C struct hack

struct_hack.c

#include <stdlib.h>
#include <stddef.h>

/* 
 * C struct hack - the last member of the struct is of variable length
 *
 * C99 however introduces the concept of a flexible array member, which
 * allows the size of an array to be omitted if it is the last member
 * in a structure
 */

struct s {
    int  n;
    char a[1];
};

int
main(void)
{
    struct s *p;
    int i, n = 100;
    size_t size;

    size = offsetof(struct s, a) + n * sizeof(p->a[0]);
    /* size = sizeof(*p) - 1 + n * sizeof(p->a[0]); */

    p = malloc(size);
    p->n = n;

    for (i = 0; i < n; i++) {
        p->a[i] = 'A' + (i % 26);
    }

    return 0;
}