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From: mouse@thunder.mcrcim.mcgill.edu (der Mouse)
Newsgroups: comp.unix.programmer
Subject: Re: get character without waiting
Message-ID: <1991Jun15.205136.4545@thunder.mcrcim.mcgill.edu>
Date: 15 Jun 91 20:51:36 GMT
References: <1991Jun12.001056.24021@watdragon.waterloo.edu>
Organization: McGill Research Centre for Intelligent Machines
Lines: 32

In article <1991Jun12.001056.24021@watdragon.waterloo.edu>, nkkwan@crocus.uwaterloo.ca writes:

> How can I check whether there is character waiting in standard input
> or not?

The only real answer is "there is no portable way".  Depending on
precisely what you mean by "standard input", there may be a
not-horribly-nonportable way.

If you mean "readable from file descriptor 0", you can try one or more
of the following, which have varying degrees of portability:

- FIONREAD ioctl
- select(2)
- poll(2)
- non-blocking mode via the FIONBIO ioctl
- non-blocking mode via the F_SETFL fcntl
- set a timeout (with alarm, or maybe setitimer) and do a read()
- if you care about only ttys, and you have the SysV tty driver
   interface, disable ICANON and set VTIME
- probably others I don't know of or have forgotten

If you mean "waiting in the stdio input buffer", there is no good
solution; about the closest you can come is to check stdin->_cnt and
hope your stdio implementation (a) has such a field and (b) keeps
something useful there.  (Neither of those is guaranteed by anything.)

					der Mouse

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