Question

I have run gdb, as my code show segfault. I understand that the code that give the error is

210         if (colour_s->colorlist.colorlist_id == 1)
(gdb) print colour_s
$1 = (car_colour_list_t_slot *) 0x21
(gdb) print colorlist
$2 = (car_colour_list_t *) 0x12d1a80
(gdb) print colour_s->colorlist
Cannot access memory at address 0x21

typedef struct {
    int colorlist_id;
} car_colour_list_t;


typedef struct _car_colour_list_t_slot {
    car_colour_list_t colorlist;
    struct _car_colour_list_t_slot *next, *prev;
} car_colour_list_t_slot;


car_colour_list_t_slot *colour_s;
colour_s = (car_colour_list_t_slot *)malloc(sizeof(car_colour_list_t_slot));
car_colour_list_t *colorlist;
colorlist = (car_colour_list_t *)malloc(sizeof(car_colour_list_t));

I have already assigned those pointers memory, but when I tried to print colour_s->colorlist it fails (can not access memory at address 0x21)

Can anyone understand why this happens, although I have already assign memory to each poitner?

Thank you

Answer 1

The colour_s pointer is definitely invalid. There is no doubt about it. GDB is simply informing you of this fact. (Any pointer near 0x0 is invalid, and any pointer with a residue modulo 4 is invalid for your structure.) The malloc function will never return 0x21.

There are lots of ways your program can do this. They boil down to two basic types of errors in your program:

• colour_s was not initialized.
• colour_s was initialized, but it was accidentally overwritten by an error in a different part of your code.
• there is an error in GDB or your compiler (not worth considering)

Run your program with Valgrind or set a watchpoint on colour_s. These will help you discover where the error is in your code.

As an alternative exercise, see if you can find the error just by reading your code, without running it. Read carefully. This kind of exercise will make you a better programmer.