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From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: comp.lang.c
Subject: Re: Perror (was Re: Accessing argc & argv from a functi)
Message-ID: <1875@ttrdc.UUCP>
Date: Mon, 24-Aug-87 03:59:25 EDT
Article-I.D.: ttrdc.1875
Posted: Mon Aug 24 03:59:25 1987
Date-Received: Tue, 25-Aug-87 02:28:25 EDT
References: <22@flmis06.ATT.COM> <28700015@ccvaxa> <853@mcgill-vision.UUCP> <518@sugar.UUCP>
Organization: AT&T, Skokie, IL
Lines: 47
Summary: picky point

In article <518@sugar.UUCP>, peter@sugar.UUCP (Peter da Silva) writes:
> You shouldn't pass null to perror. It's not guaranteed to work. It might
> even crash you.
> 
> It's easy enough to do this:
> 
> 	extern int errno;
> 	extern char **sys_errlst;

	/* Better:  */	extern char *sys_errlist[];
					     ^

	Note that "extern char **sys_errlst" would declare the location
	sys_errlst as containing a pointer to pointer to char, whereas
	"extern char *sys_errlist[]" declares the location sys_errlist
	as (more or less; purists will not agree) containing the first
	member of an array of pointer to char.  The results you get
	from trying to interchange the two in an extern declaration are
	surprising, to say the least.


> 
> 	fprintf(stderr, "%this, %that: %s\n", this, that, sys_errlst[errno]);

Note too that errno is not guaranteed to lie within the valid range of
sys_errlist.  sys_errlist is defined from errno==0 to errno==sys_nerr-1.

Perhaps a STRERR() macro would be helpful:

	extern int errno, sys_nerr;	/* declare these at the top */
	extern char *sys_errlist[];	/* of your module */

#define STRERR()	((errno >=0 && errno < sys_nerr) ? \
				sys_errlist[errno] : "Unknown error")

	...

	fprintf(stderr,"%this, %that: %s\n", this, that, STRERR());
	...

> -- Peter da Silva `-_-' ...!seismo!soma!uhnix1!sugar!peter (I said, NO PHOTOS!)

-- 
|------------Dan Levy------------|  Path: ..!{akgua,homxb,ihnp4,ltuxa,mvuxa,
|         an Engihacker @        |		vax135}!ttrdc!ttrda!levy
| AT&T Computer Systems Division |  Disclaimer:  i am not a Yvel Nad
|--------Skokie, Illinois--------|