Information Zero length arrays or Flexible array members

[justif]Zero length arrays are commonly used in system programming or otherwise if the size of an array cannot be determined prior to its usage and pointers may not be useful in that situation because of unknown memory space.[/justif]

Check out the code below.
Gcc extensions provide such capabilities, but c99 has made provision for FAMs
Important thing to understand about FAM is that sizeof() operator cannot be used on FAMs.

Code:

  1 #include <stdio.h>
  2 #include <string.h>
  3 #include <stdlib.h>
  4struct zero_len {
  5        int one;
  6        char two;
  7        unsigned int length;
  8        char empty[]; //<--- this here is Flexible array member
  9 };
10
11 int main (void)
12 {
13        struct zero_len *new_zero;
14        struct zero_len *old_zero;
15
16        new_zero = (struct zero_len *)malloc(sizeof (struct zero_len));
17        old_zero = malloc(sizeof(struct zero_len) + strlen("the new string") + 1); // add extra space
18        printf("%d %d %d %d\n", sizeof(*new_zero), sizeof(new_zero->one), sizeof(new_zero->two),
19                          sizeof(new_zero->length));
20        strcpy(old_zero->empty , "the new string"); // this is the assignment
21        puts(old_zero->empty);
22        printf("%d %d %d %d\n", sizeof(*old_zero), sizeof(old_zero->one), sizeof(old_zero->two),
23                          sizeof(old_zero->length));
24        return 0;
25 }

For more details on where this idea is used in reality:
Robert Love's answer to C (programming language): What is the advantage of using zero-length arrays in C? - Quora

This is GCC link about zero length arrays:
Zero Length - Using the GNU Compiler Collection (GCC)

 

It's not a hack, it's GCC extension and yes FAM were not implemented until C99. Beside at kernel level such things don't matter much.