Does union support flexible array members?

I have declared a flexible array member in union, like this:

#include  <stdio.h>

union ut
{
    int i;
    int a[]; // flexible array member
};

int main(void)
{
    union ut s;
    return 0;
}

and compiler gives an error :

source_file.c:8:9: error: flexible array member in union
     int a[];

But, Declared array zero size like this :

union ut
{
    int i;
    int a[0]; // Zero length array
};

And it's working fine.

Why does zero length array work fine union?

No, unions do not support flexible array members, only structs. C11 6.7.2.1 §18

As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member.

In addition, zero-length arrays is not valid C, that's a gcc non-standard extension. The reason why you get this to work is because your compiler, gcc, is configured to compile code for the "non-standard GNU language". If you would rather have it compile code for the C programming language, you need to add compiler options -std=c11 -pedantic-errors.