Passing struct entries to string and vice-versa

Question

Let's say I have a struct like this:

typedef struct {
   unsigned s_i, s_j;
   unsigned d_i, d_j;
} obs_t

I want to define a function:

obs_t * string_to_struct (char * s);

Which reads a string formatted, for example, like this: "1 1 4 6" and passes the four number to the struct obs_t. So far I was not able of avoiding segmentation fault, i'm using the following code:

obstacle_t * string_to_obstacle (char * s){
obstacle_t *ostacolo = malloc(sizeof(obstacle_t));
unsigned int **format;
sscanf(s, "%u %u %u %u", format[0], format[1], format[2], format[3]);
ostacolo -> s_i = *format[0];
ostacolo -> s_j = *format[1];
ostacolo -> d_i = *format[2];
ostacolo -> d_j = *format[3];
}

Answer 1

You never initialize the pointer value format , so it has indeterminate value and dereferencing it has undefined behavior. With a pointer to pointer to int you'd have to allocate space for 4 pointers to int and point format at that, and then allocate space for 4 integers and set the pointers to point to those.

You also do not check the return values of malloc and sscanf, so you might be either dereferencing a NULL pointer, or assigning/leaving unspecified values to ostacolo's fields, if the sscanf fails. You also have no return statement in your non-void function.

Here's a version of string_to_obstacle that addresses these issues:

obstacle_t * string_to_obstacle (char * s)
{
        obstacle_t *ostacolo = malloc(sizeof *ostacolo);

        if (!ostacolo) {
                return NULL;
        }

        if (sscanf(s, "%u %u %u %u", &ostacolo->s_i, &ostacolo->s_j, &ostacolo->d_i, &ostacolo->d_j) < 4) {
                free(ostacolo);
                return NULL;
        }

        return ostacolo;
}

It's good to keep in mind that sscanf has no way to handle passing larger integers than fit in an unsigned integer in the string. The behavior is undefined .