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GDB stands for GNU Project Debugger.

It allows you to observe what does happen when a program runs or by using a core file when a program crashed.

Install GDB

Install GDB in Debian:
$ sudo aptitude install gdb

Run a program

Debug a program:
$ gdb ./my_program

A (gdb) prompt appears when gdb executes:

Pass some arguments:
(gdb) set args -o file_name

Set a breakpoint at a function:
(gdb) break function_name

Run the program:
(gdb) run
Starting program: my_program -o file_name

Quit GDB

(gdb) quit
(gdb) q

Show GDB help

GDB man page:
$ man gdb

Get help inside GDB:
(gdb) help
(gdb) h
List of classes of commands:

aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without stopping the program
user-defined -- User-defined commands



Get help about a specific command:
(gdb) help run
(gdb) h run
(gdb) h r
Start debugged program. You may specify arguments to give it.
Args may include "*", or "[...]"; they are expanded using "sh".
Input and output redirection with ">", "<", or ">>" are also allowed.

With no arguments, uses arguments last specified (with "run" or "set args").
To cancel previous arguments and run with no arguments,
use "set args" without arguments.

Show source code

List source code:
(gdb) list
(gdb) l
130 /**
131 * @brief Process a running thread
132 */
133 void* processing_thread(void *_thread_id)
134 {
135 long thread_id = (long) _thread_id;
136 char pcap_error_buffer[PCAP_ERRBUF_SIZE];
137
138 if (!quiet_mode) {
139 printf("Running thread %ld...\n", thread_id);


List around a specific line of source code:
(gdb) list 120
(gdb) l 120

info command

(gdb) help info
Generic command for showing things about the program being debugged.


List all breakpoints:
(gdb) info break


Show local variables of current stack frame:
(gdb) i locals
rkm = 0x55fa3d905f40
err =
errnox = 8732417

Debug a crash using a core file

You may have cores disabled for your shell:
$ ulimit -c
0

To enable them:
$ ulimit -c unlimited

Systemd places coredum by default in /var/lib/systemd/coredump directory.

If core is in lz4 format, to decompress it:
$ unlz4 core.my_program.1000.a5a9801317064638a15481fbebfa686f.2010317.1582046615000000000000.lz4

Run GDB using that core file:
$ gdb my_program core_file
E.g: $ gdb my_program core.my_program.1000.a5a9801317064638a15481fbebfa686f.2010317.1582046615000000000000

Show stack

backtrace (bt) or where command shows the call stack:
(gdb) bt

or

(gdb) where

Show full stack

Show call stack and local variables:
(gdb) bt full

or

(gdb) where full

Breakpoints

Help about commands related to breakpoints:
(gdb) help breakpoints


Set a breakpoint at the beginning of a function:
(gdb) break function_name
(gdb) b function_name


List current breakpoints:
(gdb) info b
(gdb) i b
Num Type Disp Enb Address What
1 breakpoint keep n 0x00007ffff7c14210 in rd_kafka_msg_partitioner
at rdkafka_msg.c:945
breakpoint already hit 1 time



Continue execution after a breakpoint:
(gdb) continue
(gdb) c

Disable breakpoints

Disable all breakpoints:
(gdb) disable breakpoints

Disable breakpoint number 1:
(gdb) disable breakpoints 1

Enable breakpoints

Enable all breakpoints:
(gdb) enable breakpoints

Enable breakpoint number 1:
(gdb) enable breakpoints 1

Step through the source code

Step into next line but do not enter in functions:
next
n

You can also execute a number of lines:
next number
n 5 # Execute 5 lines.

Step into next line but enter inside functions:
step
s
s 3 # Execute 3 lines.

Print value of a variable

(gdb) print my_variable
(gdb) p my_variable

REFERENCE

$ man gdb

https://www.gnu.org/software/gdb/

https://darkdust.net/files/GDB%20Cheat%20Sheet.pdf