arm-2011.03/share/doc/arm-arm-none-linux-gnueabi/html/libc/Waiting-for-I_002fO.html - nest-cam/v350/arm-none-linux-gnueabi-i686-pc-linux-gnu - Git at Google

 <html lang="en">
 <head>
 <title>Waiting for I/O - The GNU C Library</title>
 <meta http-equiv="Content-Type" content="text/html">
 <meta name="description" content="The GNU C Library">
 <meta name="generator" content="makeinfo 4.13">
 <link title="Top" rel="start" href="index.html#Top">
 <link rel="up" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO" title="Low-Level I/O">
 <link rel="prev" href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO" title="Memory-mapped I/O">
 <link rel="next" href="Synchronizing-I_002fO.html#Synchronizing-I_002fO" title="Synchronizing I/O">
 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
 <!--
 This file documents the GNU C library.

 This is Edition 0.12, last updated 2007-10-27,
 of `The GNU C Library Reference Manual', for version
 2.8 (Sourcery G++ Lite 2011.03-41).

 Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
 2003, 2007, 2008, 2010 Free Software Foundation, Inc.

 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
 any later version published by the Free Software Foundation; with the
 Invariant Sections being ``Free Software Needs Free Documentation''
 and ``GNU Lesser General Public License'', the Front-Cover texts being
 ``A GNU Manual'', and with the Back-Cover Texts as in (a) below.  A
 copy of the license is included in the section entitled "GNU Free
 Documentation License".

 (a) The FSF's Back-Cover Text is: ``You have the freedom to
 copy and modify this GNU manual.  Buying copies from the FSF
 supports it in developing GNU and promoting software freedom.''-->
 <meta http-equiv="Content-Style-Type" content="text/css">
 <style type="text/css"><!--
   pre.display { font-family:inherit }
   pre.format  { font-family:inherit }
   pre.smalldisplay { font-family:inherit; font-size:smaller }
   pre.smallformat  { font-family:inherit; font-size:smaller }
   pre.smallexample { font-size:smaller }
   pre.smalllisp    { font-size:smaller }
   span.sc    { font-variant:small-caps }
   span.roman { font-family:serif; font-weight:normal; }
   span.sansserif { font-family:sans-serif; font-weight:normal; }
 --></style>
 <link rel="stylesheet" type="text/css" href="../cs.css">
 </head>
 <body>
 <div class="node">
 <a name="Waiting-for-I%2fO"></a>
 <a name="Waiting-for-I_002fO"></a>
 <p>
 Next:&nbsp;<a rel="next" accesskey="n" href="Synchronizing-I_002fO.html#Synchronizing-I_002fO">Synchronizing I/O</a>,
 Previous:&nbsp;<a rel="previous" accesskey="p" href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO">Memory-mapped I/O</a>,
 Up:&nbsp;<a rel="up" accesskey="u" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO">Low-Level I/O</a>
 <hr>
 </div>

 <h3 class="section">13.8 Waiting for Input or Output</h3>

 <p><a name="index-waiting-for-input-or-output-1271"></a><a name="index-multiplexing-input-1272"></a><a name="index-input-from-multiple-files-1273"></a>
 Sometimes a program needs to accept input on multiple input channels
 whenever input arrives.  For example, some workstations may have devices
 such as a digitizing tablet, function button box, or dial box that are
 connected via normal asynchronous serial interfaces; good user interface
 style requires responding immediately to input on any device.  Another
 example is a program that acts as a server to several other processes
 via pipes or sockets.

    <p>You cannot normally use <code>read</code> for this purpose, because this
 blocks the program until input is available on one particular file
 descriptor; input on other channels won't wake it up.  You could set
 nonblocking mode and poll each file descriptor in turn, but this is very
 inefficient.

