multiple include doesn't give compile error

Hi All

I have a main file (main.c) which includes show.h and show1.h
But show1.c also includes show.h. show.h looks like

Code:

void init() ;
void show() ;
void set(int x) ;
int *get() ;

extern int  *y ;

Because it is included twice I was expecting compile errors, but the program compiles/runs fine, why ?
I thought, in this situation, I needed things like #ifndef .... #endif!

Below are the listings of all the files

main.c

Code:

#include "show.h"
#include "show1.h"

int main(int argc, char *argv[])
 {
   init() ;
   set(10) ;
   show() ; 

   set(20) ;
   show() ; 

   // -------
   show1() ;

   set(303) ;
   show1() ;
   return 0 ;
 }

show.c

Code:

#include <stdlib.h> // malloc
#include <stdio.h>  // printf

#include "show.h"

int *y ;

void init() {
  y = malloc(sizeof(int)) ;
}

void set(int x) {
  *y = x ;
}

int * get() {
  return y ;
}

void show() {
  printf("value is %d\n", *y) ;
}

show.h (see above)
show1.c

Code:

#include <stdio.h>  // printf
#include "show.h"

extern int* y ;

void show1() {
  printf("the value is %d\n", *y) ;
  show();
}

show1.h

Code:

void show1() ;

If two .c files includes one .h file, there will be no problem and its usual.

If one .c includes one .h more than once, there will be problems. You should use precompiler directives to make sure each file is included only once.

I see, so it has todo with header files including each other

cheers