[pups] screen 3.9.9 vs. 2.11BSD write() to fifo and/or select() on socket

Steven M. Schultz sms at 2BSD.COM
Fri Feb 15 02:33:42 AEST 2002

Hi -

> From: David W Talmage <talmage at cmf.nrl.navy.mil>
> I thought that I'd read in the FM that they do.  I see now that I was 
> mistaken,perhaps delusional.  I see now that <sys/stat.h> contains the gospel:
> #define S_IFIFO 0010000         /* named pipe (fifo) - Not used by 2.11BSD */


	Adding FIFOs to the kernel would make an interesting project though -
	perhaps when I become inspired/motivated I'll give it a try.

> Sounds like I'm in for some deep hacking if I continue with this.  Will 
> overlays help me here?

	Overlays will help if the code comes out to more than 64KB.   Alas,
	overlays will _not_ help the dataspace requirements.   The last time
	I looked at 'screen' I saw things like "char buf[32768];' sprinkled
	thru the code.   So you might be in for some serious hacking to trim
	back the sizes of arrays/buffers/etc.   It probably would also be
	a good idea to 'string'ify the program (there are tools to assist
	doing this - take a look at how sendmail and lint are built).

> I wonder if setitimer() will fare any better.  alarm() is obsolete.

	The gospel according to /usr/src/lib/libc/gen/alarm.c says:

 * Backwards compatible alarm.
#include <sys/time.h>
#include <unistd.h>

unsigned int
	unsigned int secs;
	struct itimerval it, oitv;
	register struct itimerval *itp = ⁢

	itp->it_value.tv_sec = secs;
	itp->it_value.tv_usec = 0;
	if (setitimer(ITIMER_REAL, itp, &oitv) < 0)
		return (-1);
	if (oitv.it_value.tv_usec)
	return (oitv.it_value.tv_sec);


> I'll send a progress report if I decide to continue this project.  Thanks for 
> your help, Mr. Schultz.

	What I think you're seeing is the race condition that fork() has - 
	there is no guarantee which process (parent or child) runs first.
	The test program does the fork() and the child runs to completion
	before the parent enters the select().  At that point the parent
	will wait until the alarm goes off.

	One thing you might try is create two separate programs.  One would
	create the socket and wait for the other program to connect and send
	a string.  If that works it shows that the UNIX domainsockets are
	working as expected and that the fork() race is indeed the problem.
	IF the client/server tests fail then there is something wrong in the
	UNIX domain socket handling that needs to be addressed.

	Good Luck!

	Steven Schultz

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