings of supported languages.

An overview of type checking
----------------------------

Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs.  These checks prevent type mismatch errors from ever causing any run-time problems.  For example,

     1 + 2 => 3
     but
     error--> 1 + 2.3

The second example fails because the CARDINAL 1 is not type-compatible with the REAL 2.3.

For expressions you use in GDBN commands, you can tell the GDBN type checker to skip checking; to treat any mismatches as errors and abandon the expression; or only issue warnings when type mismatches occur, but evaluate the expression anyway.  When you choose the last of these, GDBN evaluates expressions like the second example above, but also issues a warning.

Even though you may turn type checking off, other type-based reasons may prevent GDBN from evaluating an expression.  For instance, GDBN does not know how to add an int and a struct foo.  These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway.

Each language defines to what degree it is strict about type.  For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers.  In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators.  See Supported languages, for further details on specific languages.

GDBN provides some additional commands for controlling the type checker:

set check type auto
     Set type checking on or off based on the current working language. See Supported languages, for the default settings for each language.

set check type on
set check type off
     Set type checking on or off, overriding the default setting for the current working language.  Issue a warning if the setting does not match the language default.  If any type mismatches occur in evaluating an expression while typechecking is on, GDBN prints a message and aborts evaluation of the expression.

set check type warn
     Cause the type checker to issue warnings, but to always attempt to evaluate the expression.  Evaluating the expression may still be impossible for other reasons.  For example, GDBN cannot add numbers and structures.

show type
     Show the current setting of the type checker, and whether or not GDBN is setting it automatically.

An overview of range checking
-----------------------------

In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks.  Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.

For expressions you use in GDBN commands, you can tell GDBN to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type.  Some languages, however, do not treat overflows as an error.  In many implementations of C, mathematical overflow causes the result to ``wrap around'' to lower values---for example, if M is the largest integer value, and S is the smallest, then

     M + 1 => S

This, too, is specific to individual languages, and in some cases specific to individual compilers or machines.  See Supported languages, for further details on specific languages.

GDBN provides some additional commands for controlling the range checker:

set check range auto
     Set range checking on or off based on the current working language. See Supported languages, for the default settings for each language.

set check range on
set check range off
     Set range checking on or off, overriding the default setting for the current working language.  A warning is issued if the setting does not match the language default.  If a range error occurs, then a message is printed and evaluation of the expression is aborted.

set check range warn
     Output messages when the GDBN range checker detects a range error, but attempt to evaluate the expression anyway.  Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many UNIX systems).

show range
     Show the current setting of the range checker, and whether or not it is being set automatically by GDBN.

Supported languages
===================

GDBN 4 supports C, C++, and Modula-2.  Some GDBN features may be used in expressions regardless of the language you use: the GDBN @ and :: operators, and the {type}addr construct (see Expressions) can be used with the constructs of any of the supported languages.

The following sections detail to what degree each of these source languages is supported by GDBN.  These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDBN expression parser will accept, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.

C and C++
---------

Since C and C++ are so closely related, many features of GDBN apply to both languages.  Whenever this is the case, we discuss both languages together.

The C++ debugging facilities are jointly implemented by the GNU C++ compiler and GDBN.  Therefore, to debug your C++ code effectively, you must compile your C++ programs with the GNU C++ compiler, g++.

C and C++ operators
...................

Operators must be defined on values of specific types.  For instance, + is defined on numbers, but not on structures.  Operators are often defined on groups of types.

For the purposes of C and C++, the following definitions hold:

  * Integral types include int with any of its storage-class specifiers; char; and enum.

  * Floating-point types include float and double.

  * Pointer types include all types defined as (TYPE *).

  * Scalar types include all of the above.

The following operators are supported.  They are listed here in order of increasing precedence:

,
     The comma or sequencing operator.  Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.

=
     Assignment.  The value of an assignment expression is the value assigned.  Defined on scalar types.

OP=
     Used in an expression of the form A OP= B, and translated to A = A OP B. OP= and = have the same precendence. OP is any one of the operators |, ^, &, <<, >>, +, -, *, /, %.

?:
     The ternary operator.  A ? B : C can be thought of as:  if A then B else C.  A should be of an integral type.

||
     Logical OR.  Defined on integral types.

&&
     Logical AND.  Defined on integral types.

|
     Bitwise OR.  Defined on integral types.

^
     Bitwise exclusive-OR.  Defined on integral types.

&
     Bitwise AND.  Defined on integral types.

==, !=
     Equality and inequality.  Defined on scalar types.  The value of these expressions is 0 for false and non-zero for true.

<, >, <=, >=
     Less than, greater than, less than or equal, greater than or equal. Defined on scalar types.  The value of these expressions is 0 for false and non-zero for true.

<<, >>
     left shift, and right shift.  Defined on integral types.

@
     The GDBN ``artificial array'' operator (see Expressions).

+, -
     Addition and subtraction.  Defined on integral types, floating-point types and pointer types.

*, /, %
     Multiplication, division, and modulus.  Multiplication and division are defined on integral and floating-point types.  Modulus is defined on integral types.

++, --
     Increment and decrement.  When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place.

*
     Pointer dereferencing.  Defined on pointer types.  Same precedence as ++.

&
     Address operator.  Defined on variables.  Same precedence as ++.

     For debugging C++, GDBN implements a use of & beyond what is allowed in the C++ language itself: you can use &(&REF) (or, if you prefer, simply &&REF to examine the address where a C++ reference variable (declared with &REF) is stored.

-
     Negative.  Defined on integral and floating-point types.  Same precedence as ++.

!
     Logical negation.  Defined on integral types.  Same precedence as ++.

~
     Bitwise complement operator.  Defined on integral types.  Same precedence as ++.

., ->
     Structure member, and pointer-to-structure member.  For convenience, GDBN regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.

[]
     Array indexing.  A[I] is defined as *(A+I).  Same precedence as ->.

()
     Function parameter list.  Same precedence as ->.

::
     C++ scope resolution operator.  Defined on struct, union, and class types.

::
     Doubled colons also represent the GDBN scope operator (see Expressions). Same precedence as ::, above.

C and C++ constants
...................

GDBN allows you to express the constants of C and C++ in the following ways:

  * Integer constants are a sequence of digits.  Octal constants are specified by a leading 0 (ie. zero), and hexadecimal constants by a leading 0x or 0X.  Constants may also end with a letter l, specifying that the constant should be treated as a long value.

  * Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent.  An exponent is of the form: e[[+]|-]NNN, where NNN is another sequence of digits.  The + is optional for positive exponents.

  * Enumerated constants consist of enumerated identifiers, or their integral equivalents.

  * Character constants are a single character surrounded by single quotes ('), or a number---the ordinal value of the corresponding character (usually its ASCII value).  Within quotes, the single character may be represented by a letter or by escape sequences, which are of the form \NNN, where NNN is the octal representation of the character's ordinal value; or of the form \X, where X is a predefined special character---for example, \n for newline.

  * String constants are a sequence of character constants surrounded by double quotes (").

  * Pointer constants are an integral value.  You can also write pointers to constants using the C operator &.

  * Array constants are comma-separated lists surrounded by braces { and }; for example, {1,2,3} is a three-element array of integers, {{1,2}, {3,4}, {5,6}} is a three-by-two array, and {&"hi", &"there", &"fred"} is a three-element array of pointers.

C++ expressions
...............

GDBN expression handling has a number of extensions to interpret a significant subset of C++ expressions.

     Warning: Most of these extensions depend on the use of additional debugging information in the symbol table, and thus require a rich, extendable object code format.  In particular, if your system uses a.out, MIPS ECOFF, RS/6000 XCOFF, or Sun ELF with stabs extensions to the symbol table, these facilities are all available. Where the object code format is standard COFF, on the other hand, most of the C++ support in GDBN will not work, nor can it. For the standard SVr4 debugging format, DWARF in ELF, the standard is still evolving, so the C++ support in GDBN is still fragile; when this debugging format stabilizes, however, C++ support will also be available on systems that use it.

 1. Member function calls are allowed; you can use expressions like

          count = aml->GetOriginal(x, y)

 2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDBN allows implicit references to the class instance pointer this following the same rules as C++.

 3. You can call overloaded functions; GDBN will resolve the function call to the right definition, with one restriction---you must use arguments of the type required by the function that you want to call. GDBN will not perform conversions requiring constructors or user-defined type operators.

 4. GDBN understands variables declared as C++ references; you can use them in expressions just as you do in C++ source---they are automatically dereferenced.

     In the parameter list shown when GDBN displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified set print address off.

