{"id":323,"date":"2016-01-16T15:13:32","date_gmt":"2016-01-16T15:13:32","guid":{"rendered":"http:\/\/nenadnoveljic.com\/blog\/?p=323"},"modified":"2016-01-16T15:13:32","modified_gmt":"2016-01-16T15:13:32","slug":"how-to-implement-file-locking-in-perl","status":"publish","type":"post","link":"https:\/\/nenadnoveljic.com\/blog\/how-to-implement-file-locking-in-perl\/","title":{"rendered":"How to implement file locking in Perl"},"content":{"rendered":"<h4>Introduction<\/h4>\n<p>The serialization of read\/write access should be used to prevent file corruption which might be caused by a concurrent write operation. Furthermore, it should also be prevented that we read the file until all current changes made by concurrent processes are completed.<\/p>\n<p>The function <a href=\"http:\/\/perldoc.perl.org\/functions\/flock.html\">flock<\/a> can be used to implemented the functionality.<\/p>\n<p>In this blog post I will provide a couple of code examples for the use cases that are missing in the original documentation.<\/p>\n<h4>Lock with wait<\/h4>\n<p>In the first example we will try to acquire a lock and wait until we get it. We&#8217;ll run the same program concurrently in two different sessions and observe the output.<\/p>\n<pre><code>#!\/u00\/oracle\/orabase\/local\/perl\/bin\/perl -w\r\nuse strict;\r\nuse Fcntl qw(:flock );\r\nopen my $fh , '&gt;' , '\/tmp\/test_file' ;\r\nprint localtime . \": trying to set lock...\\n\" ; \r\n<span style=\"color: #ff0000;\">flock( $fh , LOCK_EX) ;<\/span>\r\nprint localtime . \": lock set, sleeping...\\n\" ; \r\nsleep 10 ;\r\n<span style=\"color: #ff0000;\">flock( $fh , LOCK_UN) ;\r\n<\/span>print localtime . \": lock removed, sleeping...\\n\" ; \r\n<\/code><\/pre>\n<p>The second session waited indeed for the lock until it was released by the first session:<\/p>\n<pre><code>the output of the first process:\r\nSat Jan 16 08:19:24 2016: trying to set lock...\r\nSat Jan 16 08:19:24 2016: lock set, sleeping...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:19:34 2016: lock removed, sleeping...\r\n<\/span><\/code><\/pre>\n<pre><code>the output of the second process:\r\nSat Jan 16 08:19:26 2016: trying to set lock...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:19:34 2016: lock set, sleeping...\r\n<\/span>Sat Jan 16 08:19:44 2016: lock removed, sleeping...\r\n<\/code><\/pre>\n<p>In the second example we will explore what happens if the lock holder ends before releasing the lock:<\/p>\n<pre><code>#!\/u00\/oracle\/orabase\/local\/perl\/bin\/perl -w\r\nuse strict;\r\nuse Fcntl qw(:flock );\r\nopen my $fh , '&gt;' , '\/tmp\/test_file' ;\r\nprint localtime . \": trying to set lock...\\n\" ; \r\n<span style=\"color: #ff0000;\">flock( $fh , LOCK_EX) ;\r\n<\/span>print localtime . \": lock set, sleeping...\\n\" ; \r\nsleep 10 ;\r\nprint localtime . \": exiting...\\n\" ;\r\n<\/code><\/pre>\n<p>As expected, the lock gets released when the process stops:<\/p>\n<pre><code>the output of the first process\r\nSat Jan 16 08:05:06 2016: trying to set lock...\r\nSat Jan 16 08:05:06 2016: lock set, sleeping...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:05:16 2016: exiting...\r\n<\/span><\/code><\/pre>\n<pre><code>the output of the second process\r\nSat Jan 16 08:05:08 2016: trying to set lock...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:05:16 2016: lock set, sleeping...\r\n<\/span>Sat Jan 16 08:05:26 2016: exiting...\r\n<\/code><\/pre>\n<h4>Lock without wait<\/h4>\n<p>Finally, we&#8217;ll acquire the lock but this time won&#8217;t be waiting for the lock if it cannot be acquired. This is accomplished by applying the bitwise OR on the operation parameter (LOCK_EX | LOCK_NB) of the function flock.<\/p>\n<pre><code>#!\/u00\/oracle\/orabase\/local\/perl\/bin\/perl -w\r\nuse strict;\r\nuse Fcntl qw(:flock );\r\nopen my $fh , '&gt;' , '\/tmp\/test_file' ;\r\nprint localtime . \": trying to set lock...\\n\" ; \r\n<span style=\"color: #ff0000;\">my $succeeded = flock( $fh , LOCK_EX | LOCK_NB ) ;\r\n<\/span>if ( $succeeded ) {\r\n  print localtime . \": lock set, sleeping...\\n\" ;  \r\n} else {\r\n  print localtime . \": couldn't acquire lock, sleeping...\\n\" ;\r\n}\r\nsleep 10 ;\r\nflock( $fh , LOCK_UN) ;\r\nprint localtime . \": lock removed, sleeping...\\n\" ; \r\n<\/code><\/pre>\n<pre><code>the output of the first process\r\nSat Jan 16 08:27:52 2016: trying to set lock...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:27:52 2016: lock set, sleeping...\r\n<\/span>Sat Jan 16 08:28:02 2016: lock removed, sleeping...\r\n<\/code><\/pre>\n<pre><code>the output of the second process\r\nSat Jan 16 08:27:53 2016: trying to set lock...\r\n<span style=\"color: #ff0000;\">Sat Jan 16 08:27:53 2016: couldn't acquire lock, sleeping...\r\n<\/span>Sat Jan 16 08:28:03 2016: lock removed, sleeping...\r\n<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"<p>The post explains how to implement file locking in Perl <a href=\"https:\/\/nenadnoveljic.com\/blog\/how-to-implement-file-locking-in-perl\/\" class=\"more-link\">Continue Reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[4],"tags":[],"class_list":["post-323","post","type-post","status-publish","format-standard","hentry","category-perl"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.6 - 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