    <p>A better solution is to use the <code>select</code> function.  This blocks the
 program until input or output is ready on a specified set of file
 descriptors, or until a timer expires, whichever comes first.  This
 facility is declared in the header file <samp><span class="file">sys/types.h</span></samp>.
 <a name="index-sys_002ftypes_002eh-1274"></a>
 In the case of a server socket (see <a href="Listening.html#Listening">Listening</a>), we say that
 &ldquo;input&rdquo; is available when there are pending connections that could be
 accepted (see <a href="Accepting-Connections.html#Accepting-Connections">Accepting Connections</a>).  <code>accept</code> for server
 sockets blocks and interacts with <code>select</code> just as <code>read</code> does
 for normal input.

    <p><a name="index-file-descriptor-sets_002c-for-_0040code_007bselect_007d-1275"></a>The file descriptor sets for the <code>select</code> function are specified
 as <code>fd_set</code> objects.  Here is the description of the data type
 and some macros for manipulating these objects.

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Data Type: <b>fd_set</b><var><a name="index-fd_005fset-1276"></a></var><br>
 <blockquote><p>The <code>fd_set</code> data type represents file descriptor sets for the
 <code>select</code> function.  It is actually a bit array.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Macro: int <b>FD_SETSIZE</b><var><a name="index-FD_005fSETSIZE-1277"></a></var><br>
 <blockquote><p>The value of this macro is the maximum number of file descriptors that a
 <code>fd_set</code> object can hold information about.  On systems with a
 fixed maximum number, <code>FD_SETSIZE</code> is at least that number.  On
 some systems, including GNU, there is no absolute limit on the number of
 descriptors open, but this macro still has a constant value which
 controls the number of bits in an <code>fd_set</code>; if you get a file
 descriptor with a value as high as <code>FD_SETSIZE</code>, you cannot put
 that descriptor into an <code>fd_set</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Macro: void <b>FD_ZERO</b> (<var>fd_set *set</var>)<var><a name="index-FD_005fZERO-1278"></a></var><br>
 <blockquote><p>This macro initializes the file descriptor set <var>set</var> to be the
 empty set.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Macro: void <b>FD_SET</b> (<var>int filedes, fd_set *set</var>)<var><a name="index-FD_005fSET-1279"></a></var><br>
 <blockquote><p>This macro adds <var>filedes</var> to the file descriptor set <var>set</var>.

         <p>The <var>filedes</var> parameter must not have side effects since it is
 evaluated more than once.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Macro: void <b>FD_CLR</b> (<var>int filedes, fd_set *set</var>)<var><a name="index-FD_005fCLR-1280"></a></var><br>
 <blockquote><p>This macro removes <var>filedes</var> from the file descriptor set <var>set</var>.

         <p>The <var>filedes</var> parameter must not have side effects since it is
 evaluated more than once.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Macro: int <b>FD_ISSET</b> (<var>int filedes, const fd_set *set</var>)<var><a name="index-FD_005fISSET-1281"></a></var><br>
 <blockquote><p>This macro returns a nonzero value (true) if <var>filedes</var> is a member
 of the file descriptor set <var>set</var>, and zero (false) otherwise.

         <p>The <var>filedes</var> parameter must not have side effects since it is
 evaluated more than once.
 </p></blockquote></div>

    <p>Next, here is the description of the <code>select</code> function itself.

 <!-- sys/types.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Function: int <b>select</b> (<var>int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout</var>)<var><a name="index-select-1282"></a></var><br>
 <blockquote><p>The <code>select</code> function blocks the calling process until there is
 activity on any of the specified sets of file descriptors, or until the
 timeout period has expired.

         <p>The file descriptors specified by the <var>read-fds</var> argument are
 checked to see if they are ready for reading; the <var>write-fds</var> file
 descriptors are checked to see if they are ready for writing; and the
 <var>except-fds</var> file descriptors are checked for exceptional
 conditions.  You can pass a null pointer for any of these arguments if
 you are not interested in checking for that kind of condition.

         <p>A file descriptor is considered ready for reading if a <code>read</code>
 call will not block.  This usually includes the read offset being at
 the end of the file or there is an error to report.  A server socket
 is considered ready for reading if there is a pending connection which
 can be accepted with <code>accept</code>; see <a href="Accepting-Connections.html#Accepting-Connections">Accepting Connections</a>.  A
 client socket is ready for writing when its connection is fully
 established; see <a href="Connecting.html#Connecting">Connecting</a>.