 5. GDBN supports the C++ name resolution operator ::---your expressions can use it just as expressions in your program do.  Since one scope may be defined in another, you can use :: repeatedly if necessary, for example in an expression like SCOPE1::SCOPE2::NAME.  GDBN also allows resolving name scope by reference to source files, in both C and C++ debugging (see Program variables).

C and C++ defaults
..................

If you allow GDBN to set type and range checking automatically, they both default to off whenever the working language changes to C or C++.  This happens regardless of whether you, or GDBN, selected the working language.

If you allow GDBN to set the language automatically, it sets the working language to C or C++ on entering code compiled from a source file whose name ends with .c, .C, or .cc. See Having GDBN infer the source language, for further details.

C and C++ type and range checks
...............................

By default, when GDBN parses C or C++ expressions, type checking is not used.  However, if you turn type checking on, GDBN will consider two variables type equivalent if:

  * The two variables are structured and have the same structure, union, or enumerated tag.

  * Two two variables have the same type name, or types that have been declared equivalent through typedef.

Range checking, if turned on, is done on mathematical operations.  Array indices are not checked, since they are often used to index a pointer that is not itself an array.

GDBN and C
..........

The set print union and show print union commands apply to the union type.  When set to on, any union that is inside a struct or class will also be printed. Otherwise, it will appear as {...}.

The @ operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function.  See Expressions.

GDBN features for C++
.....................

Some GDBN commands are particularly useful with C++, and some are designed specifically for use with C++.  Here is a summary:

breakpoint menus
     When you want a breakpoint in a function whose name is overloaded, GDBN breakpoint menus help you specify which function definition you want.  See Breakpoint menus.

rbreak REGEX
     Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See Setting breakpoints.

catch EXCEPTIONS
info catch
     Debug C++ exception handling using these commands.  See Breakpoints and exceptions.

ptype TYPENAME
     Print inheritance relationships as well as other information for type TYPENAME. See Examining the Symbol Table.

set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
     Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See Print settings.

set print object
show print object
     Choose whether to print derived (actual) or declared types of objects. See Print settings.

set print vtbl
show print vtbl
     Control the format for printing virtual function tables. See Print settings.

Overloaded symbol names
     You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type SYMBOL(TYPES) rather than just SYMBOL.  You can also use the GDBN command-line word completion facilities to list the available choices, or to finish the type list for you. See Command completion, for details on how to do this.

Modula-2
--------

The extensions made to GDBN to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed).  Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them will most likely result in an error as GDBN reads in the executable's symbol table.

Operators
.........

Operators must be defined on values of specific types.  For instance, + is defined on numbers, but not on structures.  Operators are often defined on groups of types.  For the purposes of Modula-2, the following definitions hold:

  * Integral types consist of INTEGER, CARDINAL, and their subranges.

  * Character types consist of CHAR and its subranges.

  * Floating-point types consist of REAL.

  * Pointer types consist of anything declared as POINTER TO TYPE.

  * Scalar types consist of all of the above.

  * Set types consist of SET and BITSET types.

  * Boolean types consist of BOOLEAN.

The following operators are supported, and appear in order of increasing precedence:

,
     Function argument or array index separator.

:=
     Assignment.  The value of VAR := VALUE is VALUE.

<, >
     Less than, greater than on integral, floating-point, or enumerated types.

<=, >=
     Less than, greater than, less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types.  Same precedence as <.

=, <>, #
     Equality and two ways of expressing inequality, valid on scalar types. Same precedence as <.  In GDBN scripts, only <> is available for inequality, since # conflicts with the script comment character.

IN
     Set membership.  Defined on set types and the types of their members. Same precedence as <.

OR
     Boolean disjunction.  Defined on boolean types.

AND, &
     Boolean conjuction.  Defined on boolean types.

@
     The GDBN ``artificial array'' operator (see Expressions).

+, -
     Addition and subtraction on integral and floating-point types, or union and difference on set types.

*
     Multiplication on integral and floating-point types, or set intersection on set types.

/
     Division on floating-point types, or symmetric set difference on set types.  Same precedence as *.

DIV, MOD
     Integer division and remainder.  Defined on integral types.  Same precedence as *.

-
     Negative. Defined on INTEGER and REAL data.

^
     Pointer dereferencing.  Defined on pointer types.

NOT
     Boolean negation.  Defined on boolean types.  Same precedence as ^.

.
     RECORD field selector.  Defined on RECORD data.  Same precedence as ^.

[]
     Array indexing.  Defined on ARRAY data.  Same precedence as ^.

()
     Procedure argument list.  Defined on PROCEDURE objects.  Same precedence as ^.

::, .
     GDBN and Modula-2 scope operators.

     Warning: Sets and their operations are not yet supported, so GDBN will treat the use of the operator IN, or the use of operators +, -, *, /, =, , <>, #, <=, and >= on sets as an error.

Built-in functions and procedures
.................................

Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:

a
     represents an ARRAY variable.

c
     represents a CHAR constant or variable.

i
     represents a variable or constant of integral type.

m
     represents an identifier that belongs to a set.  Generally used in the same function with the metavariable S.  The type of S should be SET OF MTYPE (where MTYPE is the type of M.

n
     represents a variable or constant of integral or floating-point type.

r
     represents a variable or constant of floating-point type.

t
     represents a type.

v
     represents a variable.

x
     represents a variable or constant of one of many types.  See the explanation of the function for details.

All Modula-2 built-in procedures also return a result, described below.

ABS(N)
     Returns the absolute value of N.

CAP(C)
     If C is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument

CHR(I)
     Returns the character whose ordinal value is I.

DEC(V)
     Decrements the value in the variable V.  Returns the new value.

DEC(V,I)
     Decrements the value in the variable V by I.  Returns the new value.

EXCL(M,S)
     Removes the element M from the set S.  Returns the new set.

FLOAT(I)
     Returns the floating point equivalent of the integer I.

HIGH(A)
     Returns the index of the last member of A.

INC(V)
     Increments the value in the variable V.  Returns the new value.

INC(V,I)
     Increments the value in the variable V by I.  Returns the new value.

INCL(M,S)
     Adds the element M to the set S if it is not already there.  Returns the new set.

MAX(T)
     Returns the maximum value of the type T.

MIN(T)
     Returns the minimum value of the type T.

ODD(I)
     Returns boolean TRUE if I is an odd number.

ORD(X)
     Returns the ordinal value of its argument.  For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set).  X must be of an ordered type, which include integral, character and enumerated types.

SIZE(X)
     Returns the size of its argument.  X can be a variable or a type.

TRUNC(R)
     Returns the integral part of R.

VAL(T,I)
     Returns the member of the type T whose ordinal value is I.

     Warning:  Sets and their operations are not yet supported, so GDBN will treat the use of procedures INCL and EXCL as an error.

Constants
.........

GDBN allows you to express the constants of Modula-2 in the following ways:

  * Integer constants are simply a sequence of digits.  When used in an expression, a constant is interpreted to be type-compatible with the rest of the expression.  Hexadecimal integers are specified by a trailing H, and octal integers by a trailing B.

  * Floating point constants appear as a sequence of digits, followed by a decimal point and another sequence of digits.  An optional exponent can then be specified, in the form E[+|-]NNN, where [+|-]NNN is the desired exponent.  All of the digits of the floating point constant must be valid decimal (base 10) digits.

  * Character constants consist of a single character enclosed by a pair of like quotes, either single (') or double (").  They may also be expressed by their ordinal value (their ASCII value, usually) followed by a C.

  * String constants consist of a sequence of characters enclosed by a pair of like quotes, either single (') or double ("). Escape sequences in the style of C are also allowed.  See C and C++ constants, for a brief explanation of escape sequences.

  * Enumerated constants consist of an enumerated identifier.

  * Boolean constants consist of the identifiers TRUE and FALSE.

  * Pointer constants consist of integral values only.

  * Set constants are not yet supported.

Modula-2 defaults
.................

If type and range checking are set automatically by GDBN, they both default to on whenever the working language changes to Modula-2.  This happens regardless of whether you, or GDBN, selected the working language.

If you allow GDBN to set the language automatically, then entering code compiled from a file whose name ends with .mod will set the working language to Modula-2. See Having GDBN set the language automatically, for further details.

Deviations from standard Modula-2
.................................

A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:

  * Unlike in standard Modula-2, pointer constants can be formed by integers.  This allows you to modify pointer variables during debugging.  (In standard Modula-2, the actual address contained in a pointer variable is hidden from you; it can only be modified through direct assignment to another pointer variable or expression that returned a pointer.)

  * C escape sequences can be used in strings and characters to represent non-printable characters.  GDBN will print out strings with these escape sequences embedded.  Single non-printable characters are printed using the CHR(NNN) format.

