Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pacbell!att-ih!ihnp4!bentley!cox
From: cox@bentley.UUCP (MH Cox)
Newsgroups: comp.lang.c
Subject: Re: global data in C programming... (another way)
Message-ID: <1089@bentley.UUCP>
Date: 18 Mar 88 19:35:34 GMT
References: <7506@ncoast.UUCP> <3032@haddock.ISC.COM> <10780@ut-sally.UUCP>
Reply-To: cox@bentley.UUCP (59463-MH Cox)
Organization: AT&T Bell Laboratories, Liberty Corner
Lines: 59
Keywords: global data, extern, C programming

In article <10780@ut-sally.UUCP> nather@ut-sally.UUCP writes:
>In article <3032@haddock.ISC.COM>, karl@haddock.ISC.COM (Karl Heuer) writes:
>> In article <7506@ncoast.UUCP> btb@ncoast.UUCP (Brad Banko) writes:
>> >i thought that you just create a header file containing the global data
>> >declarations and then #include it into each of your source files, but
>> >when i try this (using Mark Williams C on an Atari ST), I get
>> >a "symbol redefined" message from the linker.
>> 
>> Yes.  After the preprocessing phase is through, what you have is a set of
>> source files each of which contains "int i, j;".  Although this is accepted by
>> some implementations, it is not portable.
>> 
>> $ Another popular style, which I do not recommend, is to have x.h contain
>> "global int i, j;" where "global" is a macro defined with a null expansion in
>> extern.c but expanding to "extern" for everyone else.
>
>I find that this, too, has limitations, since pre-initialized definitions don't
>work.  I finally solved the problem (to my satisfaction anyway) by writing a
>program I call "tglob" which takes the "master" set of global definitions,
>and turns it into a second file with "extern" inserted in the right places,
>and with definitions removed.  It creates a file called "sglob" which can
>then be #included in all files except main(), which #includes the definitions.
>
>This way, if I change anything in the gloabl definition file, I needn't also
>remember to change it in the other file.  I was ALWAYS making that mistake.
>I use "make" and include the dependency that re-creates "sglob" whenever the
>global definition file has been changed.

A better way (my opinion) would be to have one file, e.g. globals.c or in
main.c, with all the initialized definitions.  Then create a header
file with all the "extern" declarations without the initializations.
Include this in all your files, INCLUDING the globals.c or main.c.  The{
compiler will then catch any errors of the type:

globals.h:

extern int an_extern_int;


globals.c:

long an_extern_int = 0;		/* wrong type */


A second suggestion is to put all the extern global declarations be
put into one header file.  That makes it easy to "grep" all the files
in the directory to determine which ones access global variables (the
compiler should warn you about files that access global variables but
don't include the header file).

-- 

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