Redeclaring opaque struct in C file

Question

I'm working on a header file that declares some opaque struct s which is supposed to be defined in the corresponding C file. Here it is:

decl.h

#ifndef DECL_H
#define DECL_H

typedef struct test_t test;

#endif //

Some library that is supposed to be used in the implementation defines another opaque struct in its header lib.h :

//...
typedef struct _library_struct_t library_struct;
//...

Now in my decl.c file I want to make struct test_t to be the same (or compatible) with the library_struct . I tried this:

decl.c

//...
typedef library_struct test; //error: conflicting types for ‘test’
//...

But it does not compile. So the only way to go I can see now is

struct test_t{
    library_struct *lib_struct_ptr;
};

Is there shorter or more convenient way? Both test and library_struct are opaque. Why can't I make the test to be the same as library_struct ? Can macros be helpful here?

Answer 1

your code is equivalent to

typedef struct test_t test; /* from decl.h */
typedef library_struct test; /* in decl.c */

So you redefine test and of course the compiler doesn't accept that

I don't know what you expect to do through macros but redefinition is not allowed.

In the worst case you can hide the type of a pointer with a void * then casting to the type you (hope) have, but this is obviously dangerous because the compiler will follow you at your own risk .

The compiler does not check the types against you but to help you to see your errors at compile time ...