  * The assignment operator (:=) returns the value of its right-hand argument.

  * All built-in procedures both modify and return their argument.

Modula-2 type and range checks
..............................

     Warning: in this release, GDBN does not yet perform type or range checking.

GDBN considers two Modula-2 variables type equivalent if:

  * They are of types that have been declared equivalent via a TYPE T1 = T2 statement

  * They have been declared on the same line.  (Note:  This is true of the GNU Modula-2 compiler, but it may not be true of other compilers.)

As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.

Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.

The scope operators :: and .
............................

There are a few subtle differences between the Modula-2 scope operator (.) and the GDBN scope operator (::).  The two have similar syntax:

     MODULE . ID
     SCOPE :: ID

where SCOPE is the name of a module or a procedure, MODULE the name of a module, and ID is any declared identifier within your program, except another module.

Using the :: operator makes GDBN search the scope specified by SCOPE for the identifier ID.  If it is not found in the specified scope, then GDBN will search all scopes enclosing the one specified by SCOPE.

Using the . operator makes GDBN search the current scope for the identifier specified by ID that was imported from the definition module specified by MODULE.  With this operator, it is an error if the identifier ID was not imported from definition module MODULE, or if ID is not an identifier in MODULE.

GDBN and Modula-2
.................

Some GDBN commands have little use when debugging Modula-2 programs. Five subcommands of set print and show print apply specifically to C and C++: vtbl, demangle, asm-demangle, object, and union.  The first four apply to C++, and the last to the C union type, which has no direct analogue in Modula-2.

The @ operator (see Expressions), while available while using any language, is not useful with Modula-2.  Its intent is to aid the debugging of dynamic arrays, which cannot be created in Modula-2 as they can in C or C++.  However, because an address can be specified by an integral constant, the construct {TYPE}ADREXP is still useful.  (see Expressions)

In GDBN scripts, the Modula-2 inequality operator # is interpreted as the beginning of a comment.  Use <> instead.

Examining the Symbol Table
**************************

The commands described in this section allow you to inquire about the symbols (names of variables, functions and types) defined in your program.  This information is inherent in the text of your program and does not change as your program executes.  GDBN finds it in your program's symbol table, in the file indicated when you started GDBN (see Choosing files), or by one of the file-management commands (see Commands to specify files).

Occasionally, you may need to refer to symbols that contain unusual characters, which GDBN ordinarily treats as word delimiters.  The most frequent case is in referring to static variables in other source files (see Program variables).  File names are recorded in object files as debugging symbols, but GDBN would ordinarily parse a typical file name, like foo.c, as the three words foo . c.  To allow GDBN to recognize foo.c as a single symbol, enclose it in single quotes; for example,

     p 'foo.c'::x

looks up the value of x in the scope of the file foo.c.

info address SYMBOL
     Describe where the data for SYMBOL is stored.  For a register variable, this says which register it is kept in.  For a non-register local variable, this prints the stack-frame offset at which the variable is always stored.

     Note the contrast with print &SYMBOL, which does not work at all for a register variables, and for a stack local variable prints the exact address of the current instantiation of the variable.

whatis EXP
     Print the data type of expression EXP.  EXP is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. See Expressions.

whatis
     Print the data type of $, the last value in the value history.

ptype TYPENAME
     Print a description of data type TYPENAME.  TYPENAME may be the name of a type, or for C code it may have the form struct STRUCT-TAG, union UNION-TAG or enum ENUM-TAG.

ptype EXP
ptype
     Print a description of the type of expression EXP.  ptype differs from whatis by printing a detailed description, instead of just the name of the type.

     For example, for this variable declaration:

          struct complex {double real; double imag;} v;

     the two commands give this output:

          (GDBP) whatis v
          type = struct complex
          (GDBP) ptype v
          type = struct complex {
              double real;
              double imag;
          }

     As with whatis, using ptype without an argument refers to the type of $, the last value in the value history.

info types REGEXP
info types
     Print a brief description of all types whose name matches REGEXP (or all types in your program, if you supply no argument).  Each complete typename is matched as though it were a complete line; thus, i type value gives information on all types in your program whose name includes the string value, but i type ^value$ gives information only on types whose complete name is value.

     This command differs from ptype in two ways: first, like whatis, it does not print a detailed description; second, it lists all source files where a type is defined.

info source
     Show the name of the current source file---that is, the source file for the function containing the current point of execution---and the language it was written in.

info sources
     Print the names of all source files in your program for which there is debugging information, organized into two lists: files whose symbols have already been read, and files whose symbols will be read when needed.

info functions
     Print the names and data types of all defined functions.

info functions REGEXP
     Print the names and data types of all defined functions whose names contain a match for regular expression REGEXP. Thus, info fun step finds all functions whose names include step; info fun ^step finds those whose names start with step.

info variables
     Print the names and data types of all variables that are declared outside of functions (i.e., excluding local variables).

info variables REGEXP
     Print the names and data types of all variables (except for local variables) whose names contain a match for regular expression REGEXP.

maint print symbols FILENAME
maint print psymbols FILENAME
maint print msymbols FILENAME
     Write a dump of debugging symbol data into the file FILENAME. These commands are used to debug the GDBN symbol-reading code.  Only symbols with debugging data are included.  If you use maint print
     symbols, GDBN includes all the symbols for which it has already collected full details: that is, FILENAME reflects symbols for only those files whose symbols GDBN has read.  You can use the command info sources to find out which files these are.  If you use maint print psymbols instead, the dump shows information about symbols that GDBN only knows partially---that is, symbols defined in files that GDBN has skimmed, but not yet read completely.  Finally, maint print msymbols dumps just the minimal symbol information required for each object file from which GDBN has read some symbols. The description of symbol-file explains how GDBN reads symbols; both info source and symbol-file are described in Commands to specify files.

Altering Execution
******************

Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run.  You can find the answer by experiment, using the GDBN features for altering execution of the program.

For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function to its caller.

Assignment to variables
=======================

To alter the value of a variable, evaluate an assignment expression. See Expressions.  For example,

     print x=4

stores the value 4 into the variable x, and then prints the value of the assignment expression (which is 4). See Using GDBN with Different Languages, for more information on operators in supported languages.

If you are not interested in seeing the value of the assignment, use the set command instead of the print command.  set is really the same as print except that the expression's value is not printed and is not put in the value history (see Value history).  The expression is evaluated only for its effects.

If the beginning of the argument string of the set command appears identical to a set subcommand, use the set variable command instead of just set.  This command is identical to set except for its lack of subcommands.  For example, if your program has a variable width, you get an error if you try to set a new value with just set width=13, because GDBN has the command set width:

     (GDBP) whatis width
     type = double
     (GDBP) p width
     $4 = 13
     (GDBP) set width=47
     Invalid syntax in expression.

The invalid expression, of course, is =47.  In order to actually set the program's variable width, use

     (GDBP) set var width=47

GDBN allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter.

To store values into arbitrary places in memory, use the {...} construct to generate a value of specified type at a specified address (see Expressions).  For example, {int}0x83040 refers to memory location 0x83040 as an integer (which implies a certain size and representation in memory), and

     set {int}0x83040 = 4

stores the value 4 into that memory location.

Continuing at a different address
=================================

Ordinarily, when you continue your program, you do so at the place where it stopped, with the continue command.  You can instead continue at an address of your own choosing, with the following commands:

jump LINESPEC
     Resume execution at line LINESPEC.  Execution will stop immediately if there is a breakpoint there.  See Printing source lines, for a description of the different forms of LINESPEC.

     The jump command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter.  If line LINESPEC is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables.  For this reason, the jump command requests confirmation if the specified line is not in the function currently executing.  However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program.

jump *ADDRESS
     Resume execution at the instruction at address ADDRESS.

You can get much the same effect as the jump command by storing a new value into the register $pc.  The difference is that this does not start your program running; it only changes the address where it will run when it is continued.  For example,

     set $pc = 0x485

causes the next continue command or stepping command to execute at address 0x485, rather than at the address where your program stopped. See Continuing and stepping.

The most common occasion to use the jump command is to back up, perhaps with more breakpoints set, over a portion of a program that has already executed, in order to examine its execution in more detail.

Giving your program a signal
============================

signal SIGNALNUM
     Resume execution where your program stopped, but give it immediately the signal number SIGNALNUM.

     Alternatively, if SIGNALNUM is zero, continue execution without giving a signal.  This is useful when your program stopped on account of a signal and would ordinary see the signal when resumed with the continue command; signal 0 causes it to resume without a signal.

     signal does not repeat when you press RET a second time after executing the command.

Returning from a function
=========================

return
return EXPRESSION
     You can cancel execution of a function call with the return command.  If you give an EXPRESSION argument, its value is used as the function's return value.

