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

I was wondering about this too. Turns out that the struct NOTHING_LIKE_xyz * z is forward declaring struct NOTHING_LIKE_xyz. As a convoluted example,

typedef struct {
    struct foo * bar;
    int j;
} foo;

struct foo {
    int i;
};

void foobar(foo * f)
{
    f->bar->i;
    f->bar->j;
}

Here f->bar refers to the type struct foo, not typedef struct { ... } foo. The first line will compile fine, but the second will give an error. Not much use for a linked list implementation then.