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From: guy@gorodish.UUCP
Newsgroups: comp.unix.questions
Subject: Re: Stdio flaws?
Message-ID: <19977@sun.uucp>
Date: Thu, 28-May-87 20:02:53 EDT
Article-I.D.: sun.19977
Posted: Thu May 28 20:02:53 1987
Date-Received: Sat, 30-May-87 06:01:47 EDT
References: <345@phoenix.PRINCETON.EDU>
Sender: news@sun.uucp
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>     For some unknown reason, fflush(3) only flushes output buffers,
> not input buffers.

The reason is quite well known.  "flushing" a stream means writing
all buffered output to the underlying device.  This has nothing to do
with input.  Purging an input buffer is a completely separate
operation; I agree that it might be a good thing if standard I/O
permitted you to do this, but it is emphatically NOT the operation
that "fflush" should do.  (No, please don't say "well, have 'fflush'
do a purge on a stream opened for reading"; what about something open
for reading *and* writing?)

Another problem is that you may want to flush anything buffered *by
the underlying device* as well; this would be a lot harder to do in a
portable and device-independent fashion.

> 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!

The fact that many systems do not support "ftruncate" should explain
why the standard I/O library *can't* help you here.  If you don't
have a name for a file, there *is* no portable way to truncate it to
zero length!  What could standard I/O possibly do on a system without
"ftruncate", or some other way to truncate a file given only a
"handle" for the file (e.g. a file descriptor on UNIX), rather than
a name?  The standard I/O library is not capable of performing
miracles....
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com