When you use return, GDBN discards the selected stack frame (and all frames within it).  You can think of this as making the discarded frame return prematurely.  If you wish to specify a value to be returned, give that value as the argument to return.

This pops the selected stack frame (see Selecting a frame), and any other frames inside of it, leaving its caller as the innermost remaining frame.  That frame becomes selected.  The specified value is stored in the registers used for returning values of functions.

The return command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned.  In contrast, the finish command (see Continuing and stepping) resumes execution until the selected stack frame returns naturally.

Calling program functions
=========================

call EXPR
     Evaluate the expression EXPR without displaying void returned values.

You can use this variant of the print command if you want to execute a function from your program, but without cluttering the output with void returned values.  The result is printed and saved in the value history, if it is not void.

Patching programs
=================

By default, GDBN opens the file containing your program's executable code (or the corefile) read-only.  This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary.

If you'd like to be able to patch the binary, you can specify that explicitly with the set write command.  For example, you might want to turn on internal debugging flags, or even to make emergency repairs.

set write on
set write off
     If you specify set write on, GDBN will open executable and core files for both reading and writing; if you specify set write
     off (the default), GDBN will open them read-only.

     If you have already loaded a file, you must load it again (using the exec-file or core-file command) after changing set write, for your new setting to take effect.

show write
     Display whether executable files and core files will be opened for writing as well as reading.

GDBN Files
**********

GDBN needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDBN the name of the core dump file.

Commands to specify files
=========================

The usual way to specify executable and core dump file names is with the command arguments given when you start GDBN (see Getting In and Out of GDBN.

Occasionally it is necessary to change to a different file during a GDBN session.  Or you may run GDBN and forget to specify a file you want to use.  In these situations the GDBN commands to specify new files are useful.

file FILENAME
     Use FILENAME as the program to be debugged.  It is read for its symbols and for the contents of pure memory.  It is also the program executed when you use the run command.  If you do not specify a directory and the file is not found in the GDBN working directory, GDBN uses the environment variable PATH as a list of directories to search, just as the shell does when looking for a program to run.  You can change the value of this variable, for both GDBN and your program, using the path command.

     On systems with memory-mapped files, an auxiliary symbol table file FILENAME.syms may be available for FILENAME.  If it is, GDBN will map in the symbol table from FILENAME.syms, starting up more quickly.  See the descriptions of the options -mapped and -readnow (available on the command line, and with the commands file, symbol-file, or add-symbol-file), for more information.

file
     file with no argument makes GDBN discard any information it has on both executable file and the symbol table.

exec-file [ FILENAME ]
     Specify that the program to be run (but not the symbol table) is found in FILENAME.  GDBN will search the environment variable PATH if necessary to locate your program.  Omitting FILENAME means to discard information on the executable file.

symbol-file [ FILENAME ]
     Read symbol table information from file FILENAME.  PATH is searched when necessary.  Use the file command to get both symbol table and program to run from the same file.

     symbol-file with no argument clears out GDBN information on your program's symbol table.

     The symbol-file command causes GDBN to forget the contents of its convenience variables, the value history, and all breakpoints and auto-display expressions.  This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDBN.

     symbol-file will not repeat if you press RET again after executing it once.

     When GDBN is configured for a particular environment, it will understand debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions.  Best results are usually obtained from GNU compilers; for example, using GCC you can generate debugging information for optimized code.

     On some kinds of object files, the symbol-file command does not normally read the symbol table in full right away.  Instead, it scans the symbol table quickly to find which source files and which symbols are present.  The details are read later, one source file at a time, as they are needed.

     The purpose of this two-stage reading strategy is to make GDBN start up faster.  For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read.  (The set verbose command can turn these pauses into messages if desired.  See Optional warnings and messages.)

     We have not implemented the two-stage strategy for COFF yet.  When the symbol table is stored in COFF format, symbol-file reads the symbol table data in full right away.

symbol-file FILENAME [ -readnow ] [ -mapped ]
file FILENAME [ -readnow ] [ -mapped ]
     You can override the GDBN two-stage strategy for reading symbol tables by using the -readnow option with any of the commands that load symbol table information, if you want to be sure GDBN has the entire symbol table available.

     If memory-mapped files are available on your system through the mmap system call, you can use another option, -mapped, to cause GDBN to write the symbols for your program into a reusable file.  Future GDBN debugging sessions will map in symbol information from this auxiliary symbol file (if the program has not changed), rather than spending time reading the symbol table from the executable program.  Using the -mapped option has the same effect as starting GDBN with the -mapped command-line option.

     You can use both options together, to make sure the auxiliary symbol file has all the symbol information for your program.

     The auxiliary symbol file for a program called MYPROG is called MYPROG.syms.  Once this file exists (so long as it is newer than the corresponding executable), GDBN will always attempt to use it when you debug MYPROG; no special options or commands are needed.

     The .syms file is specific to the host machine where you run GDBN.  It holds an exact image of the internal GDB symbol table.  It cannot be shared across multiple host platforms.

core-file [ FILENAME ]
     Specify the whereabouts of a core dump file to be used as the ``contents of memory''.  Traditionally, core files contain only some parts of the address space of the process that generated them; GDBN can access the executable file itself for other parts.

     core-file with no argument specifies that no core file is to be used.

     Note that the core file is ignored when your program is actually running under GDBN.  So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running.  To do this, use the kill command (see Killing the child process).

load FILENAME

     load will not repeat if you press RET again after using it.

add-symbol-file FILENAME ADDRESS
add-symbol-file FILENAME ADDRESS [ -readnow ] [ -mapped ]
     The add-symbol-file command reads additional symbol table information from the file FILENAME.  You would use this command when FILENAME has been dynamically loaded (by some other means) into the program that is running.  ADDRESS should be the memory address at which the file has been loaded; GDBN cannot figure this out for itself.

     The symbol table of the file FILENAME is added to the symbol table originally read with the symbol-file command.  You can use the add-symbol-file command any number of times; the new symbol data thus read keeps adding to the old.  To discard all old symbol data instead, use the symbol-file command.

     add-symbol-file will not repeat if you press RET after using it.

     You can use the -mapped and -readnow options just as with the symbol-file command, to change how GDBN manages the symbol table information for FILENAME.

info files
info target
     info files and info target are synonymous; both print the current target (see Specifying a Debugging Target), including the names of the executable and core dump files currently in use by GDBN, and the files from which symbols were loaded.  The command help targets lists all possible targets rather than current ones.

All file-specifying commands allow both absolute and relative file names as arguments.  GDBN always converts the file name to an absolute path name and remembers it that way.

GDBN supports SunOS, SVR4, and IBM RS/6000 shared libraries. GDBN automatically loads symbol definitions from shared libraries when you use the run command, or when you examine a core file. (Before you issue the run command, GDBN will not understand references to a function in a shared library, however---unless you are debugging a core file).

info share
info sharedlibrary
     Print the names of the shared libraries which are currently loaded.

sharedlibrary REGEX
share REGEX
     This is an obsolescent command; you can use it to explicitly load shared object library symbols for files matching a UNIX regular expression, but as with files loaded automatically, it will only load shared libraries required by your program for a core file or after typing run.  If REGEX is omitted all shared libraries required by your program are loaded.

Errors reading symbol files
===========================

While reading a symbol file, GDBN will occasionally encounter problems, such as symbol types it does not recognize, or known bugs in compiler output.  By default, GDBN does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers.  If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDBN to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDBN to print more messages, to see how many times the problems occur, with the set complaints command (see Optional warnings and messages).

The messages currently printed, and their meanings, are:

inner block not inside outer block in SYMBOL

     The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements).  This error indicates that an inner scope block is not fully contained in its outer scope blocks.

     GDBN circumvents the problem by treating the inner block as if it had the same scope as the outer block.  In the error message, SYMBOL may be shown as ``(don't know)'' if the outer block is not a function.

block at ADDRESS out of order

     The symbol information for symbol scope blocks should occur in order of increasing addresses.  This error indicates that it does not do so.

     GDBN does not circumvent this problem, and will have trouble locating symbols in the source file whose symbols it is reading.  (You can often determine what source file is affected by specifying set verbose on.  See Optional warnings and messages.)

bad block start address patched

     The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line.  This is known to occur in the SunOS 4.1.1 (and earlier) C compiler.

     GDBN circumvents the problem by treating the symbol scope block as starting on the previous source line.

bad string table offset in symbol N

     Symbol number N contains a pointer into the string table which is larger than the size of the string table.

