From: Rafael Garcia-Suarez <rgarciasuarez@gmail.com>
Date: Sun, 25 Jun 2006 22:23:46 +0000 (+0000)
Subject: Nits about lock() in perlthrtut by Daniel Frederick Crisman,
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=0b390a8204ce53b7d71cfce2604bf3aa78e34f74;p=p5sagit%2Fp5-mst-13.2.git

Nits about lock() in perlthrtut by Daniel Frederick Crisman,
plus other nits by rgs

p4raw-id: //depot/perl@28427
---

diff --git a/pod/perlthrtut.pod b/pod/perlthrtut.pod
index 8e19411..8e5b2a8 100644
--- a/pod/perlthrtut.pod
+++ b/pod/perlthrtut.pod
@@ -11,7 +11,7 @@ and any data sharing between threads must be explicit.
 
 There is another older Perl threading flavour called the 5.005 model,
 unsurprisingly for 5.005 versions of Perl.  The old model is known to
-have problems, deprecated, and will probably be removed around release
+have problems, is deprecated, and support for it will be removed in release
 5.10. You are strongly encouraged to migrate any existing 5.005
 threads code to the new model as soon as possible.
 
@@ -219,12 +219,12 @@ threads.
 
 The simplest, most straightforward way to create a thread is with new():
 
-    use threads; 
+    use threads;
 
     $thr = threads->new(\&sub1);
 
-    sub sub1 { 
-        print "In the thread\n"; 
+    sub sub1 {
+        print "In the thread\n";
     }
 
 The new() method takes a reference to a subroutine and creates a new
@@ -235,17 +235,17 @@ If you need to, your program can pass parameters to the subroutine as
 part of the thread startup.  Just include the list of parameters as
 part of the C<threads::new> call, like this:
 
-    use threads; 
+    use threads;
 
-    $Param3 = "foo"; 
-    $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3); 
-    $thr = threads->new(\&sub1, @ParamList); 
+    $Param3 = "foo";
+    $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3);
+    $thr = threads->new(\&sub1, @ParamList);
     $thr = threads->new(\&sub1, qw(Param1 Param2 Param3));
 
-    sub sub1 { 
-        my @InboundParameters = @_; 
-        print "In the thread\n"; 
-        print "got parameters >", join("<>", @InboundParameters), "<\n"; 
+    sub sub1 {
+        my @InboundParameters = @_;
+        print "In the thread\n";
+        print "got parameters >", join("<>", @InboundParameters), "<\n";
     }
 
 
@@ -262,12 +262,12 @@ Since threads are also subroutines, they can return values.  To wait
 for a thread to exit and extract any values it might return, you can
 use the join() method:
 
-    use threads; 
+    use threads;
 
     $thr = threads->new(\&sub1);
 
-    @ReturnData = $thr->join; 
-    print "Thread returned @ReturnData"; 
+    @ReturnData = $thr->join;
+    print "Thread returned @ReturnData";
 
     sub sub1 { return "Fifty-six", "foo", 2; }
 
@@ -292,19 +292,19 @@ In this case, you use the detach() method.  Once a thread is detached,
 it'll run until it's finished, then Perl will clean up after it
 automatically.
 
-    use threads; 
+    use threads;
 
     $thr = threads->new(\&sub1); # Spawn the thread
 
     $thr->detach; # Now we officially don't care any more
 
     sub sub1 {
-        $a = 0; 
-        while (1) { 
-            $a++; 
-            print "\$a is $a\n"; 
-            sleep 1; 
-        } 
+        $a = 0;
+        while (1) {
+            $a++;
+            print "\$a is $a\n";
+            sleep 1;
+        }
     }
 
 Once a thread is detached, it may not be joined, and any return data
@@ -375,12 +375,12 @@ explained in the next section.
 While threads bring a new set of useful tools, they also bring a
 number of pitfalls.  One pitfall is the race condition:
 
-    use threads; 
+    use threads;
     use threads::shared;
 
-    my $a : shared = 1; 
-    $thr1 = threads->new(\&sub1); 
-    $thr2 = threads->new(\&sub2); 
+    my $a : shared = 1;
+    $thr1 = threads->new(\&sub1);
+    $thr2 = threads->new(\&sub2);
 
