how to create nested struct in c

Question

I have started to learn C and I am finding that creating complex data structures can be quite challenging!

Here is the background:

I have a created a struct in a header file foo.h and made its contents public:

struct frame {
     char *name;
     int width;
     int height;
     //other stuffs
}

extern const struct frame
     vid_1080p,
     vid_720p;

The instances of frame are constant and can be accessed from other c flies. foo.c look like this:

const struct frame vid_1080p = {
    .name                 = "1080p",
    .width                = 1920,
    .height               = 1080,
};
const struct frame vid_720p = {
    .name                 = "720p",
    .width                = 1280,
    .height               = 720,
};

I am want to create another struct within struct frame which elements will be calculated at startup of my program and will be possible to modify this if necessary. I am not sure how to approach this, I have tried this approach below and it does not work.

My failed approach:

I have modified foo.h as such:

struct frame_calc {
     int ratio;
     //other stuffs
}

struct frame {
     char *name;
     int width;
     int height;
     //other stuffs
     struct frame_calc *calc;
}

And foo.c is also modified:

 const struct frame vid_1080p = {
        .name                 = "1080p",
        .width                = 1920,
        .height               = 1080,
        .calc                 =  malloc(sizeof(struct frame_calc)) //compiler complains here
    };
    const struct frame 720p = {
        .name                 = "720p",
        .width                = 1280,
        .height               = 720,
        .calc                 =  malloc(sizeof(struct frame_calc))
    };

And then init() is called once at the beginning of my program and it fills out the calc structure:

void init(void)
{
     vid_1080p.calc.ratio = vid_1080p.height / vid_1080p.width;
     vid_720p.calc.ratio  = vid_720p.height  / vid_720p.width;
}

This approach gives me a few compiler errors. I am also not sure how to initialize my nested struct appropriately. Another concern is, I am using malloc , that means I will need to free this at the right places. I would like to avoid this. I am sure all the pro c programmers out there know how to tackle this better!

Last question, how do I access this ratio member for vid_1080p instance from other c files? I am thinking vid_1080p->frame->calc->ratio .

Hopefully, I have managed to explain what I want to do? If not, I would appreciate constructive criticism on how to modify this question better in StackOverflow, given that it is my first question!

Answer 1

You don't need to malloc the calc member because an actual instance is embedded - it is not a pointer.

If for some reason you need it to be a pointer then you need:

struct frame {
     ...
     struct frame_calc* calc;
}

And access would be var.calc->ratio = something;

If you are trying to modify the struct after creation (via init() ) why are the structs const ? Were you trying to get around the const struct issue by making the struct hold a pointer so you wouldn't have to change the pointer but could change the value it points to?

I'd suggest not using const structs:

struct frame vid_1080p {
    ...
}

Your init function can then do vid_1080p.calc.ratio = vid_1080p.height / vid_1080p.width; If you really want to enforce constness access the structs via a pointer to a const struct. const frame *p_1080p .