     GDBN circumvents the problem by considering the symbol to have the name foo, which may cause other problems if many symbols end up with this name.

unknown symbol type 0xNN

     The symbol information contains new data types that GDBN does not yet know how to read.  0xNN is the symbol type of the misunderstood information, in hexadecimal.

     GDBN circumvents the error by ignoring this symbol information.  This will usually allow your program to be debugged, though certain symbols will not be accessible.  If you encounter such a problem and feel like debugging it, you can debug GDBP with itself, breakpoint on complain, then go up to the function read_dbx_symtab and examine *bufp to see the symbol.

stub type has NULL name
     GDBN could not find the full definition for a struct or class.

const/volatile indicator missing (ok if using g++ v1.x), got...

     The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it.

info mismatch between compiler and debugger

     GDBN could not parse a type specification output by the compiler.

Specifying a Debugging Target
*****************************

A target is the execution environment occupied by your program. Often, GDBN runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the file or core commands.  When you need more flexibility---for example, running GDBN on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection---you can use the target command to specify one of the target types configured for GDBN (see Commands for managing targets).

Active targets
==============

There are three classes of targets: processes, core files, and executable files.  GDBN can work concurrently on up to three active targets, one in each class.  This allows you to (for example) start a process and inspect its activity without abandoning your work on a core file.

For example, if you execute gdb a.out, then the executable file a.out is the only active target.  If you designate a core file as well---presumably from a prior run that crashed and coredumped---then GDBN has two active targets and will use them in tandem, looking first in the corefile target, then in the executable file, to satisfy requests for memory addresses.  (Typically, these two classes of target are complementary, since core files contain only a program's read-write memory---variables and so on---plus machine status, while executable files contain only the program text and initialized data.)

When you type run, your executable file becomes an active process target as well.  When a process target is active, all GDBN commands requesting memory addresses refer to that target; addresses in an active core file or executable file target are obscured while the process target is active.

Use the core-file and exec-file commands to select a new core file or executable target (see Commands to specify files).  To specify as a target a process that is already running, use the attach command (see Debugging an already-running process).

Commands for managing targets
=============================

target TYPE PARAMETERS
     Connects the GDBN host environment to a target machine or process.  A target is typically a protocol for talking to debugging facilities.  You use the argument TYPE to specify the type or protocol of the target machine.

     Further PARAMETERS are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates.

     The target command will not repeat if you press RET again after executing the command.

help target
     Displays the names of all targets available.  To display targets currently selected, use either info target or info files (see Commands to specify files).

help target NAME
     Describe a particular target, including any parameters necessary to select it.

Here are some common targets (available, or not, depending on the GDB configuration):

target exec PROGRAM
     An executable file.  target exec PROGRAM is the same as exec-file PROGRAM.

target core FILENAME
     A core dump file.  target core FILENAME is the same as core-file FILENAME.

Remote debugging
================

If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging.  For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.

Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets.  In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs---the code that will run on the remote system to communicate with GDB.

Other remote targets may be available in your configuration of GDB; use help targets to list them.

Controlling GDBN
****************

You can alter the way GDBN interacts with you by using the set command.  For commands controlling how GDBN displays data, see Print settings; other settings are described here.

Prompt
======

GDBN indicates its readiness to read a command by printing a string called the prompt.  This string is normally (GDBP).  You can change the prompt string with the set prompt command.  For instance, when debugging GDBN with GDBN, it is useful to change the prompt in one of the GDBN sessions so that you can always tell which one you are talking to.

set prompt NEWPROMPT
     Directs GDBN to use NEWPROMPT as its prompt string henceforth.

show prompt
     Prints a line of the form: Gdb's prompt is: YOUR-PROMPT

Command editing
===============

GDBN reads its input commands via the readline interface.  This GNU library provides consistent behavior for programs which provide a command line interface to the user.  Advantages are emacs-style or vi-style inline editing of commands, csh-like history substitution, and a storage and recall of command history across debugging sessions.

You may control the behavior of command line editing in GDBN with the command set.

set editing
set editing on
     Enable command line editing (enabled by default).

set editing off
     Disable command line editing.

show editing
     Show whether command line editing is enabled.

Command history
===============

GDBN can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened.  Use these commands to manage the GDBN command history facility.

set history filename FNAME
     Set the name of the GDBN command history file to FNAME.  This is the file from which GDBN will read an initial command history list or to which it will write this list when it exits.  This list is accessed through history expansion or through the history command editing characters listed below.  This file defaults to the value of the environment variable GDBHISTFILE, or to ./.gdb_history if this variable is not set.

set history save
set history save on
     Record command history in a file, whose name may be specified with the set history filename command.  By default, this option is disabled.

set history save off
     Stop recording command history in a file.

set history size SIZE
     Set the number of commands which GDBN will keep in its history list. This defaults to the value of the environment variable HISTSIZE, or to 256 if this variable is not set.

History expansion assigns special meaning to the character !.

Since ! is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the set history expansion on command, you may sometimes need to follow ! (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded.  The readline history facilities will not attempt substitution on the strings != and !(, even when history expansion is enabled.

The commands to control history expansion are:

set history expansion on
set history expansion
     Enable history expansion.  History expansion is off by default.

set history expansion off
     Disable history expansion.

     The readline code comes with more complete documentation of editing and history expansion features.  Users unfamiliar with emacs or vi may wish to read it.

show history
show history filename
show history save
show history size
show history expansion
     These commands display the state of the GDBN history parameters. show history by itself displays all four states.

show commands
     Display the last ten commands in the command history.

show commands N
     Print ten commands centered on command number N.

show commands +
     Print ten commands just after the commands last printed.

Screen size
===========

Certain commands to GDBN may produce large amounts of information output to the screen.  To help you read all of it, GDBN pauses and asks you for input at the end of each page of output.  Type RET when you want to continue the output, or q to discard the remaining output.  GDBN also uses the screen width setting to determine when to wrap lines of output.  Depending on what is being printed, it tries to break the line at a readable place, rather than simply letting it overflow onto the following line.

Normally GDBN knows the size of the screen from the termcap data base together with the value of the TERM environment variable and the stty rows and stty cols settings. If this is not correct, you can override it with the set height and set width commands:

set height LPP
show height
set width CPL
show width
     These set commands specify a screen height of LPP lines and a screen width of CPL characters.  The associated show commands display the current settings.

     If you specify a height of zero lines, GDBN will not pause during output no matter how long the output is.  This is useful if output is to a file or to an editor buffer.

Numbers
=======

You can always enter numbers in octal, decimal, or hexadecimal in GDBN by the usual conventions: octal numbers begin with 0, decimal numbers end with ., and hexadecimal numbers begin with 0x. Numbers that begin with none of these are, by default, entered in base 10; likewise, the default display for numbers---when no particular format is specified---is base 10.  You can change the default base for both input and output with the set radix command.

set radix BASE
     Set the default base for numeric input and display.  Supported choices for BASE are decimal 2, 8, 10, 16.  BASE must itself be specified either unambiguously or using the current default radix; for example, any of

          set radix 1010
          set radix 012
          set radix 10.
          set radix 0xa

     will set the base to decimal.  On the other hand, set radix 10 will leave the radix unchanged no matter what it was.

show radix
     Display the current default base for numeric input and display.

Optional warnings and messages
==============================

By default, GDBN is silent about its inner workings.  If you are running on a slow machine, you may want to use the set verbose command. It will make GDBN tell you when it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that the symbol table for a source file is being read; see symbol-file in Commands to specify files.

set verbose on
     Enables GDBN output of certain informational messages.

set verbose off
     Disables GDBN output of certain informational messages.

show verbose
     Displays whether set verbose is on or off.

By default, if GDBN encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see Errors reading symbol files).

set complaints LIMIT
     Permits GDBN to output LIMIT complaints about each type of unusual symbols before becoming silent about the problem.  Set LIMIT to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.

show complaints
     Displays how many symbol complaints GDBN is permitted to produce.

By default, GDBN is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands.  For example, if you try to run a program which is already running:

     (GDBP) run
     The program being debugged has been started already.
     Start it from the beginning? (y or n)

If you are willing to unflinchingly face the consequences of your own commands, you can disable this ``feature'':

set confirm off
     Disables confirmation requests.

set confirm on
     Enables confirmation requests (the default).

show confirm
     Displays state of confirmation requests.

Some systems allow individual object files that make up your program to be replaced without stopping and restarting your program. If you are running on one of these systems, you can allow GDBN to reload the symbols for automatically relinked modules:

set symbol-reloading on
     Replace symbol definitions for the corresponding source file when an object file with a particular name is seen again.

set symbol-reloading off
     Do not replace symbol definitions when re-encountering object files of the same name.  This is the default state; if you are not running on a system that permits automatically relinking modules, you should leave symbol-reloading off, since otherwise GDBN may discard symbols when linking large programs, that may contain several modules (from different directories or libraries) with the same name.

show symbol-reloading
     Show the current on or off setting.

