Debugging with gdb

Our Demo Code:

#include <stdio.h>
#include <stdint.h>

int g_myvar = 42;
#define MYCONST 99

void sideeffect(void){
    g_myvar += MYCONST;
}

uint8_t myfunction(uint8_t x){
    return x+42;
}

int main(void){

    uint8_t foo = 23;
    uint8_t bla = 23;

    uint8_t x = foo + bla;
    uint8_t i;

    uint8_t y = myfunction(bla);

    uint8_t * verybad = 0;

    printf("%d\n", foo);
    printf("%d\n", y);
    // use this printf to cause a segfault
    // printf("%d\n", *verybad);

    for(i = 0; i < 10; ++i){
        y = myfunction(y);
        sideeffect();
    }
    printf("%d\n", y);


    return 0;
}

Resources

• Vorlesungsfolien Betriebssysteme UE auf der TU Wien: Debugging
• Online Tutorial auf beej.us

Basics

• gcc -g test.c -o test
• gdb test
• gdb test core
• gdb --args

Commands in GDB

• break
• continue
• next
• info breakpoints
• watch
• list
• print
• bt full
• catch signal
• run
• break .. if ...
• delete
• disable
• enable
• step
• stepi
• target record-full
• reverse-next
• set
• quit

Show preprocessor macros

See Thread on SO

gcc -g3 test.c
gdb a.out
(gdb) info macro MYCONST
(gdb) macro expand MYCONST

Fun with print

• print myfunction(y+23/7)/23.0
• print printf("%d\n", 23 > 42 ? 111 : 222)

Enable history

create a file .gdbinit in ~ and put this line in:

set history save on