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From: lgy@pupthy.UUCP
Newsgroups: comp.unix.questions
Subject: Stdio flaws?
Message-ID: <345@phoenix.PRINCETON.EDU>
Date: Thu, 28-May-87 14:10:58 EDT
Article-I.D.: phoenix.345
Posted: Thu May 28 14:10:58 1987
Date-Received: Sat, 30-May-87 04:42:17 EDT
Sender: news@phoenix.PRINCETON.EDU
Reply-To: lgy@pupthy.PRINCETON.EDU (Larry Yaffe)
Distribution: world
Organization: Physics Dept, Princeton Univ
Lines: 41
Summary: (1) flushing stdin, (2) truncating files


    There does not appear to be any clean/portable way to either
(a) flush an input stream, such as stdin, or (b) truncate an existing,
open file.  By portable, I mean sufficiently portable to be used on
non-Unix systems (e.g., Vax VMS or Cyber VSOS) which provide C and
Unix compatible subroutine libraries for stdio level routines,
but not for low-level I/O.  In practice, this means section 3 routines,
but not section 2.

    For some unknown reason, fflush(3) only flushes output buffers,
not input buffers.  The best solution I know uses:

    if (input = fdopen (dup (fileno (stdin)), "r"))
	fclose (stdin), *stdin = *input ;

This is less than ideal due to the use of `dup' which can cause protability
problems.  Using something like `ioctl (0,TCFLSH,0)' is not a solution since
stdin need not be coming from a terminal, and also since (as far as I know)
stdio buffering is totally independent from whatever buffer this ioctl call
refers to.

    Truncating a file also appears to be impossible using stdio level calls.
To illustrate the problem consider trying to:
	1) Create a temporary file using `tmpfile'.
	2) Storing a line of text in this file, and later rewinding the file
		and rereading the text.
	3) Storing a second line of text in the file, for later reuse.

At step (3) you must truncate the file to zero length; rewinding the file
is not sufficient since this allows the previous contents of the file to
`stick out' beyond the end to the current contents.  This may be accomplished
using `ftruncate (fileno (fp), 0)', however many systems do not support
ftruncate(2).  Using something like `freopen (...)' is no good since you
do not know the name of the file - only an open file pointer is available!

    I consider the inability to perform these tasks at the stdio level to
be a significant flaw in the stdio package.  If there are better ways to
deal with these problems, please let me know.
------------------------------------------------------------------------
Laurence G. Yaffe                          Physics Dept ; Princeton Univ
lgy@pupthy.PRINCETON.EDU                 PO Box 708 ; Princeton NJ 08544