C99: Flexible array inside union?

Question

I tried to convert something from using the struct hack to using a flexible array member, only to run into the following error message:

error: invalid use of structure with flexible array member

(GCC 4.8.1, gnu99, MinGW)

After trying to track down the cause of the message, I distilled it down to the following relatively-minimal case:

struct a {
    union {
        struct {
            int b;
            int c[];
        } d;
    } e;
};

In other words, a struct with a flexible array member doesn't see to be able to be put inside a union in a struct, even if the union is the last member of the struct.

(Note that putting a flexible array member directly inside the union does seem to work.)

Now: is there any good way to work around this besides reverting back to the struct hack (declaring c as an array of length 1)? A pointer to a struct inside the union would work, but suffers an additional layer of indirection.

Answer 1

The C11 standard (ISO/IEC 9899:2011) says:

§6.7.2.1 Structure and union specifiers

¶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 the example you have, the last member of struct a is not a flexible array member. It is a union containing a struct that has a flexible array member.

You have to work quite hard to get gcc to complain, though; it requires -pedantic amongst the compiler options.