Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!accuvax.nwu.edu!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.wizards
Subject: Re: unix question about stdin
Message-ID: <17062@mimsy.UUCP>
Date: 22 Apr 89 19:34:23 GMT
References: <3398@udccvax1.acs.udel.EDU> <5414@cs.Buffalo.EDU>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 48

In article <5414@cs.Buffalo.EDU> ugkamins@sunybcs.uucp (John Kaminski) writes:
>The original post wanted to know how to reconnect the stdin to the terminal
>after it had been attached to a file, assumedly through redirection.  What I
>just tried on a BSD system, and works:
>
>  fclose(stdin);
>  *stdin = *fopen("/dev/tty", "r");

This is a horrible thing to do.  (It does not work in some SysVs and
SunOSes.)  The original question indicated that the program was to

	read from stdin
	switch to an alternate file
	switch back to stdin

and that this was all done from within a single C program being written
by the questioner.  The simplest approach is:

	main()
	{
		FILE *fp;

		read_commands(stdin);
		if ((fp = fopen(other_file, "r")) == NULL)
			... error ...
		read_commands(fp);
		(void) fclose(fp);
		read_commands(stdin);
	}

If the `switch to a file' is due to a particular command, read_commands()
can maintain its own stack of files (possibly by being recursive).

It is not a good idea to assign to or through any of the `standard'
streams.  Instead, declare your own pointer and make it point to one
of the standard streams.  There is nothing wrong with

	register FILE *fp = stdin;
	... work with fp ...

but there is with

	fileno(stdin) = expression;

and the like.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris