Path: utzoo!mnetor!uunet!vsi!friedl
From: friedl@vsi.UUCP (Stephen J. Friedl)
Newsgroups: comp.lang.c
Subject: a couple of random questions
Message-ID: <530@vsi.UUCP>
Date: 13 Apr 88 03:57:05 GMT
Distribution: comp
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 42

Hiho there,

     Two random questions.  First, the word "entry" used to be a
reserved keyword in C but appears to never have been used.  Does
anybody know the direction it might have taken had its use been
implemented?  I recall vaguely about a similar keyword in FORTRAN
but wonder if anybody has any stories from long ago in days of old.

     Second, what is the portable way to rewind a Unix file
descriptor?  On almost every machine I have ever used:

        lseek(fd, (off_t)0, SEEK_SET);

works because the offset is a byte count, but it is inevitable
that on some machine, off_t is the pointer to some kind of
struct, or at least is *not* simply a byte count.  What other
machines might work this way?  I know that the good old BDS C
compiler for the Z-80 measured its offset in records, but
nevertheless a zero arg did the trick.

     Related to this, how about a portable way to back up one
record?  On the assumption that doing math on an off_t is not
portable, I basically save the offset just before a read:

#define tell(fd)        lseek(fd, (off_t)0, SEEK_CUR)

extern off_t    lseek( /* int fd, off_t offset, int whence */ );

{
off_t   off;

        while (off = tell(fd), read(fd, buf, SIZE) == SIZE)
                if (some.condition->here)
                        lseek(fd, off, SEEK_SET);       /* back up */

Note that this is largely an academic exercise, and I know that
using dpANS buffered I/O resolves these issues, but I'm just
curious...

-- 
Steve Friedl   V-Systems, Inc.   "Yes, I'm jeff@unh's brother"
friedl@vsi.com  {backbones}!vsi.com!friedl  attmail!vsi!friedl