From: Philip Newton <pne@cpan.org>
Date: Wed, 25 Jun 2003 17:50:22 +0000 (+0200)
Subject: Re: P and V
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=14180c03bf5269934b197b82e34fd4696ad1053d;p=p5sagit%2Fp5-mst-13.2.git

Re: P and V
From: "Philip Newton" <pnewton@gmx.de>
Message-ID: <3EF9E0DE.4786.16A6869@localhost>

p4raw-id: //depot/perl@19853
---

diff --git a/lib/Thread/Semaphore.pm b/lib/Thread/Semaphore.pm
index d3ebe63..70441ab 100644
--- a/lib/Thread/Semaphore.pm
+++ b/lib/Thread/Semaphore.pm
@@ -12,14 +12,14 @@ Thread::Semaphore - thread-safe semaphores
 
     use Thread::Semaphore;
     my $s = new Thread::Semaphore;
-    $s->up;	# Also known as the semaphore V -operation.
+    $s->down;	# Also known as the semaphore P operation.
     # The guarded section is here
-    $s->down;	# Also known as the semaphore P -operation.
+    $s->up;	# Also known as the semaphore V operation.
 
     # The default semaphore value is 1.
     my $s = new Thread::Semaphore($initial_value);
+    $s->down($down_value);
     $s->up($up_value);
-    $s->down($up_value);
 
 =head1 DESCRIPTION
 
@@ -29,7 +29,7 @@ control access to anything you care to use them for.
 
 Semaphores don't limit their values to zero or one, so they can be used to
 control access to some resource that there may be more than one of. (For
-example, filehandles). Increment and decrement amounts aren't fixed at one
+example, filehandles.) Increment and decrement amounts aren't fixed at one
 either, so threads can reserve or return multiple resources at once.
 
 =head1 FUNCTIONS AND METHODS
@@ -53,6 +53,10 @@ below zero, this method will block until such time that the semaphore's
 count is equal to or larger than the amount you're C<down>ing the
 semaphore's count by.
 
+This is the semaphore "P operation" (the name derives from the Dutch
+word "pak", which means "capture" -- the semaphore operations were
+named by the late Dijkstra, who was Dutch).
+
 =item up
 
 =item up NUMBER
@@ -62,6 +66,9 @@ or by one if no number has been specified. This will unblock any thread blocked
 trying to C<down> the semaphore if the C<up> raises the semaphore count
 above the amount that the C<down>s are trying to decrement it by.
 
+This is the semaphore "V operation" (the name derives from the Dutch
+word "vrij", which means "release").
+
 =back
 
 =cut