extern doesnt go well with structs?

[someprogr]

Consider the following:

myfile.c

#include <stdio.h>

extern struct mystruct ab;

int main()
{
     ab.a = 'a';
}

myfile2.c

#include <stdio.h>

struct mystruct
{
      char a;
      char b;
} ab;


....

c> gcc myfile.c myfile2.c

When I do this I get the error:
invalid use of undefined type 'struct mystruct'

Any help would be appreciated thanks.

[bboozzoo]

how is the code of myfile supposed to know the how mystruct looks like?

[hk_mp5kpdw]

You need a header that has the definition of struct mystruct. The two source files should then include this header.

[someprogr]

how is the code of myfile supposed to know the how mystruct looks like?

if I change myfile.c to:

#include <stdio.h>

extern struct mystruct
{
      char a;
      char b;
} ab;

int main()
{
     ab.a = 'a';
}

it still wont work though.

[hk_mp5kpdw]

The general way to do this is the following:

myfile.c

#include <stdio.h>
#include "myfile.h"

extern struct mystruct ab;

int main()
{
     ab.a = 'a';
}

myfile2.c

#include <stdio.h>
#include "myfile.h"

struct mystruct ab;

myfile.h

struct mystruct
{
      char a;
      char b;
};