     $thr1->join;
     $thr2->join;
@@ -401,11 +401,11 @@ nothing has happened to the shared data between the time you access it
 and the time you update it.  Even this simple code fragment has the
 possibility of error:
 
-    use threads; 
+    use threads;
     my $a : shared = 2;
     my $b : shared;
     my $c : shared;
-    my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; }); 
+    my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; });
     my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; });
     $thr1->join;
     $thr2->join;
@@ -433,10 +433,10 @@ techniques such as queues, which remove some of the hard work involved.
 
 =head2 Controlling access: lock()
 
-The lock() function takes a shared variable and puts a lock on it.  
+The lock() function takes a shared variable and puts a lock on it.
 No other thread may lock the variable until the variable is unlocked
 by the thread holding the lock. Unlocking happens automatically
-when the locking thread exits the outermost block that contains
+when the locking thread exits the block that contains the call to the
 C<lock()> function.  Using lock() is straightforward: this example has
 several threads doing some calculations in parallel, and occasionally
 updating a running total:
@@ -466,16 +466,15 @@ updating a running total:
     $thr3->join;
     print "total=$total\n";
 
-
 lock() blocks the thread until the variable being locked is
 available.  When lock() returns, your thread can be sure that no other
-thread can lock that variable until the outermost block containing the
+thread can lock that variable until the block containing the
 lock exits.
 
 It's important to note that locks don't prevent access to the variable
 in question, only lock attempts.  This is in keeping with Perl's
 longstanding tradition of courteous programming, and the advisory file
-locking that flock() gives you.  
+locking that flock() gives you.
 
 You may lock arrays and hashes as well as scalars.  Locking an array,
 though, will not block subsequent locks on array elements, just lock
@@ -509,7 +508,7 @@ lock() on the variable goes out of scope. For example:
     } # nothing happens here
 
 Note that there is no unlock() function - the only way to unlock a
-variable is to allow it to go out of scope.  
+variable is to allow it to go out of scope.
 
 A lock can either be used to guard the data contained within the variable
 being locked, or it can be used to guard something else, like a section
@@ -525,19 +524,19 @@ properly is the key to safe shared data.  Unfortunately, locks aren't
 without their dangers, especially when multiple locks are involved.
 Consider the following code:
 
-    use threads; 
-
-    my $a : shared = 4; 
-    my $b : shared = "foo"; 
-    my $thr1 = threads->new(sub { 
-        lock($a); 
-        sleep 20; 
-        lock($b); 
-    }); 
-    my $thr2 = threads->new(sub { 
-        lock($b); 
-        sleep 20; 
-        lock($a); 
+    use threads;
+
+    my $a : shared = 4;
+    my $b : shared = "foo";
+    my $thr1 = threads->new(sub {
+        lock($a);
+        sleep 20;
+        lock($b);
+    });
+    my $thr2 = threads->new(sub {
+        lock($b);
+        sleep 20;
+        lock($a);
     });
 
 This program will probably hang until you kill it.  The only way it
@@ -573,20 +572,20 @@ end and take it out the other without having to worry about
 synchronization issues.  They're pretty straightforward, and look like
 this:
 
-    use threads; 
+    use threads;
     use Thread::Queue;
 
-    my $DataQueue = Thread::Queue->new; 
-    $thr = threads->new(sub { 
-        while ($DataElement = $DataQueue->dequeue) { 
+    my $DataQueue = Thread::Queue->new;
+    $thr = threads->new(sub {
+        while ($DataElement = $DataQueue->dequeue) {
             print "Popped $DataElement off the queue\n";
-        } 
-    }); 
+        }
+    });
 
-    $DataQueue->enqueue(12); 
-    $DataQueue->enqueue("A", "B", "C"); 
-    $DataQueue->enqueue(\$thr); 
-    sleep 10; 
+    $DataQueue->enqueue(12);
+    $DataQueue->enqueue("A", "B", "C");
+    $DataQueue->enqueue(\$thr);
+    sleep 10;
     $DataQueue->enqueue(undef);
     $thr->join;
 