         <p>&ldquo;Exceptional conditions&rdquo; does not mean errors&mdash;errors are reported
 immediately when an erroneous system call is executed, and do not
 constitute a state of the descriptor.  Rather, they include conditions
 such as the presence of an urgent message on a socket.  (See <a href="Sockets.html#Sockets">Sockets</a>,
 for information on urgent messages.)

         <p>The <code>select</code> function checks only the first <var>nfds</var> file
 descriptors.  The usual thing is to pass <code>FD_SETSIZE</code> as the value
 of this argument.

         <p>The <var>timeout</var> specifies the maximum time to wait.  If you pass a
 null pointer for this argument, it means to block indefinitely until one
 of the file descriptors is ready.  Otherwise, you should provide the
 time in <code>struct timeval</code> format; see <a href="High_002dResolution-Calendar.html#High_002dResolution-Calendar">High-Resolution Calendar</a>.  Specify zero as the time (a <code>struct timeval</code> containing
 all zeros) if you want to find out which descriptors are ready without
 waiting if none are ready.

         <p>The normal return value from <code>select</code> is the total number of ready file
 descriptors in all of the sets.  Each of the argument sets is overwritten
 with information about the descriptors that are ready for the corresponding
 operation.  Thus, to see if a particular descriptor <var>desc</var> has input,
 use <code>FD_ISSET (</code><var>desc</var><code>, </code><var>read-fds</var><code>)</code> after <code>select</code> returns.

         <p>If <code>select</code> returns because the timeout period expires, it returns
 a value of zero.

         <p>Any signal will cause <code>select</code> to return immediately.  So if your
 program uses signals, you can't rely on <code>select</code> to keep waiting
 for the full time specified.  If you want to be sure of waiting for a
 particular amount of time, you must check for <code>EINTR</code> and repeat
 the <code>select</code> with a newly calculated timeout based on the current
 time.  See the example below.  See also <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>.

         <p>If an error occurs, <code>select</code> returns <code>-1</code> and does not modify
 the argument file descriptor sets.  The following <code>errno</code> error
 conditions are defined for this function:

           <dl>
 <dt><code>EBADF</code><dd>One of the file descriptor sets specified an invalid file descriptor.

           <br><dt><code>EINTR</code><dd>The operation was interrupted by a signal.  See <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>.

           <br><dt><code>EINVAL</code><dd>The <var>timeout</var> argument is invalid; one of the components is negative
 or too large.
 </dl>
         </p></blockquote></div>

    <p><strong>Portability Note:</strong>  The <code>select</code> function is a BSD Unix
 feature.

    <p>Here is an example showing how you can use <code>select</code> to establish a
 timeout period for reading from a file descriptor.  The <code>input_timeout</code>
 function blocks the calling process until input is available on the
 file descriptor, or until the timeout period expires.

 <pre class="smallexample">     #include &lt;errno.h&gt;
      #include &lt;stdio.h&gt;
      #include &lt;unistd.h&gt;
      #include &lt;sys/types.h&gt;
      #include &lt;sys/time.h&gt;

      int
      input_timeout (int filedes, unsigned int seconds)
      {
        fd_set set;
        struct timeval timeout;

        /* <span class="roman">Initialize the file descriptor set.</span> */
        FD_ZERO (&amp;set);
        FD_SET (filedes, &amp;set);

        /* <span class="roman">Initialize the timeout data structure.</span> */
        timeout.tv_sec = seconds;
        timeout.tv_usec = 0;

        /* <code>select</code><span class="roman"> returns 0 if timeout, 1 if input available, -1 if error.</span> */
        return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                           &amp;set, NULL, NULL,
                                           &amp;timeout));
      }

      int
      main (void)
      {
        fprintf (stderr, "select returned %d.\n",
                 input_timeout (STDIN_FILENO, 5));
        return 0;
      }
 </pre>
    <p>There is another example showing the use of <code>select</code> to multiplex
 input from multiple sockets in <a href="Server-Example.html#Server-Example">Server Example</a>.

