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help #include

Anh Phan 

a.h:
struct A {
   int a;
};

b.h:
struct B {
   struct A ba;
};

c.c:
#include "a.h"
#include "b.h"

</prev>

compiler complains struct A in file b.h is undefined.  I would think that by include a.h before b.h in c.c, it would see the struct A definition?

I know I could make it compile by adding:
struct A; in b.h but just want to know why it doesn't work the way above.

Thanks,

Anh

Parents
  • 0

    Your b.h is buggy. A header using data types defined in another one
    has to include that other header, i.e. b.h should read:

    #ifndef MY_B_H
#define MY_B_H

#include "a.h"
struct B {
  struct A ab;
};
#endif /* MY_B_H */

    (The #ifndef/#define stuff is standard practice to make headers safe for multiple inclusion.)

    As to the compiler error you got: that shouldn't happen unless you tried to compile b.h on its
    own --- e.g. by adding it to your uVision project as a source file to be compiled,
    which you really shouldn't do.

Reply
  • 0

    Your b.h is buggy. A header using data types defined in another one
    has to include that other header, i.e. b.h should read:

    #ifndef MY_B_H
#define MY_B_H

#include "a.h"
struct B {
  struct A ab;
};
#endif /* MY_B_H */

    (The #ifndef/#define stuff is standard practice to make headers safe for multiple inclusion.)

    As to the compiler error you got: that shouldn't happen unless you tried to compile b.h on its
    own --- e.g. by adding it to your uVision project as a source file to be compiled,
    which you really shouldn't do.

Children
  • 0 in reply to HansBernhard Broeker

    "A header using data types defined in another one has to include that other header"

    But in the example he posted, Anh had included both the headers:

    c.c:
#include "a.h"
#include "b.h"
    So, although what you say may be good practice in general, the code as posted should've been OK?

    "unless you tried to compile b.h on its own --- e.g. by adding it to your uVision project as a source file to be compiled -- which you really shouldn't do."

    Quite so.

  • 0 in reply to Andy Neil

    Well, I just tried

    #include <stdio.h>

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

void main( void )
{
	printf( "Hello, world!\n" );
}
    and it compiled without error!

  • 0 in reply to Andy Neil

    But in the example he posted, Anh had included both the headers:

    ... in the .c file. That's not solving the problem, it's just avoiding the symptoms. File b.h, as-is, is incomplete. You can't just put

    #include "b.h"
    in any given C source file --- you have to remember to
    #include "a.h"
    first. That's a deficiency of b.h. It may not seem too bad in this simple case, but it'll bite back as soon as you try that same tactics in larger projects.

  • 0 in reply to HansBernhard Broeker

    Thanks guys,

    I think I forgot to save the changes to "a.h". It works now.... "a.h" contains a lot of things including one struct needed for "b.h". I am more inclined to include headers from other development groups in .c rather than .h (person reasons).

    Anh