Gdb

References

• 7.2 Example Debugging Session: Segmentation Fault Example (unkownroad.com)
• Debugging with GDB (delorie.com)
• GDB commands (tutorialspoint.com)

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

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

kill

Kill current program

b FILE:LINE

break FILE:LINE

Insert a breakpoint at file FILE, line LINE

next

Step to next instruction

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
info break	list breakpoints
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

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