Is using flexible array members in C bad practice?

Question

I recently read that using flexible array members in C was poor software engineering practice. However, that statement was not backed by any argument. Is this an accepted fact?

(Flexible array members are a C feature introduced in C99 whereby one can declare the last element to be an array of unspecified size. For example: )

struct header {
    size_t len;
    unsigned char data[];
};

Answer 1

It is an accepted "fact" that using goto is poor software engineering practice. That doesn't make it true. There are times when goto is useful, particularly when handling cleanup and when porting from assembler.

Flexible array members strike me as having one main use, off the top of my head, which is mapping legacy data formats like window template formats on RiscOS. They would have been supremely useful for this about 15 years ago, and I'm sure there are still people out there dealing with such things who would find them useful.

If using flexible array members is bad practice, then I suggest that we all go tell the authors of the C99 spec this. I suspect they might have a different answer.