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I'm trying to figure out, how to use C headers with #ifndef and #include. Lets say I have these two header files:

headerA.h:

#ifndef HEADERA_H
#define HEADERA_H

#include "headerB.h"

typedef int MyInt;
TFoo foo;
... some other structures from headerB.h ...

#endif

headerB.h

#ifndef HEADERB_H
#define HEADERB_H

#include "headerA.h"

typedef struct foo{
    MyInt x;
} TFoo;

#endif

headerA.c

#include "headerA.h"

... some code ...

headerB.c

#include "headerB.h"

... some code ...

When compiling headerB.c, it says 

In file included from headerB.h,
                 from headerB.c:
headerA.h: error: unknown type name ‘MyInt’

I think, it's becouse when headerB.h is compiling, it defines HEADERB_H and then, when headerA.h wants to include headerB.h, the #ifndef HEADERA_H is false = skips including.

What is the best practice here? I just read, that best practice is to do all #include directives in header files, but in this situation it looks like a problem.

EDIT: Ok, sorry for misleading you. This is just and example from bigger project with more files.

Risinek
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2 Answers2

1

You have a circular dependency. Header file headerA.h depends on headerB.h which depends on headerA.h and so on and on.

You need to break that dependency, for example by not including headerB.h in headerA.h. It's not needed (nothing in headerA.h needs anything from headerB.h).

If you have to include headerB.h (as stated in your recent edit) then you first should reconsider how you use your header files, and what definition you place where. Perhaps move the definition of MyInt to headerB.h? Or have more header files, like one for type-aliases (like your MyInt which I personally see no use for), one for structures and one for variable declarations?

If that's not possible then you could try by changing the order of definitions and the include, like

#ifndef HEADERA_H
#define HEADERA_H

// Define type alias first, and other things needed by headerB.h
typedef int MyInt;

// Then include the header file needing the above definitions
#include "headerB.h"

TFoo foo;
... some other structures from headerB.h ...

#endif
Some programmer dude
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  • Thanks, that answered my question. So There is no simple solution than remaking HEADER files or breaking best practices and include headerA.h before headerB.h in headerB.c – Risinek Nov 16 '16 at 10:44
  • @Risinek Circular dependency is bad design. If you include something you derive something from there, there is no reason why the thing you include also needs something from you. – Kami Kaze Nov 16 '16 at 13:23
0

Just drop the line

#include "headerB.h"

from file headerA.h

Juan Chô
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  • thats not solution for my problem, becouse in headerA.h I need other structures and functions from headerA.h. I edited question – Risinek Nov 16 '16 at 10:25