Does union support flexible array members?

Question

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

#include  

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

int m         


        

Answer 1

int a[] is the C standard notation (since C99).

int a[0] is GNU C syntax, which predates C99. Other compilers might also support it, I don't know.

Your compiler seems to default to C90 standard with GNU extensions, which is why latter compiles, but first one does.

Furthermore, as stated in Lundin's answer, standard C does not support flexible array members in union at all.

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Try adding -std=c99 or -std=c11 to your compiler options (gcc docs here).

Also -pedantic or -pedantic-errors is probably a good idea too, it'll enforce more strict standard compliance.

And, obligatory aside, -Wall -Werror won't hurt either...