Canned Sequences of Commands
****************************

Aside from breakpoint commands (see Breakpoint command lists), GDBN provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

User-defined commands
=====================

A user-defined command is a sequence of GDBN commands to which you assign a new name as a command.  This is done with the define command.

define COMMANDNAME
     Define a command named COMMANDNAME.  If there is already a command by that name, you are asked to confirm that you want to redefine it.

     The definition of the command is made up of other GDBN command lines, which are given following the define command.  The end of these commands is marked by a line containing end.

document COMMANDNAME
     Give documentation to the user-defined command COMMANDNAME.  The command COMMANDNAME must already be defined.  This command reads lines of documentation just as define reads the lines of the command definition, ending with end.  After the document command is finished, help on command COMMANDNAME will print the documentation you have specified.

     You may use the document command again to change the documentation of a command.  Redefining the command with define does not change the documentation.

help user-defined
     List all user-defined commands, with the first line of the documentation (if any) for each.

show user
show user COMMANDNAME
     Display the GDBN commands used to define COMMANDNAME (but not its documentation).  If no COMMANDNAME is given, display the definitions for all user-defined commands.

User-defined commands do not take arguments.  When they are executed, the commands of the definition are not printed.  An error in any command stops execution of the user-defined command.

Commands that would ask for confirmation if used interactively proceed without asking when used inside a user-defined command.  Many GDBN commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

User-defined command hooks
==========================

You may define hooks, which are a special kind of user-defined command.  Whenever you run the command foo, if the user-defined command hook-foo exists, it is executed (with no arguments) before that command.

In addition, a pseudo-command, stop exists.  Defining (hook-stop) makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define:

     define hook-stop
     handle SIGALRM nopass
     end

     define hook-run
     handle SIGALRM pass
     end

     define hook-continue
     handle SIGLARM pass
     end

You can define a hook for any single-word command in GDBN, but not for command aliases; you should define a hook for the basic command name, e.g.  backtrace rather than bt. If an error occurs during the execution of your hook, execution of GDBN commands stops and GDBN issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you will get a warning from the define command.

Command files
=============

A command file for GDBN is a file of lines that are GDBN commands.  Comments (lines starting with #) may also be included.  An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.

When you start GDBN, it automatically executes commands from its init files.  These are files named GDBINIT.  GDBN reads the init file (if any) in your home directory and then the init file (if any) in the current working directory.  (The init files are not executed if you use the -nx option; see Choosing modes.)

You can also request the execution of a command file with the source command:

source FILENAME
     Execute the command file FILENAME.

The lines in a command file are executed sequentially.  They are not printed as they are executed.  An error in any command terminates execution of the command file.

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file.  Many GDBN commands that normally print messages to say what they are doing omit the messages when called from command files.

Commands for controlled output
==============================

During the execution of a command file or a user-defined command, normal GDBN output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition.  This section describes three commands useful for generating exactly the output you want.

echo TEXT
     Print TEXT.  Nonprinting characters can be included in TEXT using C escape sequences, such as \n to print a newline.  No newline will be printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space.  This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print  and foo = , use the command echo \ and foo = \ .

     A backslash at the end of TEXT can be used, as in C, to continue the command onto subsequent lines.  For example,

          echo This is some text\n\
          which is continued\n\
          onto several lines.\n

     produces the same output as

          echo This is some text\n
          echo which is continued\n
          echo onto several lines.\n

output EXPRESSION
     Print the value of EXPRESSION and nothing but that value: no newlines, no $NN = .  The value is not entered in the value history either.  See Expressions, for more information on expressions.

output/FMT EXPRESSION
     Print the value of EXPRESSION in format FMT.  You can use the same formats as for print.  See Output formats, for more information.

printf STRING, EXPRESSIONS...
     Print the values of the EXPRESSIONS under the control of STRING.  The EXPRESSIONS are separated by commas and may be either numbers or pointers.  Their values are printed as specified by STRING, exactly as if your program were to execute

          printf (STRING, EXPRESSIONS...);

     For example, you can print two values in hex like this:

          printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

     The only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.

Using GDBN under GNU Emacs
**************************

A special interface allows you to use GNU Emacs to view (and edit) the source files for the program you are debugging with GDBN.

To use this interface, use the command M-x gdb in Emacs.  Give the executable file you want to debug as an argument.  This command starts GDBN as a subprocess of Emacs, with input and output through a newly created Emacs buffer.

Using GDBN under Emacs is just like using GDBN normally except for two things:

  * All ``terminal'' input and output goes through the Emacs buffer.

This applies both to GDBN commands and their output, and to the input and output done by the program you are debugging.

This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way.

All the facilities of Emacs' Shell mode are available for interacting with your program.  In particular, you can send signals the usual way---for example, C-c C-c for an interrupt, C-c C-z for a stop.

  * GDBN displays source code through Emacs.

Each time GDBN displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (=>) at the left margin of the current line.  Emacs uses a separate buffer for source display, and splits the screen to show both your GDBN session and the source.

Explicit GDBN list or search commands still produce output as usual, but you probably will have no reason to use them.

     Warning: If the directory where your program resides is not your current directory, it can be easy to confuse Emacs about the location of the source files, in which case the auxiliary display buffer will not appear to show your source.  GDBN can find programs by searching your environment's PATH variable, so the GDBN input and output session will proceed normally; but Emacs does not get enough information back from GDBN to locate the source files in this situation.  To avoid this problem, either start GDBN mode from the directory where your program resides, or specify a full path name when prompted for the M-x gdb argument.

     A similar confusion can result if you use the GDBN file command to switch to debugging a program in some other location, from an existing GDBN buffer in Emacs.

By default, M-x gdb calls the program called gdb.  If you need to call GDBN by a different name (for example, if you keep several configurations around, with different names) you can set the Emacs variable gdb-command-name; for example,

     (setq gdb-command-name "mygdb")

(preceded by ESC ESC, or typed in the *scratch* buffer, or in your .emacs file) will make Emacs call the program named ``mygdb'' instead.

In the GDBN I/O buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:

C-h m
     Describe the features of Emacs' GDBN Mode.

M-s
     Execute to another source line, like the GDBN step command; also update the display window to show the current file and location.

M-n
     Execute to next source line in this function, skipping all function calls, like the GDBN next command.  Then update the display window to show the current file and location.

M-i
     Execute one instruction, like the GDBN stepi command; update display window accordingly.

M-x gdb-nexti
     Execute to next instruction, using the GDBN nexti command; update display window accordingly.

C-c C-f
     Execute until exit from the selected stack frame, like the GDBN finish command.

M-c
     Continue execution of your program, like the GDBN continue command.

     Warning: In Emacs v19, this command is C-c C-p.

M-u
     Go up the number of frames indicated by the numeric argument (see Numeric Arguments (The GNU Emacs Manual)), like the GDBN up command.

     Warning: In Emacs v19, this command is C-c C-u.

M-d
     Go down the number of frames indicated by the numeric argument, like the GDBN down command.

     Warning: In Emacs v19, this command is C-c C-d.

C-x &
     Read the number where the cursor is positioned, and insert it at the end of the GDBN I/O buffer.  For example, if you wish to disassemble code around an address that was displayed earlier, type disassemble; then move the cursor to the address display, and pick up the argument for disassemble by typing C-x &.

     You can customize this further by defining elements of the list gdb-print-command; once it is defined, you can format or otherwise process numbers picked up by C-x & before they are inserted.  A numeric argument to C-x & will both indicate that you wish special formatting, and act as an index to pick an element of the list.  If the list element is a string, the number to be inserted is formatted using the Emacs function format; otherwise the number is passed as an argument to the corresponding list element.

In any source file, the Emacs command C-x SPC (gdb-break) tells GDBN to set a breakpoint on the source line point is on.

If you accidentally delete the source-display buffer, an easy way to get it back is to type the command f in the GDBN buffer, to request a frame display; when you run under Emacs, this will recreate the source buffer if necessary to show you the context of the current frame.

The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way.  You can edit the files with these buffers if you wish; but keep in mind that GDBN communicates with Emacs in terms of line numbers.  If you add or delete lines from the text, the line numbers that GDBN knows will cease to correspond properly with the code.

Reporting Bugs in GDBN
**********************

Your bug reports play an essential role in making GDBN reliable.

Reporting a bug may help you by bringing a solution to your problem, or it may not.  But in any case the principal function of a bug report is to help the entire community by making the next version of GDBN work better.  Bug reports are your contribution to the maintenance of GDBN.

