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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>select</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_008_261"> </a>NAME</h4><blockquote>select - synchronous I/O multiplexing</blockquote><h4><a name = "tag_000_008_262"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="systime.h.html">sys/time.h</a>>int select(int <i>nfds</i>, fd_set *<i>readfds</i>, fd_set *<i>writefds</i>, fd_set *<i>errorfds</i>, struct timeval *<i>timeout</i>);void FD_CLR(int <i>fd</i>, fd_set *<i>fdset</i>);int FD_ISSET(int <i>fd</i>, fd_set *<i>fdset</i>);void FD_SET(int <i>fd</i>, fd_set *<i>fdset</i>);void FD_ZERO(fd_set *<i>fdset</i>);</code></pre></blockquote><h4><a name = "tag_000_008_263"> </a>DESCRIPTION</h4><blockquote>The<i>select()</i>function indicates which of the specified file descriptors is ready forreading, ready for writing, or has an error condition pending. If thespecified condition is false for all of the specified file descriptors,<i>select()</i>blocks, up to the specified timeout interval, until the specified condition istrue for at least one of the specified file descriptors.<p>The<i>select()</i>function supports regular files, terminal and pseudo-terminal devices,STREAMS-based files, FIFOs and pipes. The behaviour of<i>select()</i>on file descriptors that refer to other types of file is unspecified.<p>The <i>nfds</i> argument specifies the range of file descriptors to be tested.The<i>select()</i>function tests file descriptors in the range of 0 to <i>nfds</i>-1.<p>If the <i>readfds</i> argument is not a null pointer, it points to anobject of type <b>fd_set</b> that on input specifies the filedescriptors to be checked for being ready to read, and on outputindicates which file descriptors are ready to read.<p>If the <i>writefds</i> argument is not a null pointer, it points to anobject of type <b>fd_set</b> that on input specifies the filedescriptors to be checked for being ready to write, and on outputindicates which file descriptors are ready to write.<p>If the <i>errorfds</i> argument is not a null pointer, it points to anobject of type <b>fd_set</b> that on input specifies the filedescriptors to be checked for error conditions pending, and on outputindicates which file descriptors have error conditions pending.<p>On successful completion, the objects pointed to by the <i>readfds</i>,<i>writefds</i>, and <i>errorfds</i> arguments are modified to indicatewhich file descriptors are ready for reading, ready for writing, orhave an error condition pending, respectively. For each filedescriptor less than <i>nfds</i>, the corresponding bit will be set onsuccessful completion if it was set on input and the associatedcondition is true for that file descriptor.<p>If the <i>timeout</i> argument is not a null pointer, it points to anobject of type <b>struct timeval</b> that specifies a maximum intervalto wait for the selection to complete.If the <i>timeout</i> argument points to an object of type<b>struct timeval</b> whose members are 0,<i>select()</i>does not block.If the <i>timeout</i> argument is a null pointer,<i>select()</i>blocks until an event causes one of the masks to be returned with a valid(non-zero) value. If the time limit expires before any event occurs thatwould cause one of the masks to be set to a non-zero value,<i>select()</i>completes successfully and returns 0.<p>The use of a timeout does not affect any pending timers set up by<i><a href="alarm.html">alarm()</a></i>,<i><a href="ualarm.html">ualarm()</a></i>or<i><a href="setitimer.html">setitimer()</a></i>.<p>On successful completion, the object pointed to by the <i>timeout</i>argument may be modified.<p>Implementations may place limitations on the maximum timeout intervalsupported.On all implementations,the maximum timeout interval supportedwill be at least 31 days.If the <i>timeout</i> argument specifies a timeout interval greaterthan the implementation-dependent maximum value, themaximum value will be used as the actualtimeout value. Implementations may also place limitations on thegranularity of timeout intervals. If the requested timeout intervalrequires a finer granularity than the implementation supports, theactual timeout interval will be rounded up to the next supportedvalue.