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.

Stop treating Array as Pointer. You never allocate memory for an Array not even for a Pointer. You allocate a memory block an you use a Pointer to point there. Please explain me how does your "Array" pointing there ? — Michi
– Michi, Commented Jan 29, 2016 at 17:15

R_Kapp · Accepted Answer · 2016-02-18 15:39:25Z

5

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);

edited Feb 18, 2016 at 15:39

answered Jan 29, 2016 at 16:45

R_Kapp

2,8793 gold badges21 silver badges34 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Nawar Ismail Over a year ago

Thats what I figured, I just didn't know how to fix it. Thanks! I couldn't find anyone doing a for loop anywhere!

LPs · Accepted Answer · 2016-01-29 16:49:02Z

2

room->groundItems = malloc(sizeof(item) * 10);

The code above allocate a 10 items for only for room[0].

room[1 to 5] are not allocated and access them is UB.

You must allocate all your pointers before to access them, like showed below

roomLayout * room = malloc(sizeof(roomLayout) * 6 )
for (int i=0; i<6; i++)
{
    room[i].groundItems = malloc(sizeof(item) * 10); //     10 ground items
    room[i].doors       = malloc(sizeof(pos)  * 10); //     10 doors
}

edited Jan 29, 2016 at 16:49

answered Jan 29, 2016 at 16:43

LPs

16.2k8 gold badges34 silver badges64 bronze badges

Comments

Rabbid76 · Accepted Answer · 2016-01-29 16:52:28Z

1

You allocated groundItems and doors only for the first room, But you have to allocate it for all 6 rooms. Each room needs its own list of groundItems and doors. Adapt your code like this:

roomLayout * room = malloc(sizeof(roomLayout) * 6 ); //     6 rooms
for ( int i = 0; i < 6; i ++ )
{
    room[i].groundItems = malloc(sizeof(item)       * 10); //     10 ground items
    room[i].doors       = malloc(sizeof(pos)        * 10); //     10 doors
}

Note room->groundItems is similar to room[0].groundItems. So in your case room[0].groundItems[0].objectID worked, but room[1].groundItems[0].objectID failed, because room[1].groundItems was never initialized.

edited Jan 29, 2016 at 16:52

answered Jan 29, 2016 at 16:45

Rabbid76

212k30 gold badges163 silver badges205 bronze badges

Comments

Aviv · Accepted Answer · 2016-01-29 17:22:50Z

0

You are missing allocation to room[1], room[2] and so on. by room->groundItems = malloc... you implcitly say room[0].groundItems = malloc ....

Maybe the allocation of six places of roomLayout confused you. It allocated the pointers to groundItems and doors arrays, but not the space they required themselves.

edited Jan 29, 2016 at 17:22

answered Jan 29, 2016 at 16:54

Aviv

4244 silver badges18 bronze badges

Collectives™ on Stack Overflow

Malloc 'ing array of struct in struct

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related