Opaque Structure Pointer

Question

In my library, I have an instance structure, which contains everything needed for the library, this is so you can define multiple instances of the library. The library requires the user to define their own extension, or custom variables.

This is what I tried:

Library.h

typedef struct custom_s *custom;

typedef struct {
    int a;
    int b;
    custom customs;
} instance;

And then the user can just do:

Main.c

// User sets their own custom structure
struct custom_s {
    int c;
};

int main(void) {
    instance test;
    test.customs.c = 1;
}

However I get the error of "Segmentation fault".

Answer 1

Shouldn't it be:

test.customs->c = 1

Since you type'd it in

typedef struct custom_s *custom;

and Used as

custom in the instance structure.

Which is never allocated...

Answer 2

typedef struct custom_s *custom;

Defines a pointer to a custom struct. In your example this is an undefined pointer that is never allocated, so a segmentation fault occurs when you try to access it.

One side effect of opaque structures is that the size is unknown to client code. This means that you must create your own functions for allocating/creating them.

Make something like:

instance test;
test.customs = customs_create();
test.customs.c = 1;