Question

The following code causes a SIGSEGV, but only while debugging.

#include <stdio.h>
#include <stdlib.h>

typedef struct enemy_desc
{
int type;
int x;
int y;
}enemy;

int main()
{
    enemy **enemies;
    enemies=(enemy **)malloc(sizeof(enemy *)*16);

    enemies[0]->type=23;

    printf("%i",enemies[0]->type);
    return 0;
}

Answer 1

You have allocated memory for 16 enemy * pointers, but you have not allocated room for the 16 enemy structs themselves. There are two ways to fix this. One is to add a loop that allocates each of the 16 enemy structs one by one:

int main()
{
    enemy **enemies;
    int i;

    enemies = (enemy **) malloc(sizeof(enemy *) * 16);

    for (i = 0; i < 16; ++i) {
        enemies[i] = (enemy *) malloc(sizeof(enemy));
    }

    enemies[0]->type = 23;

    printf("%i",enemies[0]->type);
    return 0;
}

The other is to remove one level of indirection. If you declare enemy *enemies then you can allocate the 16 structs at once and forgo a loop. If there’s no need for the double indirection this would be my preferred solution:

int main()
{
    enemy *enemies;
    enemies = (enemy *) malloc(sizeof(enemy) * 16);

    enemies[0].type=23;

    printf("%i",enemies[0].type);
    return 0;
}

Notice that the -> operator switches to ..