How is it legal to reference an undefined type inside a structure?

Question

As part of answering another question, I came across a piece of code like this, which gcc compiles without complaint.

typedef struct {
    struct xyz *z;
} x         


        

Answer 1

When a variable or field of a structure type is declared, the compiler has to allocate enough bytes to hold that structure. Since the structure may require one byte, or it may require thousands, there's no way for the compiler to know how much space it needs to allocate. Some languages use multi-pass compilers which would be able find out the size of the structure on one pass and allocate the space for it on a later pass; since C was designed to allow for single-pass compilation, however, that isn't possible. Thus, C forbids the declaration of variables or fields of incomplete structure types.

On the other hand, when a variable or field of a pointer-to-structure type is declared, the compiler has to allocate enough bytes to hold a pointer to the structure. Regardless of whether the structure takes one byte or a million, the pointer will always require the same amount of space. Effectively, the compiler can tread the pointer to the incomplete type as a void* until it gets more information about its type, and then treat it as a pointer to the appropriate type once it finds out more about it. The incomplete-type pointer isn't quite analogous to void*, in that one can do things with void* that one can't do with incomplete types (e.g. if p1 is a pointer to struct s1, and p2 is a pointer to struct s2, one cannot assign p1 to p2) but one can't do anything with a pointer to an incomplete type that one could not do to void*. Basically, from the compiler's perspective, a pointer to an incomplete type is a pointer-sized blob of bytes. It can be copied to or from other similar pointer-sized blobs of bytes, but that's it. the compiler can generate code to do that without having to know what anything else is going to do with the pointer-sized blobs of bytes.