Gdb

References

• 7.2 Example Debugging Session: Segmentation Fault Example (unkownroad.com)
• Debugging with GDB (delorie.com)
• GDB commands (tutorialspoint.com)
• An Interactive Guide to Faster, Less Frustrating Debugging
• Norm Matloff's Debugging Tutorial (also guide to fast editing and gui debuggers DDD, GVD)

GDB front-ends

• DDD
• KDBG
• cgdb (text mode with vi keymap)

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:

• GDB manual
• https://beej.us/guide/bggdb/

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.
until [NUMBER]	Execute until the program reaches a source line greater than current.

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
up	Go up a level in the stack (frame calling current frame).
do down	Go down a level in the stack (frame called by current frame).

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