Opaque datatypes and structures in c

Question

I have a very simple question. If I have a source code file like this:

#include<stdio.h>
#include"example.h"


struct mystructure{
    //Data Variables
    int a;
};

int main(){
   mystructure somename;
   somename.a = 1;
   
   printf("%d\n", somename.a);
    return 0;
}

With a header file like this:

#ifndef EXAMPLE_HEADER_H
#define EXAMPLE_HEADER_H

// Opaque declaration.
struct mystructure;
typedef struct mystructure mystructure;

#endif

The code will compile fine. I could also define the structure in the header file and it will compile fine.

However, if I define the structure in a different source code file and attempt to compile it with the main source code file it will keep throwing errors about forward declarations.

Is there a way to define a structure in a source code file give it a typedef in a header file and use it elsewhere in other source files?

I have been doing this awhile now by defining the structure in the header file, but I would like to know how to do this in a more opaque way.

The structure needs to be in the header. The only alternative is operating on opaque pointers by providing a bunch of functions. One that allocates an object and gives a pointer, another that destroys it, and functions to access every member that needs to be public, etc. — HolyBlackCat, Dec 31 '21 at 22:06
If more than one file needs access to the internals of the opaque type, then you can have a public header which declares the opaque type and a private header which defines the structure. Privileged code includes the private header; unprivileged code uses the facilities declared in the public header, which must provide whatever access general users need to the actual data in the opaque type. The private header should include the public header to ensure that things stay in sync — then the privileged code only includes the private header. — Jonathan Leffler, Dec 31 '21 at 23:36
This is rather interesting. I'll have to read more about that: there is a post on private headers [here](https://stackoverflow.com/a/958872/17606354) — NorthStar, Jan 01 '22 at 01:19

Vlad from Moscow · Accepted Answer · 2021-12-31T22:09:50.457

2

To perform this declaration and statement

mystructure somename;
somename.a = 1;

the compiler needs to know the complete definition of the structure struct mystructure. For instance it needs to know how much memory to allocate for the object somename and whether the structure has the data member a.

So the definition of the structure must be visible in the module with main where these declaration and statement are present.

To hide the structure definition you could declare a pointer of the type struct mystructure * and call functions to initialize this pointer and data members of the pointed object. The corresponding functions must know the structure definition.

edited Dec 31 '21 at 22:09

answered Dec 31 '21 at 22:04

Vlad from Moscow

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I guess I will keep defining the structures in the header file (I am writing applications that need to set the data members elsewhere in other source files). But at least I know now. However, your solution of using a pointer with special functions to access the members is very enlightening. – NorthStar Dec 31 '21 at 22:26

Opaque datatypes and structures in c

1 Answers1