CODEWITHSUNDEEP
cpointersstructsegmentation-fault
user1824918
user1824918

Reputation:

Array of pointer to structs printing

I have an array of pointers to structs:

item** items = NULL;

These are the functions I wrote for allocating and printing the array:

void allocateItems(item** items, int numItems) {
    items =malloc(numItems*sizeof(item*));
    for (int i = 0; i < numItems; i++) {
        items[i]=malloc(sizeof(item*));
        items[i]->data = i + 1;
        items[i]->data2 = (i + 1) % 2;
    }
}

void printItems(item** items, int numItems) {
    for (int i = 0; i < numItems; i++) {
        printf("%d : %d\n", items[i]->data, items[i]->data2);
    }
}

But when I try to print that array of structs, I get a segmentation fault. Can someone tell me where the mistake is?

Upvotes: 0

Views: 1046

Answers (1)

hmjd
hmjd

Reputation: 121971

This is only allocating enough memory for an item*:

items[i]=malloc(sizeof(item*));

it must allocate memoryfor an item:

items[i]=malloc(sizeof(item));

In order for the changes made with allocateItems() to be visible to the caller then address of items must be passed in:

allocateItems(&items, 4);

which the changes the argument type and means items must be dereferenced within the function:

void allocateItems(item*** items, int numItems)
{
    *items = malloc(numItem * sizeof(item*));
    for (int i = 0; i < numItems; i++)
    {
        (*items)[i] = malloc(sizeof(item));
        (*items)[i]->data = i + 1;
        (*items)[i]->data2 = (i + 1) % 2;
    }
}

This could be simplified by allocating an array of item instead of an array of item*. It would also mean that the list of items are in a contiguous block of memory instead of being fragmented with the multiple malloc() for allocating each element in the array:

void allocateItems(item** items, const int numItems)
{
    *items = malloc(numItems * sizeof(item));
    if (*items)
    {
        for (int i = 0; i < numItems; i++)
        {
            (*items)[i].data  = i + 1;
            (*items)[i].data2 = (i + 1) % 2;
        }
    }
}

item* items;
allocateItems(&items, 4);

if (items)
{
    for (int i = 0; i < 4; i++)
        printf("[%d]: (%d, %d)\n",
               i,
               items[i].data,
               items[i].data2);

    free(items);
}

Upvotes: 6

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