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Full Discussion: C language Header files
Top Forums Programming C language Header files Post 302476739 by Corona688 on Thursday 2nd of December 2010 11:20:41 AM
Old 12-02-2010
All header files do is declare the existence of a function. For instance, you have a library 'flogify' or something.

Code:
// flogify.c
#include <stdio.h>
#include "flogify.h"

int flogify(const char *oops)
{
        return(printf("flogify(%s)\n", oops));
}

Code:
//flogify.h
// #ifndef/#define/#endif prevents accidental multiple inclusion.
#ifndef __FLOGIFY_H__
#define __FLOGIFY_H__

extern int flogify(const char *);

#endif/*__FLOGIFY_H__*/

so a.c and b.c will know the function exists. But you have to link it, too, otherwise it'll complain it can't find it:

Code:
$ gcc a.c b.c -o flogify
/tmp/ccEYKx87.o: In function `main':
a.c:(.text+0x19): undefined reference to `flogify'
/tmp/ccNOvwnB.o: In function `other':
b.c:(.text+0xe): undefined reference to `flogify'
collect2: ld returned 1 exit status
$ gcc a.c b.c flogify.c -o flogify
$

In other words, header files just tell code how to use functions, they don't actually make the functions exist.
 

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