Pointer to a structure that has not been declared

Question

Possible Duplicate:
undefined C struct forward declaration

How is it possible to declare a pointer to structure even when I do not declare a structure?

#include<stdio.h>

int main(){
    struct s{
     struct p *ptr;
   }; 
}

Why does the above compile successfully?

http://stackoverflow.com/questions/326205/what-is-forward-reference-in-c — Mitch Wheat, Feb 05 '12 at 09:43

score 10 · Accepted Answer · answered Feb 05 '12 at 09:41

10

It's possible because the compiler doesn't need to know anything about the structure if it only deals with a pointer to it.

This is a commonly used technique and is usually called an “opaque pointer”.

answered Feb 05 '12 at 09:41

dreamlax

can you please link me to a detailed explanation ? – Bazooka Feb 05 '12 at 09:42
2

@Parminder There's a section called "Self-referential Structures" in *The C Programming Language*. – cnicutar Feb 05 '12 at 09:43
3

Effectively, the `struct p` line _is_ a declaration. – J. C. Salomon Feb 05 '12 at 09:48
@dreamlax: I think good explanation by you... But a new thing I learnt today is Opaque Pointer... Anyhow Thanks – Rasmi Ranjan Nayak Feb 05 '12 at 10:13
@cnicutar: I think the above code is not a self referential structure.. – Rasmi Ranjan Nayak Feb 05 '12 at 10:15
1

@RasmiRanjanNayak True but that section explains why this is possible (forward declaration stuff). – cnicutar Feb 05 '12 at 10:17

score 0 · Answer 2 · answered Feb 05 '12 at 10:20

0

Look into the below link

answered Feb 05 '12 at 10:20

2 Answers2