Gdb

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Revision as of 10:55, 22 November 2016 by Mip (talk | contribs) (→‎GDB commands)
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References

GDB front-ends

There is also the built-in Text User Interface to GDB (C-x C-a: http://davis.lbl.gov/Manuals/GDB/gdb_21.html

Prepare debug session

  • Compile with debug symbols, use option -g:
gcc -g program.c               # -g : debug symbols
gcc -g -O0 program.c           #  ... -O0: disable optimization
  • Force core dumps (see bash help ulimit):
ulimit -c unlimited
./a.out
# Segmentation fault (core dumped)

GDB invocation

gdb a.out
gdb a.out core.1234           # If coredump available

GDB commands

Reference:


RETURN
repeat last command
Break points and watch points

b [+-][NUMBER]
break [+-][NUMBER]

Set a breakpoint at current line, at given line NUMBER or NUMBER lines after/before current line.

b LOCATION
break LOCATION

Set breakpoint at LOCATION.
b main sets a breakpoint at beginning of function main().
b foo.c:42 sets a breakpoint at file foo.c, line 42.

watch EXPR

Stop execution when EXPR changes

awatch EXPR

Stop execution when EXPR is accessed

i b
info b
info break

list breakpoints

cl LOCATION
clear LOCATION

Clear breakpoint by LOCATION

d
delete

Delete all breakpoints

d NUMBER
delete NUMBER

Clear breakpoint by NUMBER (as listed by i b)

dis NUMBER
disable NUMBER

Disable breakpoint by NUMBER (as listed by i b)

save b FILE
save breakpoints FILE

Save current breakpoints as script FILE. Use source to reload.
Execute program

run run [ARGS]

Start (or restart) program. Arguments may include wildcards (*) and redirections (<, <<...)

kill

Kill current program.

c cont

Continue and interrupted program.

s [NUMBER]
step [NUMBER]

Step (into) current line, or NUMBER lines.

n
next

Run to next line (over current line)

f
finish

Execute till returning from current selected frame.

advance LOCATION

Run until temporary breakpoint set at LOCATION.
View stack

bt
bt [COUNT]
backtrace [COUNT]
where [COUNT]

Print backtrace of all stack frames, or innermost (outermost) COUNT frames if COUNT>0 (COUNT<0)

f [FRAME]
frame [FRAME]

Select frame FRAME and print stack frame

u
up

Go up a level in the stack

d
down

Go down a level in the stack
View memory

display EXPR

Display EXPR at each prompt (if within scope).

i locals
i args
info locals info args

Print information on local variables / function arguments in the current frame

print EXPR

print EXPR.

undisplay NUMBER

Undisplay expression by NUMBER.
View code

l
list

List (10 by default) lines of current frame

disassemble /m
disassemble /s
disassemble 'foo.c'::bar

Disassemble a specified section of memory
Miscellaneous

q
quit

Quit gdb.

help COMMAND
apropos WORD

Get help on COMMAND, or search commands related to WORD.

source FILE

Source script FILE.

GDB examples

Simple Segmentation Fault Example

(From [1])

Example program segfault.c:
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
  char *buf;

  buf = malloc(1<<31);

  fgets(buf, 1024, stdin);
  printf("%s\n", buf);

  return 1;
}
Compile and launch gdb:
gcc -g segfault.c
gdb a.out

The debug session

run
backtrace
frame 3
print buf
kill
break segfault.c:8
run
print buf
next
print buf

Fix the bug, then start again, watching now buf:

watch buf
# Start again, answer 'y' when asked to start from beginning
run
# Break at watch point, let's _c_ontinue
c