Can someone explain me this C structure syntax?

Question

I found on this official gnu gcc website that they initialize structures like that:

struct f1 {
  int x; int y[];
} f1 = { 1, { 2, 3, 4 } };

struct f2 {
  struct f1 f1; int data[3];
} f2 = { { 1 }, { 2, 3, 4 } };

At first I thought it was a default initialization for the struct, but I tested it and it doesn't auto-initialize the struct when declared, so what's the point of using this (I compiled my program with gcc of course).

The code I tried:

#include <stdio.h>

struct a{
int x;
int y;
} a = {42, 42};

int main(void)
{
  struct a foo;

  printf("%d\n%d\n", foo.x, foo.y);
  return (0);
}

And it outputs random uninitialized data instead of

42
42

Answer 1

Your confusion seems to stem from the fact that you do not understand the syntax.

struct f1 {
  int x; int y[];
} f1 = { 1, { 2, 3, 4 } };

defines a variable named f1 with it's members initialized.

In your example, you also have a variable a initialized in the global scope, but you also have the same variable a in main scope, which is left uninitialized.

Answer 2

I misunderstood, this is equivalent to struct f1 f1 = { 1, {2, 3, 4}}; , the fact that they used the same name f1 made me think that this was a new syntax.