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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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LEARN ABOUT DEBIAN

mixal

MIXAL(1)							Mixal user's manual							  MIXAL(1)

NAME

       mixal - a load-and-go MIX assembler

SYNOPSIS

       mixal [ file ...  ]

DESCRIPTION

       Mixal  is  an  implementation of the hypothetical MIX computer and its assembly language called MIXAL.  The computer was designed by Donald
       Knuth for use in his monumental and yet to be finished book series The Art of Computer Programming.  All programs and all programming exer-
       cises in the book are written in the MIXAL language.

       This  implementation  is  a  load-and-go  assembler,  meaning that you provide it with a MIXAL program source, which it translates into MIX
       machine code, which it promptly executes by acting as a MIX emulator.

       You give Mixal zero or more program source files in the command line, which the program interprets.   If  you  give  it	no  arguments,	it
       expects	to  find  a  program  in  the standard input stream.  After the program has executed, the final state of the machine registers are
       printed to the standard output stream.

       The card punch and line printer devices are connected to the standard input and output stream, respectively.  Console input and output  are
       connected  to  standard	input and output, and the disk devices are connected to files named diskN in the current directory, where N is the
       device number.  Those files are created on demand.

BUGS

       This MIXAL implementation does not do floating-point.  The tape devices are not implemented.

AUTHOR

       This MIXAL implementation was designed and written by Darius Bacon, and then ported to Unixish systems and debugged  by	Eric  S.  Raymond.
       This  version includes corrections to multiplication and division by Larry Gately.  This manual page was written for Debian by Antti-Juhani
       Kaijanaho, with changes by Darius Bacon.

SEE ALSO

       The files /usr/share/doc/mixal/READ.ME and /usr/share/doc/mixal/NOTES.gz contain some information about this MIXAL implementation.  Be sure
       to read /usr/share/doc/mixal/README.Debian, too.

       A  description  of  the MIX system and the MIXAL language can be found in Donald E. Knuth's book The Art of Computer Programming, Volume 1:
       Fundamental   Algorithms;   3rd	 Edition   (Addison-Wesley   1997).    (Or   see   the	 home	 page	 at    http://www-cs-faculty.stan-
       ford.edu/%7Eknuth/taocp.html.)

Debian project							    18 May 2003 							  MIXAL(1)
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