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/

help

Get help on commands

run [ARGS]

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

backtrace [COUNT]

bt [COUNT]

where [COUNT]

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

frame [FRAME]

Select and print stack frame

info locals

info args

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

print VAR

Print value of variable VAR

list

List specified function or line (see help line for more information)

kill

Kill current program

b FILE:LINE

break FILE:LINE

Insert a breakpoint at file FILE, line LINE

watch expr

stop execution when expr. changes

awatch expr

stop execution when expression is accessed (read or write)

next

Step to next instruction

cont

continue execution

help command

apropos word

type help followed by command name for full documentation

type apropos wrod to serrch for commands related to word

Command name abbreviations are allowed if unambiguous

source script

source given script

save breakpoints file

save b file

save current breakpoints as script

RETURN

repeat last command

b main	Put a breakpoint at the beginning of the program
b	Put a breakpoint at the current line
b N	Put a breakpoint at line N
b +N	Put a breakpoint N lines down from the current line
b fn	Put a breakpoint at the beginning of function "fn"
d N	delete breakpoint number N
r	Run the program until a breakpoint or error
c	continue running the program until the next breakpoint or error
f	Run until the current function is finished
s	run the next line of the program
s N	run the next N lines of the program
n	like s, but don't step into functions
u N	run until you get N lines in front of the current line
p var	print the current value of the variable "var"
bt	print a stack trace
u	go up a level in the stack
d	go down a level in the stack
q	Quit gdb

Break points

b _location_ break _location_	Set breakpoint at _location_.
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)

Execute program

run	Start execution (or restart)
s step	Step into current line
n next	Run to next line (step over current line)
finish	Execute till returning from current selected frame.
advance _location_	Run until temporary breakpoint set at _location_.

Display and print

display _expr_	Display _expr_ at each prompt (if within scope).
undisplay _number_	Undisplay expression by _number_.

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