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

Reference list:

• GDB Front Ends

Ideally front-ends must use the GDB/MI2 interface.

These are using the old "annotation" mechanisms:

• 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

GDB configuratino

GDB reads file ~/.gdbinit at start.

Some references:

• http://stackoverflow.com/questions/209534/how-to-highlight-and-color-gdb-output-during-interactive-debugging

Bare minimum configuration

From StackOverflow [1]:

set history save on
set print pretty
set output-radix 16
set height 0

GDB dashboard

GDB dashboard is a modular visual interface for GDB in Python.

To install simply copy .gdbinit as ~/.gdbinit

cp gdb-dashboard/.gdbinit ~/.gdbinit

Alternatively, source it from ~/.gdbinit:

source ~/.gdbinit-dashboard

Install pygments

Install pygments to get source highlighting

sudo pip install Pygments        # Globally
pip install Pygments             # Locally

If GDB uses python3 (ldd $(which gdb))), you'll need to install with pip3:

sudo pip3 install Pygments       # Globally
pip3 install Pygments            # Locally

To get the list of available styles:

python from pygments.styles import get_all_styles as styles
python for s in styles(): print(s)

Alternative styles:

• gruvbox

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/

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
r [ARGS] 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
disp EXPR 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.
RETURN	Repeat last command.

GDB examples

Simple Segmentation Fault Example

(From [2])

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