Confusion with malloc. Creating an array of strings with 4 slots

Question

int main() {

  char *p[] = {"hello", "goodbye"};

  char **a;
  a = malloc(4 * 8);

}

I want a to have double the slots of p. How would I successfully do that without manually putting in numbers. All IK is the size of p should be there and x 2 for the double. How would I get the 8?

Answer 1

I think what you are asking is how to figure out how many strings have been put in p . To do this, you can count the number of elements in p :

char *p[] = { "hello", "goodbye" };
printf("%zu\n", sizeof(p) / sizeof(*p));

sizeof(p) is the total size of p

sizeof(*p) is the size of a single element of p

so dividing them gives you the number of elements in p

To malloc() a second array, twice as large :

int main() {

    char *p[] = { "hello", "goodbye", "lala" };

    size_t size = sizeof(p) / sizeof(*p);

    char **a;
    a = malloc(size * 2 * sizeof(*a));

}

Answer 2

Using the sizeof operator which returns you the size in bytes that a type/variable needs.

In your case

int main() {

  // because "hello" and "goodbye" are const char*
  const char *p[] = {"hello", "goodbye"};

  size_t len_p = sizeof p / sizeof p[0];

  char **a;
  a = malloc( 2 * len_p * sizeof *a );

  ...


  free(a);

}

sizeof p / sizeof p[0]; returns you the number of elements in the p array. sizeof p returns you the total amount of bytes, sizeof p[0] returns you the size of single element. Hence total size / size of element returns you the number of elements. But be aware that this method only works for arrays, not pointer. Consider this:

void add_one(int *ptr)
{
    size_t len = sizeof ptr / sizeof ptr[0];

    for(size_t i = 0; i < len; ++i)
        ptr[i] += 1;
}

int main(void)
{
    int arr[] = { 1,2 3 };

    add_one(arr);

    return 0;
}

is wrong, because ptr is pointer, not an array, so sizeof ptr returns you the size of a pointer, not the total amount of bytes needed by arr . That's why when you write a function that takes a pointer where you pass an array, the function should also take the size of the array, otherwise the size cannot be calculated, for example:

void add_one(int *ptr, size_t len)
{
    for(size_t i = 0; i < len; ++i)
        ptr[i] += 1;
}

int main(void)
{
    int arr[] = { 1,2 3 };

    add_one(arr, sizeof arr / sizeof arr[0]);

    return 0;
}

Answer 3

void allocarr(char** matrix, int bytes, int slots)
{
    int i = 0;
    while(i <= slots)
    {
        matrix[i] = (char*)calloc(1, bytes);
        ++i;
    }
}

...

char** s = malloc(4*sizeof(char*);
s = allocarr(s, 512, 4);