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<HTML><HEAD><TITLE>Recipe 8.14. Reading a String from a Binary 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="1999-07-02T01:38:50Z"><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="ch08_01.htm"TITLE="8. File Contents"><LINKREL="prev"HREF="ch08_14.htm"TITLE="8.13. Updating a Random-Access File"><LINKREL="next"HREF="ch08_16.htm"TITLE="8.15. Reading Fixed-Length Records"></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="ch08_14.htm"TITLE="8.13. Updating a Random-Access File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.13. Updating a Random-Access File"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch08_01.htm"TITLE="8. File Contents"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch08_16.htm"TITLE="8.15. Reading Fixed-Length Records"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.15. Reading Fixed-Length Records"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch08-23548">8.14. Reading a String from a Binary File</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1266">Problem<ACLASS="indexterm"NAME="ch08-idx-1000004688-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004688-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004688-2"></A></A></H3><PCLASS="para">You want to read a NUL-terminated string from a file, starting at a particular address.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1272">Solution</A></H3><PCLASS="para">Set <CODECLASS="literal">$/</CODE> to an ASCII NUL, and read the string with <>:</P><PRECLASS="programlisting">$old_rs = $/; # save old $/$/ = "\0"; # NULLseek(FH, $addr, SEEK_SET) or die "Seek error: $!\n";$string = <FH>; # read stringchomp $string; # remove NULL$/ = $old_rs; # restore old $/</PRE><PCLASS="para">You can use <CODECLASS="literal">local</CODE> to save and restore <CODECLASS="literal">$/</CODE> if you want:</P><PRECLASS="programlisting">{ local $/ = "\0"; # ...} # $/ is automatically restored</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1300">Discussion</A></H3><PCLASS="para">The example program shown in <ACLASS="xref"HREF="ch08_15.htm#ch08-15070"TITLE="bgets">Example 8.5</A>, <EMCLASS="emphasis">bgets</EM><ACLASS="indexterm"NAME="ch08-idx-1000004689-0"></A>, accepts a filename and one or more byte addresses as arguments. Decimal, octal, or hexadecimal addresses may be specified. For each address, the program reads and prints the NULL- or EOF-terminated string at that position:</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch08-15070">Example 8.5: bgets</A></H4><PRECLASS="programlisting">#!/usr/bin/perl # <ACLASS="indexterm"NAME="ch08-idx-1000004830-0"></A>bgets - get a string from an address in a binary fileuse IO::Seekable;($file, @addrs) = @ARGV or die "usage: $0 addr ...";open(FH, $file) or die "cannot open $file: $!";$/ = "\000";foreach $addr (@addrs) { $addr = oct $addr if $addr =~ /^0/; seek(FH, $addr, SEEK_SET) or die "can't seek to $addr in $file: $!"; printf qq{%#x %#o %d "%s"\n}, $addr, $addr, $addr, scalar <>;}</PRE></DIV><PCLASS="para">Here's a simple implementation of the Unix <EMCLASS="emphasis">strings</EM> program:</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch08-12126">Example 8.6: strings</A></H4><PRECLASS="programlisting">#!/usr/bin/perl# <ACLASS="indexterm"NAME="ch08-idx-1000004832-0"></A>strings - pull strings out of a binary file$/ = "\0";while (<>) { while (/([\040-\176\s]{4,})/g) { print $1, "\n"; }}<ACLASS="indexterm"NAME="ch08-idx-1000004691-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004691-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004691-2"></A></PRE></DIV></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1348">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">seek</CODE>, <CODECLASS="literal">getc</CODE>, and <CODECLASS="literal">ord</CODE> functions in <ICLASS="filename">perlfunc </I>(1) and in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the discussion of <CODECLASS="literal">qq//</CODE> in the "Quote and Quote-Like Operators" section of the <ICLASS="filename">perlop </I>(1) manpage, and in the <ACLASS="olink"HREF="../prog/ch02_03.htm#PERL2-CH-2-SECT-3.2.3">"Pick your own quotes"</A> section of <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></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="ch08_14.htm"TITLE="8.13. Updating a Random-Access File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.13. Updating a Random-Access 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="ch08_16.htm"TITLE="8.15. Reading Fixed-Length Records"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.15. Reading Fixed-Length Records"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">8.13. Updating a Random-Access 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">8.15. 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