Including header files more than once

Question

#include <stdio.h>
#include <stdio.h>

int main ()
{
   printf ("hello world");
   return 0;
}

when I compile this, the compiler doesn't give any warning/error for including stdio.h twice. Why is it so? Aren't the functions scanf, printf etc. declared and defined twice now?

Thanks, in advance

Answer 1

Typically, header files are written similar to the below example to prevent this problem:

#ifndef MYHEADER
#define MYHEADER

...


#endif

Then, if included more than once, then 2nd instance skips the content.

Answer 2

In addition to the use of include guards, as pointed out by Mark Tolonen's answer, there is no problem with declaring a function more than once, as long as the declarations are compatible. This is perfectly fine:

int foo(int, char *);
int foo(int a, char *p);
extern int foo(int x, char y[]);

In fact, since every definition is also a declaration, whenever you "forward-declare" a function declared in the same file, you are declaring the function twice.

What is not OK is to create multiple external definitions of a function; but well-written header files should not create external definitions - only declarations. The (single) definition of the printf and scanf functions should be in an object file, which is linked with your program when it is built.