In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.

Have you found a bug?
=====================

If you are not sure whether you have found a bug, here are some guidelines:

  * If the debugger gets a fatal signal, for any input whatever, that is a GDBN bug.  Reliable debuggers never crash.

  * If GDBN produces an error message for valid input, that is a bug.

  * If GDBN does not produce an error message for invalid input, that is a bug.  However, you should note that your idea of ``invalid input'' might be our idea of ``an extension'' or ``support for traditional practice''.

  * If you are an experienced user of debugging tools, your suggestions for improvement of GDBN are welcome in any case.

How to report bugs
==================

A number of companies and individuals offer support for GNU products. If you obtained GDBN from a support organization, we recommend you contact that organization first.

You can find contact information for many support companies and individuals in the file etc/SERVICE in the GNU Emacs distribution.

In any event, we also recommend that you send bug reports for GDBN to one of these addresses:

     bug-gdb@prep.ai.mit.edu
     {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gdb

Do not send bug reports to info-gdb, or to
help-gdb, or to any newsgroups. Most users of GDBN do not want to receive bug reports.  Those that do, have arranged to receive bug-gdb.

The mailing list bug-gdb has a newsgroup gnu.gdb.bug which serves as a repeater.  The mailing list and the newsgroup carry exactly the same messages.  Often people think of posting bug reports to the newsgroup instead of mailing them.  This appears to work, but it has one problem which can be crucial: a newsgroup posting often lacks a mail path back to the sender.  Thus, if we need to ask for more information, we may be unable to reach you.  For this reason, it is better to send bug reports to the mailing list.

As a last resort, send bug reports on paper to:

     GNU Debugger Bugs
     Free Software Foundation
     545 Tech Square
     Cambridge, MA 02139

The fundamental principle of reporting bugs usefully is this: report all the facts.  If you are not sure whether to state a fact or leave it out, state it!

Often people omit facts because they think they know what causes the problem and assume that some details do not matter.  Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it does not, but one cannot be sure.  Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the debugger into doing the right thing despite the bug.  Play it safe and give a specific, complete example.  That is the easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us.  It is not as important as what happens if the bug is already known.  Therefore, always write your bug reports on the assumption that the bug has not been reported previously.

Sometimes people give a few sketchy facts and ask, ``Does this ring a bell?''  Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.

To enable us to fix the bug, you should include all these things:

  * The version of GDBN.  GDBN announces it if you start with no arguments; you can also print it at any time using show version.

     Without this, we will not know whether there is any point in looking for the bug in the current version of GDBN.

  * The type of machine you are using, and the operating system name and version number.

  * What compiler (and its version) was used to compile GDBN---e.g. ``GCC--2.0''.

  * What compiler (and its version) was used to compile the program you are debugging---e.g.  ``GCC--2.0''.

  * The command arguments you gave the compiler to compile your example and observe the bug.  For example, did you use -O?  To guarantee you will not omit something important, list them all.  A copy of the Makefile (or the output from make) is sufficient.

     If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.

  * A complete input script, and all necessary source files, that will reproduce the bug.

  * A description of what behavior you observe that you believe is incorrect.  For example, ``It gets a fatal signal.''

     Of course, if the bug is that GDBN gets a fatal signal, then we will certainly notice it.  But if the bug is incorrect output, we might not notice unless it is glaringly wrong.  We are human, after all.  You might as well not give us a chance to make a mistake.

     Even if the problem you experience is a fatal signal, you should still say so explicitly.  Suppose something strange is going on, such as, your copy of GDBN is out of synch, or you have encountered a bug in the C library on your system.  (This has happened!)  Your copy might crash and ours would not.  If you told us to expect a crash, then when ours fails to crash, we would know that the bug was not happening for us.  If you had not told us to expect a crash, then we would not be able to draw any conclusion from our observations.

  * If you wish to suggest changes to the GDBN source, send us context diffs.  If you even discuss something in the GDBN source, refer to it by context, not by line number.

     The line numbers in our development sources will not match those in your sources.  Your line numbers would convey no useful information to us.

Here are some things that are not necessary:

  * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.

     This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.

     Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us.  Errors in the output will be easier to spot, running under the debugger will take less time, etc.

     However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.

  * A patch for the bug.

     A patch for the bug does help us if it is a good one.  But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need.  We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.

     Sometimes with a program as complicated as GDBN it is very hard to construct an example that will make the program follow a certain path through the code.  If you do not send us the example, we will not be able to construct one, so we will not be able to verify that the bug is fixed.

     And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it.  A test case will help us to understand.

  * A guess about what the bug is or what it depends on.

     Such guesses are usually wrong.  Even we cannot guess right about such things without first using the debugger to find the facts.

Formatting Documentation
************************

The GDB 4 release includes an already-formatted reference card, ready for printing with PostScript or GhostScript, in the gdb subdirectory of the main source directory (In
gdb-GDBVN/gdb/refcard.ps of the version GDBVN
release.).  If you can use PostScript or GhostScript with your printer, you can print the reference card immediately with refcard.ps.

The release also includes the source for the reference card.  You can format it, using TeX, by typing:

     make refcard.dvi

The GDB reference card is designed to print in landscape mode on US ``letter'' size paper; that is, on a sheet 11 inches wide by 8.5 inches high.  You will need to specify this form of printing as an option to your DVI output program.

All the documentation for GDB comes as part of the machine-readable distribution.  The documentation is written in Texinfo format, which is a documentation system that uses a single source file to produce both on-line information and a printed manual.  You can use one of the Info formatting commands to create the on-line version of the documentation and TeX (or texi2roff) to typeset the printed version.

GDB includes an already formatted copy of the on-line Info version of this manual in the gdb subdirectory.  The main Info file is gdb-VERSION-NUMBER/gdb/gdb.info, and it refers to subordinate files matching gdb.info* in the same directory.  If necessary, you can print out these files, or read them with any editor; but they are easier to read using the info subsystem in GNU Emacs or the standalone info program, available as part of the GNU Texinfo distribution.

If you want to format these Info files yourself, you need one of the Info formatting programs, such as texinfo-format-buffer or makeinfo.

If you have makeinfo installed, and are in the top level GDB source directory (gdb-GDBVN, in the case of version GDBVN), you can make the Info file by typing:

     cd gdb
     make gdb.info

If you want to typeset and print copies of this manual, you need TeX, a program to print its DVI output files, and texinfo.tex, the Texinfo definitions file.

TeX is a typesetting program; it does not print files directly, but produces output files called DVI files.  To print a typeset document, you need a program to print DVI files.  If your system has TeX installed, chances are it has such a program.  The precise command to use depends on your system; lpr -d is common; another (for PostScript devices) is dvips.  The DVI print command may require a file name without any extension or a .dvi extension.

TeX also requires a macro definitions file called texinfo.tex.  This file tells TeX how to typeset a document written in Texinfo format.  On its own, TeX cannot read, much less typeset a Texinfo file.  texinfo.tex is distributed with GDB and is located in the gdb-VERSION-NUMBER/texinfo directory.

If you have TeX and a DVI printer program installed, you can typeset and print this manual.  First switch to the the gdb subdirectory of the main source directory (for example, to gdb-GDBVN/gdb) and then type:

     make gdb.dvi

Installing GDB
**************

GDB comes with a configure script that automates the process of preparing GDB for installation; you can then use make to build the gdb program.

The GDB distribution includes all the source code you need for GDB in a single directory, whose name is usually composed by appending the version number to gdb.

For example, the GDB version GDBVN distribution is in the gdb-GDBVN directory.  That directory contains:

gdb-GDBVN/configure (and supporting files)
     script for configuring GDB and all its supporting libraries.

gdb-GDBVN/gdb
     the source specific to GDB itself

gdb-GDBVN/bfd
     source for the Binary File Descriptor library

gdb-GDBVN/include
     GNU include files

gdb-GDBVN/libiberty
     source for the -liberty free software library

gdb-GDBVN/opcodes
     source for the library of opcode tables and disassemblers

gdb-GDBVN/readline
     source for the GNU command-line interface

gdb-GDBVN/glob
     source for the GNU filename pattern-matching subroutine

gdb-GDBVN/mmalloc
     source for the GNU memory-mapped malloc package

The simplest way to configure and build GDB is to run configure from the gdb-VERSION-NUMBER source directory, which in this example is the gdb-GDBVN directory.

First switch to the gdb-VERSION-NUMBER source directory if you are not already in it; then run configure.  Pass the identifier for the platform on which GDB will run as an argument.

For example:

     cd gdb-GDBVN
     ./configure HOST
     make

where HOST is an identifier such as sun4 or decstation, that identifies the platform where GDB will run.

