c - 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;
} xyz;
int main (void) {
    return 0;
}

This is the means I've always used to construct types that point to themselves (e.g., linked lists) but I've always thought you had to name the struct so you could use self-reference. In other words, you couldn't use xyz *z within the structure because the typedef is not yet complete at that point.

But this particular sample does not name the structure and it still compiles. I thought originally there was some black magic going on in the compiler that automatically translated the above code because the structure and typedef names were the same.

But this little beauty works as well:

typedef struct {
    struct NOTHING_LIKE_xyz *z;
} xyz;

What am I missing here? This seems a clear violation since there is no struct NOTHING_LIKE_xyz type defined anywhere.

When I change it from a pointer to an actual type, I get the expected error:

typedef struct {
    struct NOTHING_LIKE_xyz z;
} xyz;

qqq.c:2: error: field `z' has incomplete type

Also, when I remove the struct, I get an error (parse error before "NOTHING ...).

Is this allowed in ISO C?

---

Update: A struct NOSUCHTYPE *variable; also compiles so it's not just inside structures where it seems to be valid. I can't find anything in the c99 standard that allows this leniency for structure pointers.

Answer 1

As the warning says in the second case, struct NOTHING_LIKE_xyz is an incomplete type, like void or arrays of unknown size. An incomplete type can only appear in a struct as a type pointed to (C17 6.7.2.1:3), with an exception for arrays of unknown size that are allowed as the last member of a struct, making the struct itself an incomplete type in this case. The code that follows cannot dereference any pointer to an incomplete type (for good reason).

Incomplete types can offer some datatype encapsulation of sorts in C... The corresponding paragraph in http://www.ibm.com/developerworks/library/pa-ctypes1/ seems like a good explanation.