Why the user-defined function needs to be before the main function for a function pointer to work

Question

Why the user-defined function needs to be before the main function for a function pointer to work.

This one works:

#include <stdio.h>

int add(int a, int b)
{
    int c;

    c = a + b;

    return c;
}

int main()
{
    int a, b, sum;
    int (*ab)(int, int) = &add; // Declaration and initialization of function pointer

    sum = (*ab)(1, 2); // Dereferencing of function pointer

    printf("1 + 2 = %d", sum);

    return 0;
}

But why this one doesn't work:

#include <stdio.h>

int main()
{
    int a, b, sum;
    int (*ab)(int, int) = &add; // Declaration and initialization of function pointer

    sum = (*ab)(1, 2); // Dereferencing of function pointer

    printf("1 + 2 = %d", sum);

    return 0;
}

#include <stdio.h>

int add(int a, int b)
{
    int c;

    c = a + b;

    return c;
}

int main()
{
    int a, b, sum;
    int (*ab)(int, int) = &add; // Declaration and initialization of function pointer

    sum = (*ab)(1, 2); // Dereferencing of function pointer

    printf("1 + 2 = %d", sum);

    return 0;
}

Answer 1

C is single pass compiler and does not support forward references. That means all variables and methods must be declared before use.

note: you add a method declarations at the start of the file and the implementation later to allow any order of method declarations. This is considered best practice.

this also why .h files are required for external symbols.

Answer 2

Hi,
From Function Declarations and Definitions
Both function and variable declarations can appear inside or outside a function definition.

add() means that you can pass any number of arguments.

Compilation may fail if parameters are added. It depends on your compiler.

Best regards,

Minxin Yu

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