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<HTML><HEAD><TITLE>Recipe 16.21. Timing Out an Operation (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="1999-07-02T01:44:15Z"><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="ch16_01.htm"TITLE="16. Process Management and Communication"><LINKREL="prev"HREF="ch16_21.htm"TITLE="16.20. Blocking Signals"><LINKREL="next"HREF="ch16_23.htm"TITLE="16.22. 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Program: sigrand"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch16-34554">16.21. Timing Out an Operation</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005795">Problem<ACLASS="indexterm"NAME="ch16-idx-1000006434-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006434-1"></A><ACLASS="indexterm"NAME="ch16-idx-1000006434-2"></A></A></H3><PCLASS="para">You want to make sure an operation doesn't take more than a certain amount of time. For instance, you're running filesystem backups and want to abort if it takes longer than an hour. Or, you want to schedule an event for the next hour.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005798">Solution</A></H3><PCLASS="para">To interrupt a long-running operation, set a <ACLASS="indexterm"NAME="ch16-idx-1000006435-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006435-1"></A>SIGALRM handler to call <CODECLASS="literal">die</CODE>. Set an alarm with <CODECLASS="literal">alarm</CODE>, then <CODECLASS="literal">eval</CODE> your code:</P><PRECLASS="programlisting">$SIG{ALRM} = sub { die "timeout" };eval { alarm(3600); # long-time operations here alarm(0);};if ($@) { if ($@ =~ /timeout/) { # timed out; do what you will here } else { alarm(0); # clear the still-pending alarm die; # propagate unexpected exception } } </PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005816">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">alarm</CODE> function takes one argument: the integer number of seconds before your process receives a SIGALRM. It may be delivered after that time in busy time-sharing systems. The default action for SIGALRM is to terminate your program, so you should install your own signal handler.</P><PCLASS="para">You cannot (usefully) give the <CODECLASS="literal">alarm</CODE> function a fractional number of seconds; if you try, it will be truncated to an integer. For precise timers, see <ACLASS="xref"HREF="ch03_10.htm"TITLE="High-Resolution Timers">Recipe 3.9</A>.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005823">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch06_02.htm#PERL2-CH-6-SECT-2.1">"Signals"</A> sections in <ACLASS="olink"HREF="../prog/ch06_01.htm">Chapter 6</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A> and in <ICLASS="filename">perlipc </I>(1); the <ACLASS="olink"HREF="../prog/ch03_005.htm"><CODECLASS="literal">alarm</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="ch03_10.htm"TITLE="High-Resolution Timers">Recipe 3.9</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="ch16_21.htm"TITLE="16.20. 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