    </body></html>
	<html lang="en">
	<head>
	<title>Waiting for I/O - The GNU C Library</title>
	<meta http-equiv="Content-Type" content="text/html">
	<meta name="description" content="The GNU C Library">
	<meta name="generator" content="makeinfo 4.13">
	<link title="Top" rel="start" href="index.html#Top">
	<link rel="up" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO" title="Low-Level I/O">
	<link rel="prev" href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO" title="Memory-mapped I/O">
	<link rel="next" href="Synchronizing-I_002fO.html#Synchronizing-I_002fO" title="Synchronizing I/O">
	<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
	<!--
	This file documents the GNU C library.

	This is Edition 0.12, last updated 2007-10-27,
	of `The GNU C Library Reference Manual', for version
	2.8 (Sourcery G++ Lite 2011.03-41).

	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
	2003, 2007, 2008, 2010 Free Software Foundation, Inc.

	Permission is granted to copy, distribute and/or modify this document
	under the terms of the GNU Free Documentation License, Version 1.3 or
	any later version published by the Free Software Foundation; with the
	Invariant Sections being ``Free Software Needs Free Documentation''
	and ``GNU Lesser General Public License'', the Front-Cover texts being
	``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A
	copy of the license is included in the section entitled "GNU Free
	Documentation License".

	(a) The FSF's Back-Cover Text is: ``You have the freedom to
	copy and modify this GNU manual. Buying copies from the FSF
	supports it in developing GNU and promoting software freedom.''-->
	<meta http-equiv="Content-Style-Type" content="text/css">
	<style type="text/css"><!--
	pre.display { font-family:inherit }
	pre.format { font-family:inherit }
	pre.smalldisplay { font-family:inherit; font-size:smaller }
	pre.smallformat { font-family:inherit; font-size:smaller }
	pre.smallexample { font-size:smaller }
	pre.smalllisp { font-size:smaller }
	span.sc { font-variant:small-caps }
	span.roman { font-family:serif; font-weight:normal; }
	span.sansserif { font-family:sans-serif; font-weight:normal; }
	--></style>
	<link rel="stylesheet" type="text/css" href="../cs.css">
	</head>
	<body>
	<div class="node">
	<a name="Waiting-for-I%2fO"></a>
	<a name="Waiting-for-I_002fO"></a>
	<p>
	Next: <a rel="next" accesskey="n" href="Synchronizing-I_002fO.html#Synchronizing-I_002fO">Synchronizing I/O</a>,
	Previous: <a rel="previous" accesskey="p" href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO">Memory-mapped I/O</a>,
	Up: <a rel="up" accesskey="u" href="Low_002dLevel-I_002fO.html#Low_002dLevel-I_002fO">Low-Level I/O</a>
	<hr>
	</div>

	<h3 class="section">13.8 Waiting for Input or Output</h3>

	<p><a name="index-waiting-for-input-or-output-1271"></a><a name="index-multiplexing-input-1272"></a><a name="index-input-from-multiple-files-1273"></a>
	Sometimes a program needs to accept input on multiple input channels
	whenever input arrives. For example, some workstations may have devices
	such as a digitizing tablet, function button box, or dial box that are
	connected via normal asynchronous serial interfaces; good user interface
	style requires responding immediately to input on any device. Another
	example is a program that acts as a server to several other processes
	via pipes or sockets.

	<p>You cannot normally use <code>read</code> for this purpose, because this
	blocks the program until input is available on one particular file
	descriptor; input on other channels won't wake it up. You could set
	nonblocking mode and poll each file descriptor in turn, but this is very
	inefficient.

	<p>A better solution is to use the <code>select</code> function. This blocks the
	program until input or output is ready on a specified set of file
	descriptors, or until a timer expires, whichever comes first. This
	facility is declared in the header file <samp><span class="file">sys/types.h</span></samp>.
	<a name="index-sys_002ftypes_002eh-1274"></a>
	In the case of a server socket (see <a href="Listening.html#Listening">Listening</a>), we say that
	“input” is available when there are pending connections that could be
	accepted (see <a href="Accepting-Connections.html#Accepting-Connections">Accepting Connections</a>). <code>accept</code> for server
	sockets blocks and interacts with <code>select</code> just as <code>read</code> does
	for normal input.

