c - Xcode - Warning: Implicit declaration of function is invalid in C99

Question

Getting a warning : Implicit declaration of function 'Fibonacci' is invalid in C99. What's wrong?

#include <stdio.h>

int main(int argc, const char * argv[])
{
    int input;
    printf("Please give me a number : ");
    scanf("%d", &input);
    getchar();
    printf("The fibonacci number of %d is : %d", input, Fibonacci(input)); //!!!

}/* main */

int Fibonacci(int number)
{
    if(number<=1){
        return number;
    }else{
        int F = 0;
        int VV = 0;
        int V = 1;
        for (int I=2; I<=getal; I++) {
            F = VV+V;
            VV = V;
            V = F;
        }
        return F;
    }
}/*Fibonacci*/

Answer 1

The function has to be declared before it's getting called. This could be done in various ways:

• Write down the prototype in a header
  Use this if the function shall be callable from several source files. Just write your prototype
  int Fibonacci(int number);
  down in a .h file (e.g. myfunctions.h) and then #include "myfunctions.h" in the C code.

• Move the function before it's getting called the first time
  This means, write down the function
  int Fibonacci(int number){..}
  before your main() function

• Explicitly declare the function before it's getting called the first time
  This is the combination of the above flavors: type the prototype of the function in the C file before your main() function

As an additional note: if the function int Fibonacci(int number) shall only be used in the file where it's implemented, it shall be declared static, so that it's only visible in that translation unit.