Path: utzoo!attcan!uunet!dino!uxc.cso.uiuc.edu!garcon!uicsrd.csrd.uiuc.edu!mcdaniel
From: mcdaniel@uicsrd.csrd.uiuc.edu (Tim McDaniel)
Newsgroups: comp.std.c
Subject: Re: struct comparison
Message-ID: <1611@garcon.cso.uiuc.edu>
Date: 29 Jul 89 05:39:45 GMT
References: <167@ssp1.idca.tds.philips.nl> <12040019@hpfcdc.HP.COM> <2288@auspex.auspex.com>
Sender: news@garcon.cso.uiuc.edu
Reply-To: mcdaniel@uicsrd.csrd.uiuc.edu (Tim McDaniel)
Organization: Center for Supercomputing R&D (Cedar), U. of Ill.
Lines: 40

In article <2288@auspex.auspex.com> guy@auspex.auspex.com (Guy Harris)
writes:
>OK, what if I want to see if one file offset is larger than, or smaller
>than, another?
...
>Perhaps add and subtract aren't as common as other operators, but I'd
>still be a bit annoyed if I couldn't add and subtract file offsets;
>unlike structure comparison, I've used addition of file offsets on a
>number of occasions.

The BSD 4.3 man page for fseek(3S) and ftell(3S) says:

     _Ftell_ returns the current value of the offset relative to
     the beginning of the file associated with the named _stream_.
     It is measured in bytes on UNIX; on some other systems it is
     a magic cookie, and the only foolproof way to obtain an
     _offset_ for _fseek_.

K&R 2, 1st edition, p. 248, says (for fseek):

     For a binary file, the position is set to _offset_ characters
     from _origin_, which may be SEEK_SET (beginning), SEEK_CUR
     (current postion), or SEEK_END (end of file).  For a text stream,
     _offset_ must be zero, or a value returned by _ftell_ (in which
     case _origin_ must be SEEK_SET).

Thus, in pANS C, a *text* stream offset may indeed be a "magic
cookie", for which comparison and arithmetic is undefined.  Binary
mode requires an actual character number.  (Personally, I see no point
in having the distinction; I'd like to see all _offset_s be magic
cookies.  It's too late, now that X3J11 has acted.)

--
"Let me control a planet's oxygen supply, and I don't care who makes
the laws." - GREAT CTHUHLU'S STARRY WISDOM BAND (via Roger Leroux)
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