C callback function argument scope - c

Question

H.ello just a hobbyist here. I started again C after a little JS detour that taught me closures, oop and other stuff. I usually make small code snippets that I can reference or reuse later. So in the code below is there a reason why the compiler says counter is undeclared, as it is referenced as an argument of the callback function argument?
To make this work,
1. I could make counter a global variable, this works but it is not best practice.
2. I could move caller function inside main, but I'd like to avoid this.
3. pass counter as a 4th seperate pointer argument, however in that case, I don't really know with what voodoo should I invoke the caller function popArr within main.
Generally I'm interested in the technique how to use callbacks with arguments without using global variables or putting everything inside main.
So here is the code:
#include <stdio.h>
// CALLBACKS
/* Caller */
void popArr(int *arr, size_t length, int (*callback)(int *counter)) {
for (size_t i=0; i<length; i++) {
arr[i] = callback(&counter);
printf("arr[%zu] is: %d\n", i, arr[i]);
}
}
/* A simple generator callback saving counter. */
int generator(int *counter) {
*counter = *counter+1;
return *counter*5;
}
/* Main function. */
int main(void) {
int counter = 1;
int arr[10];
popArr(arr, 10, &generator);
return 0;
}

Answer 1

So in the code below is there a reason why the compiler says counter
is undeclared, as it is referenced as an argument of the callback
function argument?
I presume you mean in function popArr(). Yes, of course there's a reason. In popArr(), the symbol counter appears only in the prototype for the callback function, and the scope of that appearance is limited to the prototype in which it appears. There is no symbol with that name in scope in the body of popArr().
The different appearances of the symbol counter in that prototype, in function generator(), and in main() all have different, non-overlapping scope. They are not related to each other, notwithstanding the reuse of the same name.
Generally I'm interested in the technique how to use callbacks with arguments without using global variables or putting everything inside main.
There are two main scenarios:
Callbacks with parameters that the caller (popArr() in your example) is expected to choose itself, and
Callbacks with parameters that the callback provider (main() in your example) is expected to specify.
Those are not mutually exclusive. Case (1) has no special requirements -- the caller just passes whatever argument is appropriate, at its own discretion. Case (2) isn't much harder, in principle: the code that provides the callback simply needs to provide the appropriate argument with it. For your very simple case, where the argument and callback do not need to be stored, that would look something like this:
void popArr(int *arr, size_t length, int (*callback)(int *), int *counter) {
// The name of the parameter to (*callback)() is omitted for clarity. It is
// optional in this context.
for (size_t i = 0; i < length; i++) {
arr[i] = callback(counter); // Note: passing counter, not *counter
printf("arr[%zu] is: %d\n", i, arr[i]);
}
}
/* A simple generator callback saving counter. */
int generator(int *counter) {
*counter += 1;
return *counter * 5;
}
/* Main function. */
int main(void) {
int counter = 1;
int arr[10];
// It is unnecessary, but not harmful, to use the & operator on the function
// name. I think it's better style to omit it.
// It is necessary to use the & operator on 'counter', but mainly because
// you want to pass a pointer to counter, as opposed to its value
popArr(arr, 10, generator, &counter);
return 0;
}