Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!unisoft!rembo
From: rembo@unisoft.UUCP (Tony Rems)
Newsgroups: comp.unix.programmer
Subject: Re: how to put a program into a .plan file
Message-ID: <3149@unisoft.UUCP>
Date: 5 Oct 90 22:36:32 GMT
References: <978@bbt.UUCP> <1990Sep26.201406.12663@athena.mit.edu> <1990Sep28.154733.26346@cbnewsj.att.com>
Reply-To: rembo@unisoft.UUCP (Tony Rems)
Lines: 50

In article <1990Sep28.154733.26346@cbnewsj.att.com> dwex@cbnewsj.att.com (david.e.wexelblat) writes:
>In article <1990Sep26.201406.12663@athena.mit.edu>, jik@athena.mit.edu (Jonathan I. Kamens) writes:
>> In article <978@bbt.UUCP>, rgs@bbt.UUCP (steinbeiser) writes:
>> |> Ok, how do you check the status of the file to see if it has
>> |> been open by someone else?  Its not obvious how (or if) stat() would do this.
>> |> Also, would the C program have to be running constantly in the background
>> |> someplace?
>> 
>> 1. You open the pipe for write, and do a select() on it, and when you get an
>>    OK to write from select(), you know that someone has opened the other end.
>> 
>
>Or, in System V, quoting from the open(2) man page:
>
>	When opening a FIFO with O_RDONLY or O_WRONLY set:
>
>	If O_NDELAY is set:
>		[ not relevant ]
>
>	If O_NDELAY is clear:
>		An open for reading-only will block until a process
>		opens the file for writing.  An open for writing-only
>		will block until a process opens the file for reading.
>
>So all you have to do is make .plan a FIFO, and have a program open
>it for writing (the program will have to be in an infinite loop, though).
>

There are a bunch of ways to handle this in C.  Doing a select is
really overkill.  The simplest way is to have a loop that looks 
something like:

	make fifo;
	loop forever;
		open FIFO for writing	/* This will block until 
					   someon opens it for reading */
		fork()
		if in child:
			exec a program
		if in parent 
			close FIFO
			wait for child;
		

I have a program called plan which takes one argument, the 
name of a program to exec as your plan.  This version inherits
your environment on the exec.  If anyone would like a copy
of the source, drop me a note.

-Tony