Question

I’m writing a unit test framework (see SO for more details). Or view the code at GitHub.

Safer Code describes a way to pass functions of arbitrary types.

But how do I call such a function without knowing its types beforehand? Assume f needs no input, so f() should work on its own.

Let’s say I want to populate an array using an arbitrary generator function.

void* gen_array(fp gen, size_t size) {
    int i, len = gen_int() % 100;

    void* arr = GC_MALLOC(len * size);

    for (i = 0; i < len; i++) {
        arr[i] = gen(NULL);
    }

    return arr;
}

It should look something like this, but I get compiler errors:

gcc -o example example.c qc.c qc.h -lgc
In file included from example.c:1:
qc.h:21: error: expected declaration specifiers or ‘...’ before ‘size_t’
In file included from qc.c:1:
qc.h:21: error: expected declaration specifiers or ‘...’ before ‘size_t’
qc.c:23: error: conflicting types for ‘gen_array’
qc.h:21: error: previous declaration of ‘gen_array’ was here
qc.c: In function ‘gen_array’:
qc.c:29: warning: dereferencing ‘void *’ pointer
qc.c:29: error: too many arguments to function ‘gen’
qc.c:29: error: invalid use of void expression
qc.h:21: error: expected declaration specifiers or ‘...’ before ‘size_t’
make: *** [example] Error 1

Answer 1

After thinking about some more I realize your problem your above code would never work.
You are first calling trying to call a void function with no parameters with the parameter NULL. Next you would need your code to be more generic. I placed an example below of what I mean. Now using a global variable

#include <stdio.h>
#include <stdlib.h>

typedef void (*fp)(void);


void * GEN_ARRAY_TEMP;

int gen_int() {
    return 67;
}

void* gen_array(fp gen, size_t size) {
    int i, len = gen_int() % 100;

    void* arr = malloc(len * size);
    void* arr_end = arr + len * size;
    GEN_ARRAY_TEMP = arr;

    while (GEN_ARRAY_TEMP <= arr_end) {
        gen();
        GEN_ARRAY_TEMP+=size;
    }
    return arr;
}

void make_int() {
    (*(int*)GEN_ARRAY_TEMP) = 9;
}

int main() {
    int i;
    int * gen_int_array = (int*) gen_array(make_int, sizeof(int));
    for(i=0;i<67;i++) {
        printf("%d\n",gen_int_array[i]);
    }
}