gdb

GDB

Purpose

Walk agents through GDB sessions from first launch to advanced workflows: crash diagnosis, reverse debugging, remote debugging, and multi-thread inspection.

Triggers

• "My program segfaults / crashes — how do I debug it?"
• "How do I set a breakpoint on condition X?"
• "How do I inspect memory / variables in GDB?"
• "How do I debug a remote embedded target?"
• "GDB shows ?? frames / no source"
• "How do I replay a bug deterministically?" (record/replay)

Workflow

1. Prerequisite: compile with debug info

Always compile with -g (GCC/Clang). Use -Og or -O0 for most debuggable code.

gcc -g -Og -o prog main.c

For release builds: use -g -O2 and keep the binary with symbols (strip separately with objcopy).

2. Start GDB

gdb ./prog                          # load binary
gdb ./prog core                     # load with core dump
gdb -p 12345                        # attach to running process
gdb --args ./prog arg1 arg2         # pass arguments
gdb -batch -ex 'run' -ex 'bt' ./prog  # non-interactive (CI)

3. Essential commands

Command	Shortcut	Effect
run [args]	r	Start the program
continue	c	Resume after break
next	n	Step over (source line)
step	s	Step into
nexti	ni	Step over (instruction)
stepi	si	Step into (instruction)
finish		Run to end of current function
until N		Run to line N
return [val]		Force return from function
quit	q	Exit GDB

4. Breakpoints and watchpoints

break main                          # break at function
break file.c:42                     # break at line
break *0x400abc                     # break at address
break foo if x > 10                 # conditional break
tbreak foo                          # temporary breakpoint (fires once)
rbreak ^mylib_.*                    # regex breakpoint on all matching functions

watch x                             # watchpoint: break when x changes
watch *(int*)0x601060               # watch memory address
rwatch x                            # break when x is read
awatch x                            # break on read or write

info breakpoints                    # list all breakpoints
delete 3                            # delete breakpoint 3
disable 3                           # disable without deleting
enable 3