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From: gamma@rtp47.UUCP (Michael Meissner)
Newsgroups: net.unix
Subject: Re: stdout buffering question
Message-ID: <26@rtp47.UUCP>
Date: Tue, 16-Apr-85 19:04:02 EST
Article-I.D.: rtp47.26
Posted: Tue Apr 16 19:04:02 1985
Date-Received: Sun, 21-Apr-85 01:38:38 EST
References: <749@druxr.UUCP>
Organization: Data General, RTP, NC
Lines: 27

> 
>  I am trying to exec a child with its standard in, out and error
> directed to pipes. I have managed to accomplish this in two different
> ways, closing the std stuff, duping to the pipes and by execing the shell
> with appropriate arguments. The snag I have run into is that it appears
> that when a process is execed the operating system does you a favor and
> make stdout buffered.	...

First, the decision on whether to buffer is not done at exec time, but rather
in the standard I/O library when you first write to the file.  Different
systems make different decisions on when to buffer.  Typically, if the
output is a console, it is either unbuffered (BSD?), or line buffered (system
V).  Pipes are typically fully buffered for performance reasons.  The only
three ways I can see of fixing your problem, is to:

    1)	Create a pseudo terminal that is opened for write in exec'ing the
	child, and for read in the parent.

    2)	Hack on the standard I/O library to unbuffer pipes, and expect a
	possible performance hit on pipes in general.  Or, use the hacked
	library only in the program you use.  Alternatively, hack isatty
	to always return 1 (this can be done with a reasonable patch prog.)

    3)	Try to get the program to use fflush at appropriate times.

	Michael Meissner
	Data General Corporation
	...!mcnc!rti-sel!rtp47!meissner