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From: fdg@sortac.UUCP
Newsgroups: comp.os.minix
Subject: Re: More on sh/shar
Message-ID: <498@sortac.UUCP>
Date: Wed, 3-Jun-87 07:47:30 EDT
Article-I.D.: sortac.498
Posted: Wed Jun  3 07:47:30 1987
Date-Received: Fri, 5-Jun-87 04:05:27 EDT
References: <1643@encore.UUCP> <581@uokmax.UUCP>
Reply-To: fdg@sortac.UUCP (Fred Gant)
Organization: AT&T Tech-NS, Atlanta
Lines: 75
Summary: Please get your facts straight.

In article <581@uokmax.UUCP> rmtodd@uokmax.UUCP (Richard Michael Todd) writes:
>In article <1643@encore.UUCP>, paradis@encore.UUCP (Jim Paradis) writes:
>> the code for /bin/sh.  Getting it to compile was a feat unto itself...
>> (seems the MINIX compiler handles globals and externs differently
>> from the PC/IX compiler...). 
>Yep.  Actually it's a difference between how UNIX-type linkers (of which
>PC/IX's is apparently one) and most everybody else's handle globals.
>Under UNIX, you can have multiple files each with a global var. defined,
>say:
>file foo.c:
>	int a;
>...
>
>file bar.c:
>	int a;
>...
>and the UNIX linker will see the variable declared multiple times in the
>modules and allocate space in the result file only once.  Most other linkers
>consider duplicate variable declarations to be errors.  For C programs
>on those systems, the global definition can appear in only one file, and
>the other files must use extern declarations (e.g. "extern int a;").
>On a side note, I've also gotten the shell to recompile, both under MINIX
>and under Aztec C (running the output thru dos2out).  I changed all the 
>offending definitions in sh.h (which gets included in every file) to
>read "EXTERN int whatever;" and #defined EXTERN extern in every file except
>one.  Very minimal changes were needed to get it to compile under Aztec
>C after I'd gotten it to compile under MINIX cc.
>--------------------------------------------------------------------------
>Richard Todd
The "BOOK" is on order; therefore, I do not have the MINIX code yet and can
not comment on its handling of globals.  I do, however, use C and UNIX
daily and can comment on that.  In your example, "int a" is not global in
either case.  It is in fact local to the file that declares it.  To be
global, it must be declared in a header file, such as sh.h, or be declared
outside of the function.  An example would be

int a, b;  /* These are global. */

main()
{
	extern int a, b;   /* a & b are globals used by main */
	int c, x;          /* c, x, & y are local to main    */
	char *y;
...
	foo(x,y);
...
	bar();
...
}

foo(w,m)
int w;                     /* w & m are local to foo */
char *m;
{
	extern int a;      /* a is a global used by foo */
...
}

bar()
{
	extern int b;      /* b is a global used by bar 
	int a;             ** a is local to bar - Note: the
			   ** compiler and linker should complete
			   ** but lint will warn you that a
			   ** has been redeclared here masking the
			   ** global a.
		           */
...
}

This posting is not meant to be critical.  I am attempting to clear up a
point of confusion.

Fred Gant  akgua!sortac!fdg
AT&T Network Systems