NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/11158

Text File | 1996-08-05 | 2.0 KB | 51 lines

Path: ip-pdx09-31.teleport.com!user
From: [e-mail] (Peter Miller)
Newsgroups: comp.lang.c
Subject: Re: Global variables in a multiple-sourcefile application
Date: Thu, 21 Mar 1996 22:20:58 -0700
Organization: Teleport - Portland's Public Access [phone]
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: ip-pdx09-31.teleport.com

In article <[e-mail]>, [e-mail] wrote:

> I have two questions about the use of global variables in a
> multiple-sourcefile application:
>
> 1) If I have a global variable x that is used in both source files of
> an application, but I don't need it initialised in its definition,
> I would generally put "int x;" in one and "extern int x;" in the other.
>
> However, is there anything unsafe about putting a declaration in a header
> file included by both files, without a definition in either file, or
> do I really need a definition in one of the files?
>
> i.e. is this OK?:
> Mark Bergman                       Email: [e-mail]
> Ferranti Syseca Ltd.               Tel:   [phone]
> Wythenshawe, Manchester, UK        Fax:   [phone]


1) I would expect that the modules would compile but would not link together.
No, you are going to have to define the variable in one file, and declare
it extern in the other file, otherwise the linker will not create an
executable.

so, typically, in the main module you would define the variable:
   int x;
and in the other module, declare it:
    extern int x;

2) It is very dangerous to have data with the same name declared
differently between source files.  If the actual variable is smaller than
the external declaration, then use of that variable externally will likely
cause memory reads and writes out of bounds.  If the actual variable is of
a larger size than the external declaration, then unexpected values are
likely to result.  In either case, doom and aggravation are the most
likely result.

Sincerely,

Peter Miller