a struct with a flexible array member in another struct

Question

Is something like the code below valid?

struct foo { int a; int b[]; }; struct bar { int c; struct foo d; }; struct bar *x = malloc(sizeof(struct bar) + sizeof(int [128]));

It seems ok to me, but I am a bit skeptical because compiler does not complain if I do:

struct bar { struct foo d; int c; };

@Shahbaz: No, int b[] is not the same as int *b. This construct is called a flexible array member. c-faq.com/struct/structhack.html — jamesdlin
– jamesdlin, Commented May 10, 2012 at 21:10
@jamesdlin, didn't know that! It seems like a terrible idea, though. How could they make it standard? — Shahbaz
– Shahbaz, Commented May 10, 2012 at 21:43
@Shahbaz It's not a terrible idea, it's just dangerous if people use it wrongly. It was a common hack to have arrays of length 1 as last members of structs for that purpose before it became standard. gcc had long accepted 0-length arrays as flexible array members. — Daniel Fischer
– Daniel Fischer, Commented May 10, 2012 at 21:47

Daniel Fischer · Accepted Answer · 2012-05-10 21:04:47Z

It's not okay. Section 6.7.2.1 (in n1570), point 3 says

3 A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type; such a structure (and any union containing, possibly recursively, a member that is such a structure) shall not be a member of a structure or an element of an array.

So a struct with a flexible array member may not be part of another struct.

(It may well work as the last member of a struct, though, if the compiler accepts it.)

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