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<HTML><HEAD><TITLE>Recipe 14.1. Making and Using a DBM File (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="1999-07-02T01:42:46Z"><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_01.htm"TITLE="14.0. Introduction"><LINKREL="next"HREF="ch14_03.htm"TITLE="14.2. Emptying a DBM File"></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_01.htm"TITLE="14.0. Introduction"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 14.0. Introduction"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_03.htm"TITLE="14.2. Emptying a DBM File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 14.2. Emptying a DBM File"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch14-25857">14.1. Making and Using a DBM File</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-150">Problem<ACLASS="indexterm"NAME="ch14-idx-1000004919-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004919-1"></A></A></H3><PCLASS="para">You want to create, populate, inspect, or delete values in a DBM database.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-156">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">dbmopen</CODE><ACLASS="indexterm"NAME="ch14-idx-1000004920-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004920-1"></A> or <CODECLASS="literal">tie</CODE> to open the database and make it accessible through a hash. Then use the hash as you normally would. When you're done, call <CODECLASS="literal">dbmclose</CODE><ACLASS="indexterm"NAME="ch14-idx-1000004921-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004921-1"></A> or <CODECLASS="literal">untie</CODE>.</P><DIVCLASS="sect3"><H4CLASS="sect3"><ACLASS="title"NAME="ch14-pgfId-1000004641">dbmopen</A></H4><PRECLASS="programlisting">use DB_File; # optional; overrides defaultdbmopen %HASH, $FILENAME, 0666 # open database, accessed through %HASH or die "Can't open $FILENAME: $!\n";$V = $HASH{$KEY}; # retrieve from database$HASH{$KEY} = $VALUE; # put value into databaseif (exists $HASH{$KEY}) { # check whether in database # ...}delete $HASH{$KEY}; # remove from databasedbmclose %HASH; # close the database</PRE></DIV><DIVCLASS="sect3"><H4CLASS="sect3"><ACLASS="title"NAME="ch14-pgfId-1000004663">tie</A></H4><PRECLASS="programlisting">use DB_File; # load database moduletie %HASH, "DB_File", $FILENAME # open database, to be accessed or die "Can't open $FILENAME:$!\n"; # through %HASH$V = $HASH{$KEY}; # retrieve from database$HASH{$KEY} = $VALUE; # put value into databaseif (exists $HASH{$KEY}) { # check whether in database # ...}delete $HASH{$KEY}; # delete from databaseuntie %hash; # close the database</PRE></DIV></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-1000004643">Discussion</A></H3><PCLASS="para">Accessing a database as a hash is powerful but easy, giving you a persistent hash that sticks around after the program using it has finished running. It's also much faster than loading in a new hash every time; even if the hash has a million entries, your program starts up virtually instantaneously.</P><PCLASS="para">The <ACLASS="indexterm"NAME="ch14-idx-1000004931-0"></A>program in <ACLASS="xref"HREF="ch14_02.htm#ch14-26660"TITLE="userstats">Example 14.1</A> treats the database as though it were a normal hash. You can even call <CODECLASS="literal">keys</CODE> or <CODECLASS="literal">each</CODE> on it. Likewise, <CODECLASS="literal">exists</CODE> and <CODECLASS="literal">defined</CODE> are implemented for tied DBM hashes. Unlike a normal hash, a DBM hash does not distinguish between those two functions.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch14-26660">Example 14.1: userstats</A></H4><PRECLASS="programlisting">#!/usr/bin/perl -w# userstats - generates statistics on who is logged in.# call with an argument to display totalsuse DB_File;$db = '/tmp/userstats.db'; # where data is kept between runstie(%db, 'DB_File', $db) or die "Can't open DB_File $db : $!\n";if (@ARGV) { if ("@ARGV" eq "ALL") { @ARGV = sort keys %db; } foreach $user (@ARGV) { print "$user\t$db{$user}\n"; }} else { @who = `who`; # run who(1) if ($?) { die "Couldn't run who: $?\n"; # exited abnormally } # extract username (first thing on the line) and update foreach $line (@who) { $line =~ /^(\S+)/; die "Bad line from who: $line\n" unless $1; $db{$1}++; }}untie %db;</PRE></DIV><PCLASS="para">We use <EMCLASS="emphasis">who</EM> to get a list of users logged in. This typically produces output like:</P><PRECLASS="programlisting"><BCLASS="emphasis.bold">gnat ttyp1 May 29 15:39 (coprolith.frii.com)</B></PRE><PCLASS="para">If the <EMCLASS="emphasis">userstats</EM> program is called without any arguments, it checks who's logged on and updates the database appropriately.</P><PCLASS="para">If the program is called with arguments, these are treated as usernames whose information will be presented. The special argument <CODECLASS="literal">"ALL"</CODE> sets <CODECLASS="literal">@ARGV</CODE> to a sorted list of DBM keys. For large hashes with many keys, this is prohibitively expensive - a better solution would be to use the BTREE bindings of DB_File described in <ACLASS="xref"HREF="ch14_07.htm"TITLE="Sorting Large DBM Files">Recipe 14.6</A>.<ACLASS="indexterm"NAME="ch14-idx-1000004923-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004923-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004923-2"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-240">See Also</A></H3><PCLASS="para">The documentation for the standard modules GDBM_File, NDBM_File, SDBM_File, DB_File, 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>; <ICLASS="filename">perltie </I>(1); the section on <ACLASS="olink"HREF="../prog/ch05_04.htm">"Using Tied Variables"</A> in <ACLASS="olink"HREF="../prog/ch05_01.htm">Chapter 5</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the discussion on the effect of your <CODECLASS="literal">umask</CODE> on file creation in <ACLASS="xref"HREF="ch07_02.htm"TITLE="Opening a File">Recipe 7.1</A>; <ACLASS="xref"HREF="ch13_16.htm"TITLE="Creating Magic Variables with tie">Recipe 13.15</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_01.htm"TITLE="14.0. 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