ch15_10.htm

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<HTML><HEAD><TITLE>Recipe 15.9. Checking for Waiting Input (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen &amp; Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly &amp; Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:43:22Z"><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="ch15_01.htm"TITLE="15. User Interfaces"><LINKREL="prev"HREF="ch15_09.htm"TITLE="15.8. Using POSIX termios"><LINKREL="next"HREF="ch15_11.htm"TITLE="15.10. Reading Passwords"></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="ch15_09.htm"TITLE="15.8. Using POSIX termios"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 15.8. Using POSIX termios"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch15_01.htm"TITLE="15. User Interfaces"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch15_11.htm"TITLE="15.10. Reading Passwords"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 15.10. Reading Passwords"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch15-23042">15.9. Checking for Waiting Input</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch15-pgfId-881">Problem<ACLASS="indexterm"NAME="ch15-idx-1000005114-0"></A><ACLASS="indexterm"NAME="ch15-idx-1000005114-1"></A><ACLASS="indexterm"NAME="ch15-idx-1000005114-2"></A><ACLASS="indexterm"NAME="ch15-idx-1000005114-3"></A></A></H3><PCLASS="para">You want to know whether keyboard input is waiting without actually reading it.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch15-pgfId-887">Solution</A></H3><PCLASS="para">Use the CPAN module Term::ReadKey, and try to read a key in non-blocking mode by passing it an argument of <CODECLASS="literal">-1</CODE>:</P><PRECLASS="programlisting">use Term::ReadKey;ReadMode ('cbreak');if (defined ($char = ReadKey(-1)) ) {    # input was waiting and it was $char} else {    # no input was waiting}ReadMode ('normal');                  # restore normal tty settings</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch15-pgfId-915">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">-1</CODE> parameter to <CODECLASS="literal">ReadKey</CODE> indicates a non-blocking read of a character. If no character is available, <CODECLASS="literal">ReadKey</CODE> returns <CODECLASS="literal">undef</CODE>.<ACLASS="indexterm"NAME="ch15-idx-1000005116-0"></A><ACLASS="indexterm"NAME="ch15-idx-1000005116-1"></A><ACLASS="indexterm"NAME="ch15-idx-1000005116-2"></A><ACLASS="indexterm"NAME="ch15-idx-1000005116-3"></A><ACLASS="indexterm"NAME="ch15-idx-1000005116-4"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch15-pgfId-921">See Also</A></H3><PCLASS="para">The documentation for the Term::ReadKey module from CPAN; <ACLASS="xref"HREF="ch15_07.htm"TITLE="Reading from the Keyboard">Recipe 15.6</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="ch15_09.htm"TITLE="15.8. Using POSIX termios"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 15.8. 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