Question

 char **arr;
 arr = (char **)calloc(1,sizeof(char*));

 for(i = 0; i< 16; i++)
    if(arr[i] = (char *)calloc(1, 2*sizeof(char)) == NULL)
        perror("Memory cannot be allocated to arr[i]", %d);

The above code throws an error inside the for loop, when i am trying to allocate memory to arr[i]. Is anything wrong with this allocation. Essentially, i want to store 16 strings of length 2. I’ve tried it with array of pointers too (char *arr[16]). I have tried looking for resources on double pointer initializations using malloc() and calloc() and couldn’t find many . If you could point out some links, that would be greatly appreciated.
Thanks.

Answer 1

You need to allocate enough memory for 16 pointers, not just one.

arr = (char **)calloc(16, sizeof(char*));

What happens with your code is that arr has enough memory only for one pointer, so arr[0] = <something> is correct, but arr[1] and higher is touching memory that doesn’t belong to the program.

Additionally, the way you assign the string pointers is wrong. You are assigning 0 or 1 values, depending on whether the result if calloc is NULL. You need to add parentheses there:

if ((arr[i] = (char *)calloc(1, 2*sizeof(char))) == NULL)
    perror("Memory cannot be allocated to arr[%d]", i);

Er even better:

for(i = 0; i < 16; i++) {
    arr[i] = (char *)calloc(1, 2*sizeof(char));
    if (arr[i] == NULL) {
        perror("Memory cannot be allocated to arr[%d]", i);
    }
}