@@ -615,29 +614,29 @@ semaphore's current count would decrement below zero.  This program
 gives a quick demonstration:
 
     use threads;
-    use Thread::Semaphore; 
+    use Thread::Semaphore;
 
-    my $semaphore = new Thread::Semaphore; 
+    my $semaphore = new Thread::Semaphore;
     my $GlobalVariable : shared = 0;
 
-    $thr1 = new threads \&sample_sub, 1; 
-    $thr2 = new threads \&sample_sub, 2; 
+    $thr1 = new threads \&sample_sub, 1;
+    $thr2 = new threads \&sample_sub, 2;
     $thr3 = new threads \&sample_sub, 3;
 
-    sub sample_sub { 
-        my $SubNumber = shift @_; 
-        my $TryCount = 10; 
-        my $LocalCopy; 
-        sleep 1; 
-        while ($TryCount--) { 
-            $semaphore->down; 
-            $LocalCopy = $GlobalVariable; 
-            print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n"; 
-            sleep 2; 
-            $LocalCopy++; 
-            $GlobalVariable = $LocalCopy; 
-            $semaphore->up; 
-        } 
+    sub sample_sub {
+        my $SubNumber = shift @_;
+        my $TryCount = 10;
+        my $LocalCopy;
+        sleep 1;
+        while ($TryCount--) {
+            $semaphore->down;
+            $LocalCopy = $GlobalVariable;
+            print "$TryCount tries left for sub $SubNumber (\$GlobalVariable is $GlobalVariable)\n";
+            sleep 2;
+            $LocalCopy++;
+            $GlobalVariable = $LocalCopy;
+            $semaphore->up;
+        }
     }
 
     $thr1->join;
@@ -725,15 +724,15 @@ a thread to give up the processor.
 Perl's threading package provides the yield() function that does
 this. yield() is pretty straightforward, and works like this:
 
-    use threads; 
+    use threads;
 
     sub loop {
-	    my $thread = shift;
-	    my $foo = 50;
-	    while($foo--) { print "in thread $thread\n" }
-	    threads->yield;
-	    $foo = 50;
-	    while($foo--) { print "in thread $thread\n" }
+	my $thread = shift;
+	my $foo = 50;
+	while($foo--) { print "in thread $thread\n" }
+	threads->yield;
+	$foo = 50;
+	while($foo--) { print "in thread $thread\n" }
     }
 
     my $thread1 = threads->new(\&loop, 'first');
@@ -771,7 +770,7 @@ thread that's created.
 
 =head2 Are These Threads The Same?
 
-The equal() method takes two thread objects and returns true 
+The equal() method takes two thread objects and returns true
 if the objects represent the same thread, and false if they don't.
 
 Thread objects also have an overloaded == comparison so that you can do
@@ -783,12 +782,12 @@ C<< threads->list >> returns a list of thread objects, one for each thread
 that's currently running and not detached.  Handy for a number of things,
 including cleaning up at the end of your program:
 
-    # Loop through all the threads 
-    foreach $thr (threads->list) { 
-        # Don't join the main thread or ourselves 
-        if ($thr->tid && !threads::equal($thr, threads->self)) { 
-            $thr->join; 
-        } 
+    # Loop through all the threads
+    foreach $thr (threads->list) {
+        # Don't join the main thread or ourselves
+        if ($thr->tid && !threads::equal($thr, threads->self)) {
+            $thr->join;
+        }
     }
 
 If some threads have not finished running when the main Perl thread
@@ -813,7 +812,7 @@ things we've covered.  This program finds prime numbers using threads.
     11
     12 for my $i ( 3 .. 1000 ) {
     13     $stream->enqueue($i);
-    14 } 
+    14 }
     15
     16 $stream->enqueue(undef);
     17 $kid->join;
@@ -830,7 +829,7 @@ things we've covered.  This program finds prime numbers using threads.
     28            print "Found prime $num\n";
     29	              $kid = new threads(\&check_num, $downstream, $num);
     30         }
-    31     } 
+    31     }
     32     $downstream->enqueue(undef) if $kid;
     33     $kid->join		if $kid;
     34 }