GDB

GDB stands for the GNU debugger, which helps finds errors like segmentation faults.

Canonical complication command line:

gcc [cflags] -o <output> <input

Optimize for speed: (-O2)

gcc -Wall -Werror -O2 -o myprogram main.c

Enable debugging support (-g):

gcc -Wall -Error -g myprogram main.c

- To balance performance with debugging experience use -Og - Not recommended to use debugging along with other optimizations - No optimization option is equivalent to -O8

Start GDB with the program to debug as an argument:

gdb myprogram

Running the program: - Without any argument: gdb run - With arguments: gdb run argv1, argv2...

GDB offers an interactive shell: - Can ask gdb help <argument>

Possible scenarios: 1. Program doesn't have bugs 2. Best-case scenario: Segmentation fault 3. Worst-case scenario: Doesn't crash, but wrong output

Backtrace: - First thing to do when getting a segfault - Understand what is the sequence of calls that brought us there

gdb backtrace

Breakpoints: - Stop the program during the execution at a designated point - At a line:

gdb break string-test.c.:13
Breakpoint 1 at 0x4005ef: file string-test.c, line 13.

- At a function:

gdb b main
Breakpoint 1 at 0x40059f: file string-test.c, line 8

- At a condition:

gdb b string-test.c:13 if i == 5

- Show info of all declared breakpoints:

gdb info b

Dealing with Breakpoints: - Once the program has stopped and the gdb shell is available, you can: 1. Continue the execution until the next breakpoint: gdb continue or gdb c 2. Execute only the next line of code and break again: gdb step or gdb s 3. Jump over function calls: gdb next or gdb n - Tip: Typing <enter> in the interactive gdb shell repeats the last command