How to declare an opaque structure pointer interface

Question

I'm trying to architect some C code such that the .c file and private .h file define a rather complex structure, but a public facing header file only requires pointers to that structure type. What I tried was this:

publicRadio.h

typedef struct _radio Radio;
void radioReceive(Radio *radio, ....);

privateRadio.h

#include <publicRadio.h>
struct _radio {
    ...
    ...
}

radio.c

#include <publicRadio.h>
#include <privateRadio.h>

void radioReceive(Radio *radio, ....)
{
    ...
}

When I try to compile this though, I get something like

radio.c:91:9: error: parameter 'radio' has just a forward declaration
radio.c:90:6: error: conflicting types for 'radioReceive'
publicRadio.h:29:6: note: previous declaration of 'radioReceive' was here

I'm trying to emulate the pattern I've seen in other libraries that have public facing APIs that expose their structures through functions, but hide the gory details. What is the recipe one uses to accomplish that?

Answer 1

This pattern is normally implemented like this:

radio.h

#ifndef RADIO_H
#define RADIO_H

typedef struct _radio Radio;
void radioReceive(Radio *radio, ....);

#endif

radio.c

#include "radio.h"

struct _radio {
    // _radio struct implementation
};

void radioReceive(Radio *radio, ....) {
    // radioReceive implementation
}

main.c

#include "radio.h"
.... // use Radio, radioReceive, etc.

Answer 2

OK: The following does work on both MSVC and GCC without problems.

The two important errors were:

• Missing semi-colon in privateRadio.h
• Including publicRadio.h in radio.c causes double inclusion. It is already included in privateRadio.h

Here is the code I used:

publicRadio.h

    typedef struct _radio Radio;
    void radioReceive(Radio *radio, ....);

privateRadio.h

    #include "publicRadio.h"

    struct _radio {
        ...
        ...
    };

radio.c

    #include "privateRadio.h"

    void radioReceive(Radio *radio, ....)
    {
        ...
    }

main.c

    #include "publicRadio.h"

    void main()
    {
        Radio* radio = ...;
        radioReceive(radio, ....);
    }