📄 ch14_03.htm
字号:
<HTML><HEAD><TITLE>Recipe 14.2. Emptying 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="1999-07-02T01:42:47Z"><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_02.htm"TITLE="14.1. Making and Using a DBM File"><LINKREL="next"HREF="ch14_04.htm"TITLE="14.3. Converting Between 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_02.htm"TITLE="14.1. Making and Using a DBM File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 14.1. Making and Using a DBM File"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_04.htm"TITLE="14.3. Converting Between DBM Files"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 14.3. Converting Between DBM Files"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch14-29206">14.2. Emptying a DBM File</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-250">Problem<ACLASS="indexterm"NAME="ch14-idx-1000004932-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004932-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004932-2"></A></A></H3><PCLASS="para">You want to clear out a DBM file.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-256">Solution</A></H3><PCLASS="para">Open the database and assign <CODECLASS="literal">()</CODE> to it. Use <CODECLASS="literal">dbmopen</CODE>:</P><PRECLASS="programlisting">dbmopen(%HASH, $FILENAME, 0666) or die "Can't open FILENAME: $!\n";%HASH = ();dbmclose %HASH;</PRE><PCLASS="para">or <CODECLASS="literal">tie</CODE>:</P><PRECLASS="programlisting">use DB_File;tie(%HASH, "DB_File", $FILENAME) or die "Can't open FILENAME: $!\n";%HASH = ();untie %hash;</PRE><PCLASS="para">Alternatively, delete the file and reopen with create mode:</P><PRECLASS="programlisting">unlink $FILENAME or die "Couldn't unlink $FILENAME to empty the database: $!\n";dbmopen(%HASH, $FILENAME, 0666) or die "Couldn't create $FILENAME database: $!\n";</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-290">Discussion</A></H3><PCLASS="para">It may be quicker to delete the file and create a new one than to reset it, but doing so opens you up to a race condition that trips up a careless program or makes it vulnerable to an attacker. The attacker could make a link pointing to the file <EMCLASS="emphasis">/etc/precious</EM> with the same name as your file between the time when you deleted the file and when you recreated it. When the DBM library opens the file, it clobbers <EMCLASS="emphasis">/etc/precious</EM>.</P><PCLASS="para">If you delete a DB_File database and recreate it, you'll lose any customizable settings like page size, fill-factor, and so on. This is another good reason to assign the empty list to the tied hash.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-298">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>; the <CODECLASS="literal">unlink</CODE> function in <ICLASS="filename">perlfunc </I>(1); <ACLASS="xref"HREF="ch14_02.htm"TITLE="Making and Using a DBM File">Recipe 14.1</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_02.htm"TITLE="14.1. Making and Using a DBM File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 14.1. Making and Using a DBM File"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch14_04.htm"TITLE="14.3. Converting Between DBM Files"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 14.3. Converting Between DBM Files"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">14.1. Making and Using a DBM File</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">14.3. Converting Between DBM Files</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><FONTSIZE="-1"></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -