redefinition of typedef

Question

I am possibly doing this incorrectly and this is much a question about why it works in one compiler and not the other.

I have a large C application, and I am

Answer 1

As others have already stated, you cannot redefine types in C. This error mostly indicates that there might be looped includes or other logic flaw. To avoid this, the best practice is to lock include files i.e.

#ifndef __HEADER_H__
#define __HEADER_H__

// Your code goes here

#endif

This way unnecessary includes will be omitted by this lock.
In your example you would need to include B in A and A in C. Including B in C would have no effect and would satisfy the compiler

Answer 2

Just as a matter of style I would put the typedef after the struct. i.e.:

struct B_ {
    int c;
};
typedef struct B_ B;

That way you are saying: "here is B_ and now I want to refer to it as B". It could be that the other way around fools something in the compiler.

Answer 3

One piece of the idiom is missing. The forward declarations are independent from the definitions, so they should be in a separate header file.

// a_fwd.h

#ifndef A_FWD_H
#define A_FWD_H

typedef struct A_ A;

#endif

// a.h

#ifndef A_H
#define A_H

#include "a_fwd.h"

struct A_ {
};

#endif

Now it's always safe to include any headers in any order.

---

It is illegal to have two definitions of anything. A typedef is a definition, not just a declaration, so the one compiler was being quite lax to allow the redundancy.

Answer 4

You are defining multiple times the same thing.

You can spread it across multiple header files just have to make sure that there is some B seen before struct _A is defined.

This code works:

#include <stdio.h>

typedef struct _B B;
typedef struct _A A;

struct _A {
    double a;
    B *b;
};

struct _B {
    int c;
};

void function_do_something(A* a, B* b)
{
    printf("a->a (%f) b->c (%d)\n", a->a, b->c);
}

int main()
{
   A a;
   B b;

  a.a = 3.4;
  b.c = 34;

  function_do_something(&a, &b);

  return 0;
}

Output:

> ./x
a->a (3.400000) b->c (34)

EDIT: updated for C

EDIT 2: spread into multiple header files

b.h:

#ifndef B_H
#define B_H

struct _B {
    int c;
};

#endif

a.h:

#ifndef A_H
#define A_H

typedef struct _B B;

struct _A {
    double a;
    B *b;
};

typedef struct _A A;

#endif

main.c:

#include <stdio.h>

#include "a.h"
#include "b.h"

void function_do_something(A* a, B* b)
{
    printf("a->a (%f) b->c (%d)\n", a->a, b->c);
}

int main()
{
   A a;
   B b;

  a.a = 3.4;
  b.c = 34; 

  function_do_something(&a, &b);

  return 0;
}

Answer 5

I was surprised by this because I'm fairly sure that redeclaring the same typedef in the same scope is legal in C++, but apparently it is not legal in C prior to the 2011 standard.

First, typedef names have no linkage:

ISO/IEC 9899:1999 + TC3 6.2.6/6:

The following identifiers have no linkage: an identifier declared to be anything other than an object or a function [...]

and 6.7/3:

If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a declarator or type specifier) with the same scope and in the same name space, except for tags as specified in 6.7.2.3.

So you need to ensure that each typedef declaration appears only once at file scope in each translation unit.

The 2011 C standard allows redeclaration of typedef names. 6.7 3 says:

… a typedef name may be redefined to denote the same type as it currently does, provided that type is not a variably modified type;…

Answer 6

You are redefining A and B by writing the same statement in several header files. One solution would be to drop the typedef of A and B from the A.h and B.h and use your C.h as it is.