#ifndef

when we create header files i know you can do it like this..

blah.h

Code:

---


#ifndef BLAH_H
#define BLAH_H

void blah();

#endif

---

in main.c if i was to include this header file i would do
#include "blah.h"

but if i didnt have the #ifndef stuff, this could still work couldnt it?
so then whats the point for the #ifndef stuff?

and say if blah was named blah.c i could do #include "blah.c" without the ifndef stuff aswell right?

its so that it is not included more than once.

Code:

---

#ifndef BLAH_H //if BLAH_H is not defined (BLAH_H is just a label for the following code)
#define BLAH_H //if BLAH_H is not previously defined then define it :)
 
...
 
#endif //self explanatory (i hope)

---

read this faq entry
http://faq.cprogramming.com/cgi-bin/...&id=1043284392

Simiple test:
header1.h

Code:

---

#ifndef HEADER1_H_
#define HEADER1_H_

void foo (void)
{
  printf("I'm in foo()\n");
}

#endif

---

header2.h

Code:

---

void bar (void)
{
  printf("I'm in bar()\n");
}

---

main.c

Code:

---

#include <stdio.h>
#include "header1.h"
#include "header2.h"
#include "header1.h"
#include "header2.h"

int main(void)
{
  foo();
  bar();
  return 0;
}

---

Compile and look at the results

I did exactly what Thantos said, but included #defines and int/char declartions.

Why doesn't #define and int/char declaration in header2.h file give error? Why only functions??

Please let me know..

Way to bump an old thread.

#defines don't give an error because the preprocessor doesn't care if there are multiple declartions, it just uses the newest one