Sometimes we get a bus error or a segmentation fault. But we have no idea where it is happening. The debugger program called gdb can help. Here's how to see where the bug happened in your code.
gdb file [filename] set args [arg1] [arg2] ror
gdb [filename] run arg1 arg2The r command will run the debugger.
GNU Tools gdb(1)
NAME
gdb - The GNU Debugger
SYNOPSIS
gdb [-help] [-nx] [-q] [-batch] [-cd=dir] [-f] [-b bps]
[-tty=dev] [-s symfile] [-e prog] [-se prog] [-c core]
[-x cmds] [-d dir] [prog[core|procID]]
DESCRIPTION
The purpose of a debugger such as GDB 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.
GDB can do four main kinds of things (plus other things in
support of these) to help you catch bugs in the act:
o Start your program, specifying anything that might
affect its behavior.
o Make your program stop on specified conditions.
o Examine what has happened, when your program has
stopped.
o 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 GDB to debug programs written in C, C++, and
Modula-2. Fortran support will be added when a GNU Fortran
compiler is ready.
GDB is invoked with the shell command gdb. Once started, it
reads commands from the terminal until you tell it to exit
with the GDB command quit. You can get online help from gdb
itself by using the command help.
You can run gdb with no arguments or options; but the most
usual way to start GDB is with one argument or two, specify-
ing an executable program as the argument:
gdb program
You can also start with both an executable program and a
core file specified:
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gdb program core
You can, instead, specify a process ID as a second argument,
if you want to debug a running process:
gdb program 1234
would attach GDB to process 1234 (unless you also have a
file named `1234'; GDB does check for a core file first).
Here are some of the most frequently needed GDB commands:
break [file:]function
Set a breakpoint at function (in file).
run [arglist]
Start your program (with arglist, if specified).
bt Backtrace: display the program stack.
print expr
Display the value of an expression.
c Continue running your program (after stopping, e.g. at
a breakpoint).
next Execute next program line (after stopping); step over
any function calls in the line.
step Execute next program line (after stopping); step into
any function calls in the line.
help [name]
Show information about GDB command name, or general
information about using GDB.
quit Exit from GDB.
For full details on GDB, see Using GDB: A Guide to the GNU
Source-Level Debugger, by Richard M. Stallman and Roland H.
Pesch. The same text is available online as the gdb entry
in the info program.
OPTIONS
Any arguments other than options specify an executable file
and core file (or process ID); that is, the first argument
encountered with no associated option flag is equivalent to
a `-se' option, and the second, if any, is equivalent to a
`-c' option if it's the name of a file. Many options have
both long and short forms; both are shown here. The long
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forms are also recognized 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.)
All the options and command line arguments you give are pro-
cessed in sequential order. The order makes a difference
when the `-x' option is used.
-help
-h List all options, with brief explanations.
-symbols=file
-s file
Read symbol table from file file.
-write
Enable writing into executable and core files.
-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 GDB commands from file file.
-directory=directory
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-d directory
Add directory to the path to search for source files.
-nx
-n Do not execute commands from any `.gdbinit' initializa-
tion files. Normally, the commands in these files are
executed after all the command options and arguments
have been processed.
-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 GDB commands
in the command files.
Batch mode may be useful for running GDB as a filter,
for example to download and run a program on another
computer; in order to make this more useful, the mes-
sage
Program exited normally.
(which is ordinarily issued whenever a program running
under GDB control terminates) is not issued when run-
ning in batch mode.
-cd=directory
Run GDB using directory as its working directory,
instead of the current directory.
-fullname
-f Emacs sets this option when it runs GDB as a subpro-
cess. It tells GDB 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 the program stops). This recognizable format
looks like two ` 32' characters, followed by the file
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name, line number and character position separated by
colons, and a newline. The Emacs-to-GDB interface pro-
gram uses the two ` 32' characters as a signal to
display the source code for the frame.
-b bps
Set the line speed (baud rate or bits per second) of
any serial interface used by GDB for remote debugging.
-tty=device
Run using device for your program's standard input and
output.
SEE ALSO
`gdb' entry in info; Using GDB: A Guide to the GNU Source-
Level Debugger, Richard M. Stallman and Roland H. Pesch,
July 1991.
COPYING
Copyright (c) 1991 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies
of this manual provided the copyright notice and this per-
mission notice are preserved on all copies.
Permission is granted to copy and distribute modified ver-
sions of this manual under the conditions for verbatim copy-
ing, provided that the entire resulting derived work is dis-
tributed 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 condi-
tions for modified versions, except that this permission
notice may be included in translations approved by the Free
Software Foundation instead of in the original English.
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