Running configure HOST and then running make builds the bfd, readline, mmalloc, and libiberty libraries, then gdb itself.  The configured source files, and the binaries, are left in the corresponding source directories.

configure is a Bourne-shell (/bin/sh) script; if your system does not recognize this automatically when you run a different shell, you may need to run sh on it explicitly:

     sh configure HOST

If you run configure from a directory that contains source directories for multiple libraries or programs, such as the gdb-GDBVN source directory for version GDBVN, configure creates configuration files for every directory level underneath (unless you tell it not to, with the --norecursion option).

You can run the configure script from any of the subordinate directories in the GDB distribution, if you only want to configure that subdirectory; but be sure to specify a path to it.

For example, with version GDBVN, type the following to configure only the bfd subdirectory:

     cd gdb-GDBVN/bfd
     ../configure HOST

You can install GDBP anywhere; it has no hardwired paths. However, you should make sure that the shell on your path (named by the SHELL environment variable) is publicly readable.  Remember that GDB uses the shell to start your program---some systems refuse to let GDB debug child processes whose programs are not readable.

Compiling GDB in another directory
==================================

If you want to run GDB versions for several host or target machines, you need a different gdb compiled for each combination of host and target.  configure is designed to make this easy by allowing you to generate each configuration in a separate subdirectory, rather than in the source directory.  If your make program handles the VPATH feature (GNU make does), running make in each of these directories builds the gdb program specified there.

To build gdb in a separate directory, run configure with the --srcdir option to specify where to find the source. (You also need to specify a path to find configure itself from your working directory.  If the path to configure would be the same as the argument to --srcdir, you can leave out the --srcdir option; it will be assumed.)

For example, with version GDBVN, you can build GDB in a separate directory for a Sun 4 like this:

     cd gdb-GDBVN
     mkdir ../gdb-sun4
     cd ../gdb-sun4
     ../gdb-GDBVN/configure sun4
     make

When configure builds a configuration using a remote source directory, it creates a tree for the binaries with the same structure (and using the same names) as the tree under the source directory.  In the example, you'd find the Sun 4 library libiberty.a in the directory gdb-sun4/libiberty, and GDB itself in gdb-sun4/gdb.

One popular reason to build several GDB configurations in separate directories is to configure GDB for cross-compiling (where GDB runs on one machine---the host---while debugging programs that run on another machine---the target).  You specify a cross-debugging target by giving the --target=TARGET option to configure.

When you run make to build a program or library, you must run it in a configured directory---whatever directory you were in when you called configure (or one of its subdirectories).

The Makefile that configure generates in each source directory also runs recursively.  If you type make in a source directory such as gdb-GDBVN (or in a separate configured directory configured with --srcdir=PATH/gdb-GDBVN), you will build all the required libraries, and then build GDB.

When you have multiple hosts or targets configured in separate directories, you can run make on them in parallel (for example, if they are NFS-mounted on each of the hosts); they will not interfere with each other.

Specifying names for hosts and targets
======================================

The specifications used for hosts and targets in the configure script are based on a three-part naming scheme, but some short predefined aliases are also supported.  The full naming scheme encodes three pieces of information in the following pattern:

     ARCHITECTURE-VENDOR-OS

For example, you can use the alias sun4 as a HOST argument, or as the value for TARGET in a --target=TARGET option.  The equivalent full name is sparc-sun-sunos4.

The configure script accompanying GDB does not provide any query facility to list all supported host and target names or aliases.  configure calls the Bourne shell script config.sub to map abbreviations to full names; you can read the script, if you wish, or you can use it to test your guesses on abbreviations---for example:

     % sh config.sub sun4
     sparc-sun-sunos411
     % sh config.sub sun3
     m68k-sun-sunos411
     % sh config.sub decstation
     mips-dec-ultrix42
     % sh config.sub hp300bsd
     m68k-hp-bsd
     % sh config.sub i386v
     i386-unknown-sysv
     % sh config.sub i786v
     Invalid configuration `i786v': machine `i786v' not recognized

config.sub is also distributed in the GDB source directory (gdb-GDBVN, for version GDBVN).

configure options
=================

Here is a summary of the configure options and arguments that are most often useful for building GDBN.  configure also has several other options not listed here.  See What Configure Does (configure.info), for a full explanation of configure.

     configure [--help]
               [--prefix=DIR]
               [--srcdir=PATH]
               [--norecursion] [--rm]
               [--target=TARGET] HOST

You may introduce options with a single - rather than -- if you prefer; but you may abbreviate option names if you use --.

--help
     Display a quick summary of how to invoke configure.

-prefix=DIR
     Configure the source to install programs and files under directory DIR.

--srcdir=PATH
     Warning: using this option requires GNU make, or another
     make that implements the VPATH feature.

     Use this option to make configurations in directories separate from the GDB source directories.  Among other things, you can use this to build (or maintain) several configurations simultaneously, in separate directories.  configure writes configuration specific files in the current directory, but arranges for them to use the source in the directory PATH.  configure will create directories under the working directory in parallel to the source directories below PATH.

--norecursion
     Configure only the directory level where configure is executed; do not propagate configuration to subdirectories.

--rm
     Remove the configuration that the other arguments specify.

--target=TARGET
     Configure GDB for cross-debugging programs running on the specified TARGET.  Without this option, GDB is configured to debug programs that run on the same machine (HOST) as GDB itself.

     There is no convenient way to generate a list of all available targets.

HOST ...
     Configure GDB to run on the specified HOST.

     There is no convenient way to generate a list of all available hosts.

configure accepts other options, for compatibility with configuring other GNU tools recursively; but these are the only options that affect GDB or its supporting libraries.

GNU GENERAL PUBLIC LICENSE
**************************

Version 2, June 1991

     Copyright © 1989, 1991 Free Software Foundation, Inc.
     675 Mass Ave, Cambridge, MA 02139, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

Preamble
========

The licenses for most software are designed to take away your freedom to share and change it.  By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software---to make sure the software is free for all its users.  This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it.  (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.)  You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price.  Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have.  You must make sure that they, too, receive or can get the source code.  And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software.  If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.

Finally, any free program is threatened constantly by software patents.  We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary.  To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and modification follow.

TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

 1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License.  The ``Program'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language.  (Hereinafter, translation is included without limitation in the term ``modification''.)  Each licensee is addressed as ``you''.

     Activities other than copying, distribution and modification are not covered by this License; they are outside its scope.  The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.

 2. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.

     You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.

 3. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:

      a. You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.

      b. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

      c. If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License.  (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)

     These requirements apply to the modified work as a whole.  If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works.  But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.

     Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.

     In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.

 4. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:

      a. Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,

      b. Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,

      c. Accompany it with the information you received as to the offer to distribute corresponding source code.  (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)

     The source code for a work means the preferred form of the work for making modifications to it.  For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable.  However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

     If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.

 5. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License.  Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

 6. You are not required to accept this License, since you have not signed it.  However, nothing else grants you permission to modify or distribute the Program or its derivative works.  These actions are prohibited by law if you do not accept this License.  Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.

 7. Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions.  You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License.

 8. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License.  If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all.  For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program.

     If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.

     It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices.  Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.

     This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

 9. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded.  In such case, this License incorporates the limitation as if written in the body of this License.

 10. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time.  Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

     Each version is given a distinguishing version number.  If the Program specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation.  If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.

 11. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission.  For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this.  Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.

     NO WARRANTY

 12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

 13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

END OF TERMS AND CONDITIONS

Applying These Terms to Your New Programs
=========================================

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program.  It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND AN IDEA OF WHAT IT DOES.
     Copyright (C) 19YY  NAME OF AUTHOR

     This program is free software; you can redistribute it and/or
     modify it under the terms of the GNU General Public License
     as published by the Free Software Foundation; either version 2
     of the License, or (at your option) any later version.

     This program is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program; if not, write to the
     Free Software Foundation, Inc., 675 Mass Ave,
     Cambridge, MA 02139, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode:

     Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR
     Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
     type `show w'.  This is free software, and you are welcome
     to redistribute it under certain conditions; type `show c'
     for details.

The hypothetical commands show w and show c should show the appropriate parts of the General Public License.  Of course, the commands you use may be called something other than show w and show c; they could even be mouse-clicks or menu items---whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the program, if necessary.  Here is a sample; alter the names:

     Yoyodyne, Inc., hereby disclaims all copyright
     interest in the program `Gnomovision'
     (which makes passes at compilers) written
     by James Hacker.

     SIGNATURE OF TY COON, 1 April 1989
     Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs.  If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library.  If this is what you want to do, use the GNU Library General Public License instead of this License.