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<HTML><HEAD><TITLE>Recipe 14.5. Locking DBM Files (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:42:49Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch14_01.htm"TITLE="14. Database Access"><LINKREL="prev"HREF="ch14_05.htm"TITLE="14.4. Merging DBM Files"><LINKREL="next"HREF="ch14_07.htm"TITLE="14.6. Sorting Large DBM Files"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch14_05.htm"TITLE="14.4. Merging DBM Files"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 14.4. Merging DBM Files"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch14_01.htm"TITLE="14. Database Access"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch14_07.htm"TITLE="14.6. Sorting Large DBM Files"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 14.6. Sorting Large DBM Files"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch14-chap14_locking_0">14.5. Locking DBM Files</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-460">Problem<ACLASS="indexterm"NAME="ch14-idx-1000004957-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004957-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004957-2"></A></A></H3><PCLASS="para">You need several concurrently running programs to have simultaneous access to a DBM file.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-466">Solution</A></H3><PCLASS="para">Either use the DBM implementation's locking mechanism if it has one, lock the file with <CODECLASS="literal">flock</CODE>, or use an auxiliary locking scheme as in <ACLASS="xref"HREF="ch07_22.htm"TITLE="Program: netlock">Recipe 7.21</A>.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-472">Discussion</A></H3><PCLASS="para">With SDBM or NDBM, you can't do much to lock the database itself. You must devise an auxiliary locking scheme using an extra lockfile.</P><PCLASS="para"><ACLASS="indexterm"NAME="ch14-idx-1000004958-0"></A>GDBM uses the concept of readers and writers: either many readers or one solitary writer may have a GDBM file open at any given time. You specify whether you're a reader or a writer when you open it. This can be annoying.</P><PCLASS="para">Version 1 of Berkeley DB gives you access to the file descriptor of the open database, allowing you to <CODECLASS="literal">flock</CODE> it. The lock applies to the database as a whole, not to individual records. Version 2 implements its own full transaction system with locking.</P><PCLASS="para"><ACLASS="xref"HREF="ch14_06.htm#ch14-25344"TITLE="dblockdemo">Example 14.3</A> shows an example of locking a database using Berkeley DB. Run this repeatedly in the background to see locks granted in proper order.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch14-25344">Example 14.3: dblockdemo</A></H4><PRECLASS="programlisting">#!/usr/bin/perl# <ACLASS="indexterm"NAME="ch14-idx-1000005185-0"></A>dblockdemo - demo locking dbm databasesuse DB_File;use strict;sub LOCK_SH { 1 } # In case you don't havesub LOCK_EX { 2 } # the standard Fcntl module. Yousub LOCK_NB { 4 } # should, but who can tellsub LOCK_UN { 8 } # how those chips fall?my($oldval, $fd, $db, %db, $value, $key);$key = shift || 'default';$value = shift || 'magic';$value .= " $$";$db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0666) or die "dbcreat /tmp/foo.db $!";$fd = $db->fd; # need this for lockingprint "$$: db fd is $fd\n";open(DB_FH, "+<&=$fd") or die "dup $!";unless (flock (DB_FH, LOCK_SH | LOCK_NB)) { print "$$: CONTENTION; can't read during write update! Waiting for read lock ($!) ...."; unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }}print "$$: Read lock granted\n";$oldval = $db{$key};print "$$: Old value was $oldval\n";flock(DB_FH, LOCK_UN);unless (flock (DB_FH, LOCK_EX | LOCK_NB)) { print "$$: CONTENTION; must have exclusive lock! Waiting for write lock ($!) ...."; unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }}print "$$: Write lock granted\n";$db{$key} = $value;$db->sync; # to flushsleep 10;flock(DB_FH, LOCK_UN);undef $db;untie %db;close(DB_FH);print "$$: Updated db to $key=$value\n";<ACLASS="indexterm"NAME="ch14-idx-1000005287-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000005287-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000005287-2"></A></PRE></DIV></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-594">See Also</A></H3><PCLASS="para">The documentation for the standard DB_File module, also in <ACLASS="olink"HREF="../prog/ch07_01.htm">Chapter 7</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; <ACLASS="xref"HREF="ch07_12.htm"TITLE="Locking a File">Recipe 7.11</A>; <ACLASS="xref"HREF="ch16_13.htm"TITLE="Sharing Variables in Different Processes">Recipe 16.12</A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch14_05.htm"TITLE="14.4. 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