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.

Answer 2

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) { //…