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<HTML><HEAD><TITLE>Recipe 17.16. Restarting a Server on Demand (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="1999-07-02T01:44:45Z"><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="ch17_01.htm"TITLE="17. Sockets"><LINKREL="prev"HREF="ch17_16.htm"TITLE="17.15. Making a Daemon Server"><LINKREL="next"HREF="ch17_18.htm"TITLE="17.17. Program: backsniff"></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="ch17_16.htm"TITLE="17.15. Making a Daemon Server"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 17.15. Making a Daemon Server"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch17_01.htm"TITLE="17. Sockets"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch17_18.htm"TITLE="17.17. Program: backsniff"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 17.17. Program: backsniff"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch17-33998">17.16. Restarting a Server on Demand</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch17-pgfId-2026">Problem<ACLASS="indexterm"NAME="ch17-idx-1000004877-0"></A><ACLASS="indexterm"NAME="ch17-idx-1000004877-1"></A><ACLASS="indexterm"NAME="ch17-idx-1000004877-2"></A><ACLASS="indexterm"NAME="ch17-idx-1000004877-3"></A></A></H3><PCLASS="para">You want your server to shutdown and restart when it receives a HUP signal, just like <CODECLASS="literal">inetd</CODE> or <CODECLASS="literal">httpd </CODE>.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch17-pgfId-2032">Solution</A></H3><PCLASS="para">Catch the <CODECLASS="literal">SIGHUP</CODE> signal, and re-execute your program:</P><PRECLASS="programlisting">$SELF = "/usr/local/libexec/myd"; # which program I am@ARGS = qw(-l /var/log/myd -d); # program arguments$SIG{HUP} = \&phoenix;sub phoenix { # close all your connections, kill your children, and # generally prepare to be reincarnated with dignity. exec($SELF, @ARGS) or die "Couldn't restart: $!\n";}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch17-pgfId-2058">Discussion</A></H3><PCLASS="para">It sure looks simple ("when I get a HUP signal, restart") but it's tricky. You must know your own program name, and that isn't easy to find out. You could use <CODECLASS="literal">$0</CODE> or the FindBin module. For normal programs, this is fine, but critical system utilities must be more cautious, as there's no guarantee that <CODECLASS="literal">$0</CODE> is valid. You can hardcode the filename and arguments into your program, as we do here. That's not necessarily the most convenient solution, however, so you might want to read the program and arguments from an external file (using the filesystem's protections to ensure it hasn't been tampered with).</P><PCLASS="para">Be sure and install your signal handler <EMCLASS="emphasis">after</EM> you define <CODECLASS="literal">$SELF</CODE> and <CODECLASS="literal">@ARGS</CODE>, otherwise there's a race condition when a SIGHUP could run <CODECLASS="literal">restart</CODE> but you don't know the program to run. This would cause your program to die.</P><PCLASS="para">Some servers don't want restart on receiving a SIGHUP - they just want to reread their configuration file.</P><PRECLASS="programlisting">$CONFIG_FILE = "/usr/local/etc/myprog/server_conf.pl";$SIG{HUP} = \&read_config;sub read_config { do $CONFIG_FILE;} </PRE><PCLASS="para">Some clever servers even autoload their configuration files when they notice that those files have been updated. That way you don't have to go out of your way to signal them.<ACLASS="indexterm"NAME="ch17-idx-1000004879-0"></A><ACLASS="indexterm"NAME="ch17-idx-1000004879-1"></A><ACLASS="indexterm"NAME="ch17-idx-1000004879-2"></A><ACLASS="indexterm"NAME="ch17-idx-1000004879-3"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch17-pgfId-2080">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch03_033.htm"> <CODECLASS="literal">exec</CODE></A> function 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> and in <ICLASS="filename">perlfunc </I>(1); <ACLASS="xref"HREF="ch08_17.htm"TITLE="Reading Configuration Files">Recipe 8.16</A>; <ACLASS="xref"HREF="ch08_18.htm"TITLE="Testing a File for Trustworthiness">Recipe 8.17</A>; <ACLASS="xref"HREF="ch16_16.htm"TITLE="Installing a Signal Handler">Recipe 16.15</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="ch17_16.htm"TITLE="17.15. 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