C Unknown type name 'my_structure'

Question

I have this code:

main.h

#ifndef MAINH
#define MAINH
...
#include "my_struct.h"
void some_func(my_structure *x);
...
#endif

and

my_struct.h

#ifndef UTILSH
#define UTILSH
...
#include "main.h"
...
typedef struct abcd {
    int a;
} my_structure;
...
#endif

but I'm getting this when I try to compile: error: unknown type name ‘my_structure’

Any idea why?

Answer 1

Because of how you've ordered your includes, the compiler sees void some_func(my_structure *x); before it sees typedef struct abcd { int a; } my_structure;.

Let's walk this through.

Assuming my_struct.h is processed first, we get the following sequence of events:

1. UTILSH is defined
2. MAINH is defined
3. Because UTILSH is already defined, we don't process my_struct.h again, so the typedef isn't processed
4. void some_func(my_structure *x); is processed.
5. Now the typedef is processed.

So after preprocessing, your compiler sees the following sequence of declarations:

...
void some_func(my_structure *x);
...
typedef struct abcd {...} my_structure;

Bad juju. You either need a forward declaration of my_structure in main.h, or you need to break that circular dependency (which is the much preferred option). Is there anything in main.h that my_structure.h actually uses? If so, you will want to factor it out into a separate file that both main.h and my_structure.h include.

Answer 2

You created a circular header inclusion. Circular inclusion never achieves anything. It is infinite. The #ifndef include guard will break the infinite inclusion circle at some unpredictable point (depending on which header is included into .c file first). This is what happened in your case. Basically, your circular inclusion got "resolved" into including main.h first and my_struct.h second. This is why main.h knows nothing about my_struct type.

Again, circular inclusion never achieves anything. Get rid of circular inclusion. Design your header structure hierarchically: lower-level headers included into higher-level headers, but never the other way around. In your case my_struct.h is probably a lower-level header, which means that you have to stop including main.h into my_struct.h. Redesign your headers so that my_struct.h no longer needs main.h.

Answer 3

The error message is coming from main.h while it's included in my_struct.h, before my_structure is defined. You should rethink your include paths since main.h and my_struct.h include each other.

You probably want your main.h file to just include my_struct.h, and not have my_struct.h to include anything. You're essentially instructing your C compiler to have an infinite co-include loop.