What is the significance of forward declaration in C programming?

Question

I am now learning C programming through Learn C the Hard Way by Zed A. Shaw. There is this code (taking from his website):

#include <stdio.h>
#include <ctype.h>

// forward declarations
int can_print_it(char ch);
void print_letters(char arg[]);

void print_arguments(int argc, char *argv[])
{
    int i = 0;

    for(i = 0; i < argc; i++) {
        print_letters(argv[i]);
    }
}

void print_letters(char arg[])
{
    int i = 0;

    for(i = 0; arg[i] != '\0'; i++) {
        char ch = arg[i];

        if(can_print_it(ch)) {
            printf("'%c' == %d ", ch, ch);
        }
    }

    printf("\n");
}

int can_print_it(char ch)
{
    return isalpha(ch) || isblank(ch);
}


int main(int argc, char *argv[])
{
    print_arguments(argc, argv);
    return 0;
}

Don't we just can code it like this (put the can_print_it and print_letters functions at the top and remove the need for forward declaration):

#include <stdio.h>
#include <ctype.h>

int can_print_it(char ch)
{
    return isalpha(ch) || isblank(ch);
}

void print_letters(char arg[])
{
    int i = 0;

    for(i = 0; arg[i] != '\0'; i++) {
        char ch = arg[i];

        if(can_print_it(ch)) {
            printf("'%c' == %d ", ch, ch);
        }
    }

    printf("\n");
}

Is there really times when the forward declaration is important and unavoidable?

Answer 1

Forward declarations of functions are unavoidable whenever your call graph is cyclic; that is, whenever you have (direct or indirect) recursion between functions.

They are useful if you want to separate your program into more than one translation unit, as they allow separation of declaration and definition of functions (placing the declaration in a .h header and the definition in a .c file).

Answer 2

Forward declarations of functions in C typically have two different uses.

Modules

The header of exported functions are declared in a header file which is included in a client module.

Mutual Recursion

In mutual recursion two functions call each other repeatedly. Without a forward declaration one of the two functions will be undeclared in the body of the other.

Example:

int Odd(int n);

int Even(int n)
{
    return (n == 0)? 1: Odd(n - 1);
}

int Odd(int n)
{
    return (n == 0)? 0: Even(n - 1);
}

With a function pointer though, we can do without a forward declaration:

int (*odd)(int n);

int Even(int n)
{
    return (n == 0)? 1: odd(n - 1);
}

int Odd(int n)
{
    return (n == 0)? 0: Even(n - 1);
}

void Init(void)
{
    odd = Odd;
    ...
}