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<HTML><HEAD><TITLE>Recipe 16.20. Blocking Signals (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="1999-07-02T01:44:14Z"><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_20.htm"TITLE="16.19. Avoiding Zombie Processes"><LINKREL="next"HREF="ch16_22.htm"TITLE="16.21. Timing Out an Operation"></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="ch16_20.htm"TITLE="16.19. Avoiding Zombie Processes"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 16.19. Avoiding Zombie Processes"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch16_01.htm"TITLE="16. Process Management and Communication"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch16_22.htm"TITLE="16.21. Timing Out an Operation"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 16.21. Timing Out an Operation"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch16-14904">16.20. Blocking Signals</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005762">Problem</A></H3><PCLASS="para"><ACLASS="indexterm"NAME="ch16-idx-1000006433-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006433-1"></A><ACLASS="indexterm"NAME="ch16-idx-1000006433-2"></A><ACLASS="indexterm"NAME="ch16-idx-1000006433-3"></A><ACLASS="indexterm"NAME="ch16-idx-1000006433-4"></A><ACLASS="indexterm"NAME="ch16-idx-1000006433-5"></A>You'd like to delay the reception of a signal, possibly to prevent unpredictable behavior from signals that can interrupt your program at any point.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005765">Solution</A></H3><PCLASS="para">Use the POSIX module's interface to the <CODECLASS="literal">sigprocmask</CODE> system call. This is only available if your system is POSIX conformant.</P><PCLASS="para">To block a signal around an operation:</P><PRECLASS="programlisting">use POSIX qw(:signal_h);$sigset = POSIX::SigSet->new(SIGINT); # define the signals to block$old_sigset = POSIX::SigSet->new; # where the old sigmask will be keptunless (defined sigprocmask(SIG_BLOCK, $sigset, $old_sigset)) { die "Could not block SIGINT\n";}</PRE><PCLASS="para">To unblock:</P><PRECLASS="programlisting">unless (defined sigprocmask(SIG_UNBLOCK, $old_sigset)) { die "Could not unblock SIGINT\n";}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005781">Discussion</A></H3><PCLASS="para">The POSIX standard introduced <CODECLASS="literal">sigaction</CODE> and <CODECLASS="literal">sigprocmask</CODE> to give you better control over how signals are delivered. The <CODECLASS="literal">sigprocmask</CODE> function controls delayed delivery of signals and <CODECLASS="literal">sigaction</CODE> installs handlers. If available, Perl uses <CODECLASS="literal">sigaction</CODE> when you change <CODECLASS="literal">%SIG</CODE>.</P><PCLASS="para">To use <CODECLASS="literal">sigprocmask</CODE>, first build a signal set using <CODECLASS="literal">POSIX::SigSet->new</CODE>. This takes a list of signal numbers. The POSIX module exports functions named after the signals, which return their signal numbers.</P><PRECLASS="programlisting">use POSIX qw(:signal_h);$sigset = POSIX::SigSet->new( SIGINT, SIGKILL );</PRE><PCLASS="para">Pass the POSIX::SigSet object to <CODECLASS="literal">sigprocmask</CODE> with the SIG_BLOCK flag to delay signal delivery, SIG_UNBLOCK to restore delivery of the signals, or SIG_SETMASK to block only signals in the POSIX::SigSet. The most paranoid of programmers block signals for a <CODECLASS="literal">fork</CODE> to prevent a signal handler in the child process being called before Perl can update the child's <CODECLASS="literal">$$</CODE> variable, its process id. If the signal handler were called immediately and reported <CODECLASS="literal">$$</CODE> in that handler, it could possibly report its parent's <CODECLASS="literal">$$</CODE>, not its own. This issue does not arise often.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1000005789">See Also</A></H3><PCLASS="para">Your system's <EMCLASS="emphasis">sigprocmask</EM> (2) manpage (if you have one); the documentation for the standard POSIX module 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></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_20.htm"TITLE="16.19. Avoiding Zombie Processes"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 16.19. 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