	<p><a name="index-file-descriptor-sets_002c-for-_0040code_007bselect_007d-1275"></a>The file descriptor sets for the <code>select</code> function are specified
	as <code>fd_set</code> objects. Here is the description of the data type
	and some macros for manipulating these objects.

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Data Type: <b>fd_set</b><var><a name="index-fd_005fset-1276"></a></var><br>
	<blockquote><p>The <code>fd_set</code> data type represents file descriptor sets for the
	<code>select</code> function. It is actually a bit array.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Macro: int <b>FD_SETSIZE</b><var><a name="index-FD_005fSETSIZE-1277"></a></var><br>
	<blockquote><p>The value of this macro is the maximum number of file descriptors that a
	<code>fd_set</code> object can hold information about. On systems with a
	fixed maximum number, <code>FD_SETSIZE</code> is at least that number. On
	some systems, including GNU, there is no absolute limit on the number of
	descriptors open, but this macro still has a constant value which
	controls the number of bits in an <code>fd_set</code>; if you get a file
	descriptor with a value as high as <code>FD_SETSIZE</code>, you cannot put
	that descriptor into an <code>fd_set</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Macro: void <b>FD_ZERO</b> (<var>fd_set *set</var>)<var><a name="index-FD_005fZERO-1278"></a></var><br>
	<blockquote><p>This macro initializes the file descriptor set <var>set</var> to be the
	empty set.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Macro: void <b>FD_SET</b> (<var>int filedes, fd_set *set</var>)<var><a name="index-FD_005fSET-1279"></a></var><br>
	<blockquote><p>This macro adds <var>filedes</var> to the file descriptor set <var>set</var>.

	<p>The <var>filedes</var> parameter must not have side effects since it is
	evaluated more than once.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Macro: void <b>FD_CLR</b> (<var>int filedes, fd_set *set</var>)<var><a name="index-FD_005fCLR-1280"></a></var><br>
	<blockquote><p>This macro removes <var>filedes</var> from the file descriptor set <var>set</var>.

	<p>The <var>filedes</var> parameter must not have side effects since it is
	evaluated more than once.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Macro: int <b>FD_ISSET</b> (<var>int filedes, const fd_set *set</var>)<var><a name="index-FD_005fISSET-1281"></a></var><br>
	<blockquote><p>This macro returns a nonzero value (true) if <var>filedes</var> is a member
	of the file descriptor set <var>set</var>, and zero (false) otherwise.

	<p>The <var>filedes</var> parameter must not have side effects since it is
	evaluated more than once.
	</p></blockquote></div>

	<p>Next, here is the description of the <code>select</code> function itself.

	<!-- sys/types.h -->
	<!-- BSD -->
	<div class="defun">
	— Function: int <b>select</b> (<var>int nfds, fd_set read-fds, fd_set write-fds, fd_set except-fds, struct timeval timeout</var>)<var><a name="index-select-1282"></a></var><br>
	<blockquote><p>The <code>select</code> function blocks the calling process until there is
	activity on any of the specified sets of file descriptors, or until the
	timeout period has expired.

	<p>The file descriptors specified by the <var>read-fds</var> argument are
	checked to see if they are ready for reading; the <var>write-fds</var> file
	descriptors are checked to see if they are ready for writing; and the
	<var>except-fds</var> file descriptors are checked for exceptional
	conditions. You can pass a null pointer for any of these arguments if
	you are not interested in checking for that kind of condition.

	<p>A file descriptor is considered ready for reading if a <code>read</code>
	call will not block. This usually includes the read offset being at
	the end of the file or there is an error to report. A server socket
	is considered ready for reading if there is a pending connection which
	can be accepted with <code>accept</code>; see <a href="Accepting-Connections.html#Accepting-Connections">Accepting Connections</a>. A
	client socket is ready for writing when its connection is fully
	established; see <a href="Connecting.html#Connecting">Connecting</a>.

