ISO C Void * and Function Pointers

Question

While following some tutorials and reading about function pointers I learned that evidently assigning a void pointer to a function pointer in ISO C is undefined, is there any wa

Answer 1

I encountered this problem using glib. Glib data structures, such as GSList usually have a field called void *data. I wanted to store functions in a list and got a bunch of errors similar to this:

warning: ISO C forbids passing argument 2 of ‘g_slist_append’ between function pointer and ‘void *’ [-pedantic]

This example generates a bunch of warnings using gcc -Wall -ansi -pedantic

typedef int (* func) (int);

int mult2(int x)
{
    return x + x;
}

int main(int argc, char *argv[])
{
    GSList *functions = NULL;
    func f;

    functions = g_slist_append(functions, mult2);
    f = (func *) functions->data;
    printf("%d\n", f(10));
    return 0;
}

So I wrapped the function in a struct and all the warnings go away:

struct funcstruct {
    int (* func) (int);
};

int mult2(int x)
{
    return x + x;
}

int main(int argc, char *argv[])
{
    GSList *functions = NULL;
    struct funcstruct p;
    p.func = mult2;

    functions = g_slist_append(functions, &p);
    p = * (struct funcstruct *) functions->data;
    printf("%d\n", p.func(10));
    return 0;
}

It's arguable that this is quite a bit of extra code to make a few warnings disappear, but I don't like my code to generate warnings. Also, the above are toy examples. In the real code I'm writing, it turns out to be quite useful to wrap the list of functions in a struct.

I'd be interested to hear if this is problematic or if there's a better way of doing it.