Question

I had a similar question here about allocating and initializing one pointer to struct in a subfunction. Unfortunately I can’t extend the good solution I got there to initialize an array of structs.
The first element is OK but the second (and all following) elements are zero/NULL.

Here is a commented example. Maybe someone can help me…

#include <stdio.h>
#include <stdlib.h>

typedef struct {int n;} mystruct;

void alloc_and_init_array(mystruct **s)
{
    // create an array containing two elements
    *s = calloc(sizeof(mystruct), 2);
    (*s[0]).n = 100;
    (*s[1]).n = 200;
}

int main(void)
{
    mystruct *s; // only a pointer. No memory allocation.
    alloc_and_init_array(&s);

    printf("1st element: %d\n", s[0].n); // here I get 100, that's OK
    printf("2nd element: %d\n", s[1].n); // here I get 0. Why?

    return 0;
}

Answer 1

You need some parentheses:

((*s)[1]).n = 200;
 ^ extra parentheses required

The subscript ([]) has higher precedence than the indirection (*) so without parentheses it is applied first.

You need to dereference s to get the array pointed to by it, then access the element at index one.