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From: josh@hi.UUCP
Newsgroups: comp.unix.questions
Subject: Re: A couple questions
Message-ID: <4173@hi.uucp>
Date: Tue, 14-Apr-87 13:57:39 EST
Article-I.D.: hi.4173
Posted: Tue Apr 14 13:57:39 1987
Date-Received: Wed, 15-Apr-87 05:47:02 EST
References: <3164@jade.BERKELEY.EDU>
Reply-To: josh@hi.UUCP (Josh Siegel)
Organization: U. of New Mexico, Albuquerque
Lines: 74

In article <3164@jade.BERKELEY.EDU> marcp@beryl.berkeley.edu (Marc M. Pack) writes:
>Hello!  I've a couple of questions in UNIX 4.2.
>
>1.  How do programs like "more" distinguish between text files and
>	executable files?  Hopefully, there's something surer than
>	just taking a sample of a file and testing it. 

I think this is the way it does it. First it stat's it to see
if it is a directory then it reads in some of it to
see if it is ascii.

>
>2.  Is it possible to, while in a C program, call another program and
>	put it into the background?  Actually, I know it's possible,
>	'cause I can do it with a line like:
>
>		system("cat textfile &");
>
>	This won't work, however, if I try to "more" the file instead.
>	What determines what can be put in the background and what can't?
>	Is there some way to run a program from within a program, and have
> 	it return upon completion to the original program besides "system"?
>	(The execl series, of course, doesn't return.)

This is a loaded question and can become far more complicated then
you know...

The command more(1) checks to see if it is run on
a terminal (a tty).  This is why "more" will not run in a system() call.
You CAN put almost anything in the background, it is just harder
if it requires to be run on a terminal (talk(1), more(1), etc).

A short source for system might look like (Hmm...):

system(string)
char string[];
{
	if(vfork()) {
		return(wait(0));
	} else {
		... parse string ...
		execl(prog,args);
	}
}
/* This is not a complete system() but I am writting it from memory. */

If you wish to run a more(1) or a talk(1) or rogue(6), it requires
 the program to open a pty.  This makes the program (more) think
 it is on a terminal while it really is not.  This is how
 rlogin works.  Try the following:

	1) Set up a .rhosts file in your home directory
	2) rsh to that account using the following command:

	  % rsh {machine} /bin/csh -i

	3) Try doing a "more" of a file.  Gets sick eh?
	   This is one reason why pty's were invented.

I have a whole library of routines for networking, ptys, etc
 that I can post if there are any requests for it...

>
>Thanks for your time,
>
>							Marc M. Pack


	Hope I was helpful
			--Josh Siegel
-- 
Josh Siegel		(siegel@hc.dspo.gov)
                        (505) 277-2497  (Home)
		I'm a mathematician, not a programmer!