<p>If the <i>readfds</i>, <i>writefds</i>, and <i>errorfds</i> arguments areall null pointers andthe <i>timeout</i> argumentis not a null pointer,<i>select()</i>blocks for the time specified, or until interrupted by a signal.If the <i>readfds</i>, <i>writefds</i>, and <i>errorfds</i> arguments areall null pointers and the <i>timeout</i> argument is a null pointer,<i>select()</i>blocks until interrupted by a signal.<p>File descriptors associated with regular files always select truefor ready to read, ready to write, and error conditions.<p>On failure, the objectspointed to by the <i>readfds</i>, <i>writefds</i>, and <i>errorfds</i>arguments are not modified. If the timeout interval expires withoutthe specified condition being true for any of the specified filedescriptors, theobjects pointed to by the <i>readfds</i>, <i>writefds</i>, and <i>errorfds</i>arguments have all bits set to 0.<p>File descriptor masks of type <b>fd_set</b> can be initialisedand tested with FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO().It is unspecified whether each of these is a macro or a function. If a macrodefinition is suppressed in order to access an actual function, or a programdefines an external identifier with any of these names, the behaviour isundefined.<dl compact><dt>FD_CLR(<i>fd</i>, &<i>fdset</i>)<dd>Clears the bit for the file descriptor <i>fd</i> inthe file descriptor set <i>fdset</i>.<dt>FD_ISSET(<i>fd</i>, &<i>fdset</i>)<dd>Returns a non-zero value if the bit for the file descriptor <i>fd</i> isset in the file descriptor set pointed to by <i>fdset</i>, and 0 otherwise.<dt>FD_SET(<i>fd</i>, &<i>fdset</i>)<dd>Sets the bit for the file descriptor <i>fd</i> in the filedescriptor set <i>fdset</i>.<dt>FD_ZERO(&<i>fdset</i>)<dd>Initialises the file descriptor set <i>fdset</i>to have zero bits for all file descriptors.</dl><p>The behaviour of these macros is undefined if the <i>fd</i> argument isless than 0 or greater than or equal to FD_SETSIZE, or if any of thearguments are expressions with side effects.</blockquote><h4><a name = "tag_000_008_264"> </a>RETURN VALUE</h4><blockquote>FD_CLR(), FD_SET() and FD_ZERO() return no value.FD_ISSET() a non-zero value if the bit for the file descriptor<i>fd</i>is set in the file descriptor set pointed to by<i>fdset</i>,and 0 otherwise.<p>On successful completion,<i>select()</i>returns the total number of bits set in the bit masks.Otherwise, -1 is returned, and <i>errno</i> is set to indicate the error.</blockquote><h4><a name = "tag_000_008_265"> </a>ERRORS</h4><blockquote>Under the following conditions,<i>select()</i>fails and sets <i>errno</i> to:<dl compact><dt>[EBADF]<dd>One or more of the file descriptor sets specified a file descriptor that isnot a valid open file descriptor.<dt>[EINTR]<dd>The<i>select()</i>function was interrupted before any of the selected events occurred and beforethe timeout interval expired.If SA_RESTART has been set for the interrupting signal, it isimplementation-dependent whether<i>select()</i>restarts or returns with [EINTR].<dt>[EINVAL]<dd>An invalid timeout interval was specified.<dt>[EINVAL]<dd>The <i>nfds</i> argument is less than 0 or greater thanFD_SETSIZE.<dt>[EINVAL]<dd>One of the specified file descriptors refers to a STREAM or multiplexer thatis linked (directly or indirectly) downstream from a multiplexer.</dl></blockquote><h4><a name = "tag_000_008_266"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_267"> </a>APPLICATION USAGE</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_268"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_269"> </a>SEE ALSO</h4><blockquote><i><a href="fcntl.html">fcntl()</a></i>,<i><a href="poll.html">poll()</a></i>,<i><a href="read.html">read()</a></i>,<i><a href="write.html">write()</a></i>,<i><a href="systime.h.html"><sys/time.h></a></i>.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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