NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/unix/programm/6077

Internet Message Format | 1993-01-28 | 1.6 KB

Path: sparky!uunet!ulowell!m2c!bu.edu!cvbnet!hsiegel
From: [e-mail] (mailhost)
Newsgroups: comp.unix.programmer
Subject: Re: More advanced popen()
Message-ID: <[e-mail]>
Date: 28 Jan 93 15:41:32 GMT
References: <[e-mail]>
Sender: [e-mail]
Lines: 30

From article <[e-mail]>, by [e-mail] (L Lundman):
>
> After finding out that popen() only read or wrote to a process and not both
> I was told I had to write one of my own if I wanted one. Can someone
> inform me why this code doesn't seem to work?
>
> (Knowing my luck, it's some dumb mistake ;) )
...
    [code deleted to save space]
...

In the first place, you would need _two_ pipes, one to write to the
child process and one to read from it.  This can be done, and works.

What frequently munges attempts to do this is a noxious aspect of the
way pipes work.  I quote from "man pipe" on my SunOS 4.1.3 system:

* BUGS
*    Should more than {PIPE_BUF} bytes be necessary in  any  pipe
*    among a loop of processes, deadlock will occur.

PIPE_BUF, as I recall (I haven't looked it up lately) is 256 (512?)
bytes.  This causes problems surprising often.  If you can guarantee
that you won't exceed this, you may be able to implement a double-
ended version of popen.

Howard Siegel                   [e-mail]
Computervision                  [e-mail]
Bedford, Mass.                  [e-mail]
[phone] x4064            cvbnet![e-mail]