Malloc 'ing array of struct in struct

Question

I've been doing research for a while, and I'm not finding anything that helps me.

I have the following struct declarations:

typedef struct position_struct{
    int x;
    int y;
} pos;

typedef struct item_struct{
    char member1;
    pos member2;
} item;

typedef struct room_stuct{
    item * member3;
    pos * member4;
    pos member5;
} roomLayout;

And the code to try to malloc it is (I removed the error checking for brevity):

 roomLayout *genFloor () {
                                                         // Allocate mem for Arrays of:
    roomLayout * room = malloc(sizeof(roomLayout) * 6 ); //     6 rooms
    room->member3     = malloc(sizeof(item)       * 10); //     10 member3's
    room->member4     = malloc(sizeof(pos)        * 10); //     10 member4's

    /* TESTING */
    room[0].member3[0].member1 = 'd';
    printf("Room[0] is good\n");
    room[1].member3[0].member1 = 'd';
    printf("Room[1] is good\n");           // Never prints/makes it to this line


    return room;
 }

When I try this, assigning to room[1] causes a crash, but not room[0]. My guess is that I havent actually allocated enough space for the whole array and only one spot. But I don't understand why as I believe that I'm following what I see everywhere else.

If someone could please explain to me the procedure for allocating memory for this kind of setup, that would be very helpful! Thank you.

Answer 1

You are only allocating member3 and member4 for the first room; not for all of them. In the line room->member3, room is at the beginning of the memory segment, or at room[0], which allocates space for 10 member3s. room[1], however, has not allocated any space for any member3s (similarly for doors). Work around this by allocating in a loop:

for(i = 0; i < 6; ++i)
{
    room[i].member3 = malloc(sizeof(room[i].member3) * 10);
    room[i].member4 = malloc(sizeof(room[i].member4) * 10);
}

When free-ing the memory, remember to use a similar loop, i.e.

for(i = 0; i < 6; ++i)
{
    free(room[i].member3);
    free(room[i].member4);
}

free(room);