Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!mcf!mibte!gamma!thumper!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: extern - when to use it
Message-ID: <9702@alice.UUCP>
Date: 29 Jul 89 15:14:20 GMT
References: <6420@columbia.edu>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 34

In article <6420@columbia.edu>, kearns@read.columbia.edu (Steve Kearns) writes:

> Would someone post an article explaining the proper use of
> extern when declaring function?  When is it neccessary, 
> just a good idea, a bad idea?  What about the new
> 'extern "C" ' I have been hearing about?

Saying `extern' by itself in a function declaration has no effect.
Thus

	extern void munge();

means precisely the same thing as

	void munge();

extern "C" is new in C++ 2.0 .  You must use it to declare any C function
you wish to call from C++ or any C++ function you wish to call from C:

	extern "C" double sqrt(double);

The declaration informs the C++ compiler that sqrt(double) is
a C function so that it can use a C calling sequence for it
(which is therefore no longer required to the be the same as
the C++ calling sequence).  Similarly:

	extern "C" void frob() {
		// stuff
	}

says that frob() is a function that will be called from C.
-- 
				--Andrew Koenig
				  ark@europa.att.com