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

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*/

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.

The compiler wants to know the function before it can use it

just declare the function before you call it

#include <stdio.h>

int Fibonacci(int number); //now the compiler knows, what the signature looks like. this is all it needs for now

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)
{
//…

in c The function has to be declared before it's getting called

include head file

I have the same warning (it's make my app cannot build). When I add C function in Objective-C's .m file, But forgot to declared it at .h file.