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From: gorpong@math-cs.kent.edu (Gordon C. Galligher)
Newsgroups: comp.lang.perl
Subject: Perl 3.0 PL28 problems with pipe()
Keywords: Pipe call is a problem
Message-ID: <1990Sep4.205514.13088@math-cs.kent.edu>
Date: 4 Sep 90 20:55:14 GMT
Organization: Kent State University
Lines: 100

There is some synchronization problems when dealing with pipes via the
pipe() call in Perl.  Below are code examples first in PERL and then one
in C.  The one in C works just like you would expect:
	IN THE PARENT:  PID == ...
	IN THE CHILD:  PID == 0
	CHILD-SEND:  25
	PARENT REC'd <25>
	PARENT-SEND:  255
	CHILD REC'd <255>
The Perl example, on the other hand, does this:
	IN THE CHILD:  PID == 0
	CHILD-SEND:  25
	IN THE PARENT:  PID == ...
And just sits there.

I do get some output when I change the $val = <DESC?_0> calls below to
read() calls, but I need to know exactly how many bytes are coming down
the pipe in order to not hang.  What is so drastically different between
the C program and Perl that makes it not work?  Any help would be greatly
appreciated.  Is there a way to specially treat descriptors created with
pipe() to be non-buffered?  (I even tried $| = 1 on a read descriptor,
and it had no effect (but it did not fail).)

In order to totally emulate the effect of C, I would need to pack a variable
with an integer, and force that down the pipe first, and then unpack it to
get the number of bytes coming, and so on.  That is a lot of work.  Granted,
if it needs to be done, it needs to be done, but it would make me think 
twice before choosing Perl as the tool to use.

		-- Gordon.
=========================snip here--PERL--====================================

pipe(DESC1_0, DESC1_1);
pipe(DESC2_0, DESC2_1);
$id = fork();
$| = 1;
if ( $id == 0 )
{
	sleep(1);
	close(DESC1_0);
	close(DESC2_1);
	select(DESC1_1); $| = 1; select(STDOUT);
	print "IN THE CHILD:  PID == $id\n";
	$mval = 25;
	print "CHILD-SEND:  $mval\n";
	print DESC1_1 $mval;
	$mval = <DESC2_0>;
	print "CHILD REC'd  <$mval>\n";
	exit(0);
}
else
{
	close(DESC1_1);
	close(DESC2_0);
	select(DESC2_1); $| = 1; select(STDOUT);
	print "IN THE PARENT:  PID == $id\n";
	$val = <DESC1_0>;
	print "PARENT REC'd <$val>\n";
	$val = 255;
	print "PARENT-SEND:  $val\n";
	print DESC2_1 $val;
	exit(0);
}
=========================snip here--C--=======================================
#include        <stdio.h>

main()
{
	int desc1[2], desc2[2];
	int pid, msg1, msg2;
	char buf[256];

	pipe(desc1);
	pipe(desc2);
	switch(pid = fork())
	{
		case 0:
			close(desc1[0]);
			close(desc2[1]);
			printf("IN THE CHILD:  PID == %d\n", pid);
			msg1 = 25;
			printf("CHILD-SEND:  %d\n", msg1);
			write(desc1[1], & msg1, sizeof(int));
			read(desc2[0], & msg2, sizeof(int));
			printf("CHILD REC'd <%d>\n", msg2);
			exit(0);

		default:
			close(desc1[1]);
			close(desc2[0]);
			printf("IN THE PARENT:  PID == %d\n", pid);
			read(desc1[0], & msg1, sizeof(int));
			printf("PARENT REC'd <%d>\n", msg1);
			msg2 = 255;
			printf("PARENT-SEND:  %d\n", msg2);
			write(desc2[1], & msg2, sizeof(int));
			exit(0);
	} /* SWITCH */
} /* MAIN */
===================================done snip==============================