	<p>“Exceptional conditions” does not mean errors—errors are reported
	immediately when an erroneous system call is executed, and do not
	constitute a state of the descriptor. Rather, they include conditions
	such as the presence of an urgent message on a socket. (See <a href="Sockets.html#Sockets">Sockets</a>,
	for information on urgent messages.)

	<p>The <code>select</code> function checks only the first <var>nfds</var> file
	descriptors. The usual thing is to pass <code>FD_SETSIZE</code> as the value
	of this argument.

	<p>The <var>timeout</var> specifies the maximum time to wait. If you pass a
	null pointer for this argument, it means to block indefinitely until one
	of the file descriptors is ready. Otherwise, you should provide the
	time in <code>struct timeval</code> format; see <a href="High_002dResolution-Calendar.html#High_002dResolution-Calendar">High-Resolution Calendar</a>. Specify zero as the time (a <code>struct timeval</code> containing
	all zeros) if you want to find out which descriptors are ready without
	waiting if none are ready.

	<p>The normal return value from <code>select</code> is the total number of ready file
	descriptors in all of the sets. Each of the argument sets is overwritten
	with information about the descriptors that are ready for the corresponding
	operation. Thus, to see if a particular descriptor <var>desc</var> has input,
	use <code>FD_ISSET (</code><var>desc</var><code>, </code><var>read-fds</var><code>)</code> after <code>select</code> returns.

	<p>If <code>select</code> returns because the timeout period expires, it returns
	a value of zero.

	<p>Any signal will cause <code>select</code> to return immediately. So if your
	program uses signals, you can't rely on <code>select</code> to keep waiting
	for the full time specified. If you want to be sure of waiting for a
	particular amount of time, you must check for <code>EINTR</code> and repeat
	the <code>select</code> with a newly calculated timeout based on the current
	time. See the example below. See also <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>.

	<p>If an error occurs, <code>select</code> returns <code>-1</code> and does not modify
	the argument file descriptor sets. The following <code>errno</code> error
	conditions are defined for this function:

	<dl>
	<dt><code>EBADF</code><dd>One of the file descriptor sets specified an invalid file descriptor.

	<br><dt><code>EINTR</code><dd>The operation was interrupted by a signal. See <a href="Interrupted-Primitives.html#Interrupted-Primitives">Interrupted Primitives</a>.

	<br><dt><code>EINVAL</code><dd>The <var>timeout</var> argument is invalid; one of the components is negative
	or too large.
	</dl>
	</p></blockquote></div>

	<p><strong>Portability Note:</strong> The <code>select</code> function is a BSD Unix
	feature.

	<p>Here is an example showing how you can use <code>select</code> to establish a
	timeout period for reading from a file descriptor. The <code>input_timeout</code>
	function blocks the calling process until input is available on the
	file descriptor, or until the timeout period expires.

	<pre class="smallexample"> #include <errno.h>
	#include <stdio.h>
	#include <unistd.h>
	#include <sys/types.h>
	#include <sys/time.h>

	int
	input_timeout (int filedes, unsigned int seconds)
	{
	fd_set set;
	struct timeval timeout;

	/* <span class="roman">Initialize the file descriptor set.</span> */
	FD_ZERO (&set);
	FD_SET (filedes, &set);

	/* <span class="roman">Initialize the timeout data structure.</span> */
	timeout.tv_sec = seconds;
	timeout.tv_usec = 0;

	/* <code>select</code><span class="roman"> returns 0 if timeout, 1 if input available, -1 if error.</span> */
	return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
	&set, NULL, NULL,
	&timeout));
	}

	int
	main (void)
	{
	fprintf (stderr, "select returned %d.\n",
	input_timeout (STDIN_FILENO, 5));
	return 0;
	}
	</pre>
	<p>There is another example showing the use of <code>select</code> to multiplex
	input from multiple sockets in <a href="Server-Example.html#Server-Example">Server Example</a>.

	</body></html>