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From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: fseek fread fwrite fflush
Message-ID: <669@taumet.com>
Date: 14 Apr 91 17:10:28 GMT
References: <1991Apr13.003817.12434@watdragon.waterloo.edu> <1991Apr12.194547.18080@hellgate.utah.edu>
Organization: Taumetric Corporation, San Diego
Lines: 26

tmurphy%peruvian.utah.edu@cs.utah.edu (Thomas Murphy) writes:

>In article <1991Apr13.003817.12434@watdragon.waterloo.edu> amewalduck@trillium.waterloo.edu (Andrew Walduck) writes:
>>I've got a question about fseek, fread, fwrite and fflush....
>>Is it necessary to do the following?
>>fflush(fp);
>>fseek(fp....);
>>fwrite(fp...);

>You are always safest if you flush streams btwn io phases...this goes
>for io to the console or to some file...thus if you have been reading
>you should flush before writing and again before returning to reading
>again and in addition flush before closing...

Not quite true.  The ANSI rules state that the effect of fflush() on
a file whose last operation was a read are undefined -- an implementation
is allowed to do bad things if you fflush() after an input operation.
But you are guaranteed to be able to safely switch between reading
and writing a file if you fseek() first -- even fseek() to the same
position.

For non-ANSI systems, you have to check the manual to see whether the
fflush() is safe or required.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com