Path: utzoo!utgpu!watserv1!watmath!att!cbnewsj!dwex
From: dwex@cbnewsj.att.com (david.e.wexelblat)
Newsgroups: comp.unix.programmer
Subject: Re: how to put a program into a .plan file
Summary: More trivial in System V
Message-ID: <1990Sep28.154733.26346@cbnewsj.att.com>
Date: 28 Sep 90 15:47:33 GMT
References: <978@bbt.UUCP> <1990Sep26.201406.12663@athena.mit.edu>
Organization: AT&T Bell Laboratories
Lines: 31

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).

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