c - warning: implicit declaration of function - order of includes matter? -

i have following sample code snippet of code main.c calls 3 functions, 3 headers - giving me warnings reason unknown:

#include "header1.h"  #include "header2.h"  #include "header3.h"    int main()  {    function1(); // header1    function2(); // header2    function3(); // header 3  } 

basically, after using gcc, functions 2 & 3 produce warning. however, after rearranging code this:

#include "header3.h"  #include "header1.h"  #include "header2.h"         int main()  {    function1(); // header1    function2(); // header2    function3(); // header 3  } 

it give me warning functions 1 & 2 implicit. doing wrong here?

your include file 1 should this:

#ifndef __rezon_functions1__  #define __rezon_functions1__   #endif 

the other 2 files should similar macro name changed accordingly