C Language Function Declaration and Definition

#include <stdio.h>
int addNumbers(int a, int b);         // Function declaration, describing function prototype

int main()
{
    int n1,n2,sum;

    printf("Enters two numbers: ");
    scanf("%d %d",&n1,&n2);

    sum = addNumbers(n1, n2);        // Call the function
    printf("sum = %d",sum);

    return 0;
}

int addNumbers(int a, int b)         // Definition of function
{
    int result;
    result = a+b;
    return result;                  
}

If the function definition is used directly instead of being declared, it needs to be placed before calling the function

#include <stdio.h>

int addNumbers(int a, int b)         // Definition of function
{
    int result;
    result = a+b;
    return result;                  
}
int main()
{
    int n1,n2,sum;

    printf("Enters two numbers: ");
    scanf("%d %d",&n1,&n2);

    sum = addNumbers(n1, n2);        // Call the function
    printf("sum = %d",sum);

    return 0;
}