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
• How to highlight and color gdb output during interactive debugging?

Ideally front-ends must use the GDB/MI2 interface. There is also the built-in Text User Interface to GDB (C-x C-a: http://davis.lbl.gov/Manuals/GDB/gdb_21.html

Good candidates

Mainly from SO post above:

• .gdbinit
• GDB Dashboard (my fork)
• pwndbg
• voltron

Vim:

• https://github.com/vim-scripts/Conque-GDB

Use old shell implementation, not the new neovim terminal.

• http://pyclewn.sourceforge.net/

Read some comment it was not convenient, and would require extra plugin like https://github.com/notEvil/vim-debug.

Neovim:

• https://github.com/critiqjo/lldb.nvim

Only for LLDB.

GDBInit hacks

• Using colout one may color almost any gdb output.

Old front-ends

These are using the old "annotation" mechanisms:

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

GDB configuration

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)
fin finish	Execute till returning from current selected frame.
advance LOCATION	Run until temporary breakpoint set at LOCATION.
u [NUMBER] until [NUMBER]	Execute until the program reaches a source line greater than current (very handy).

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.

Tips

Define a custom label for breakpoint in C/C++

Say we want to set a breakpoint at a specified location in source file, but this position may move over time. The easiest is to use an asm statement to define the label [2]:

#include <stdio.h>

int main () {
  void *ret_p = &&ret;
  printf("ret: %p\n", ret_p);
  goto *ret_p;

  return 1;

ret:
  asm("RET:")

    return 0;
}

This will add a symbol table entry as follows.

gcc  -Wl,--export-dynamic t.c  -ldl
readelf -s a.out | grep RET
# 41: 0804858a     0 NOTYPE  LOCAL  DEFAULT   13 RET

Use help to test a command abbreviation

Don't know if f stands for finish or frame? Just use help f:

help f
# Select and print a stack frame.
# ...

Examples

Simple Segmentation Fault Example

(From [3])

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

More GDB stuff

display i            # Display i at each stop
display/4i $pc       # Display the 4 next instruction at each stop
u                    # Continue until we reach an higher pc address

Using gdb-dashboard

da                   # Print dashboard again
da reg               # Remove register
da reg               # Add it back
da m w 0x1000 0x10   # Watch memory zone at 0x1000 for 16 bytes
da m w 0x2000 0x10   # Add a second zone at 0x2000