[PATCH] Improved documentation for flock() in perlfunc.pod

[Paul Fenwick <pjf@perltraining.com.au>]

G'day p5p,

The example for flock() in perlfunc.pod contains a few errors and
not-so-good practices.  The attached patch changes the following:

    * We now consistently use scalar filehandles rather than MBOX
    * flock() and seek() are checked for errors.
    * The SEEK_END constant is used, rather than a magic number.

Cheerio,

	Paul

-- 
Paul Fenwick <pjf@perltraining.com.au> | http://perltraining.com.au/
Director of Training                   | Ph:  +61 3 9354 6001
Perl Training Australia                | Fax: +61 3 9354 2681
>From 49330409ac3bc46823d0cbf02865f474ed198126 Mon Sep 17 00:00:00 2001
From: Paul Fenwick <pjf@perltraining.com.au>
Date: Mon, 15 Sep 2008 17:42:41 +1000
Subject: [PATCH] Improved documentation for flock() in perlfunc.pod

    * We now consistently use scalar filehandles rather than MBOX
    * flock() and seek() are checked for errors.
    * The SEEK_END constant is used, rather than a magic number
---
 pod/perlfunc.pod |   18 ++++++++++--------
 1 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index e5518af..90e6b9b 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -1936,25 +1936,27 @@ perl.
 
 Here's a mailbox appender for BSD systems.
 
-    use Fcntl ':flock'; # import LOCK_* constants
+    use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
 
     sub lock {
-	flock(MBOX,LOCK_EX);
-	# and, in case someone appended
-	# while we were waiting...
-	seek(MBOX, 0, 2);
+        my ($fh) = @_;
+	flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
+
+	# and, in case someone appended while we were waiting...
+	seek(MBOX, 0, SEEK_END) or die "Cannot seek - $!\n";
     }
 
     sub unlock {
-	flock(MBOX,LOCK_UN);
+        my ($fh) = @_;
+	flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
     }
 
     open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
 	    or die "Can't open mailbox: $!";
 
-    lock();
+    lock($mbox);
     print $mbox $msg,"\n\n";
-    unlock();
+    unlock($mbox);
 
 On systems that support a real flock(), locks are inherited across fork()
 calls, whereas those that must resort to the more capricious fcntl()
-- 
1.5.2.2

---

Follow-Ups from:

Roland Giersig <rgiersig@cpan.org>

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