fd_zero.html

posix标准英文,html格式

<p><i>FD_ISSET</i>(<i>fd</i>, <i>fdsetp</i>) shall evaluate to non-zero if the file descriptor <i>fd</i> is a member of the set
pointed to by <i>fdsetp</i>, and shall evaluate to zero otherwise.</p>

<p><i>FD_SET</i>(<i>fd</i>, <i>fdsetp</i>) shall add the file descriptor <i>fd</i> to the set pointed to by <i>fdsetp</i>. If the
file descriptor <i>fd</i> is already in this set, there shall be no effect on the set, nor will an error be returned.</p>

<p><i>FD_ZERO</i>(<i>fdsetp</i>) shall initialize the descriptor set pointed to by <i>fdsetp</i> to the null set. No error is
returned if the set is not empty at the time <i>FD_ZERO</i>() is invoked.</p>

<p>The behavior of these macros is undefined if the <i>fd</i> argument is less than 0 or greater than or equal to FD_SETSIZE, or if
<i>fd</i> is not a valid file descriptor, or if any of the arguments are expressions with side effects.</p>
</blockquote>

<h4><a name="tag_03_485_04"></a>RETURN VALUE</h4>

<blockquote>
<p>Upon successful completion, the <i>pselect</i>() and <i>select</i>() functions shall return the total number of bits set in the
bit masks. Otherwise, -1 shall be returned, and <i>errno</i> shall be set to indicate the error.</p>

<p><i>FD_CLR</i>(), <i>FD_SET</i>(), and <i>FD_ZERO</i>() do not return a value. <i>FD_ISSET</i>() shall return 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>
</blockquote>

<h4><a name="tag_03_485_05"></a>ERRORS</h4>

<blockquote>
<p>Under the following conditions, <i>pselect</i>() and <i>select</i>() shall fail and set <i>errno</i> to:</p>

<dl compact>
<dt>[EBADF]</dt>

<dd>One or more of the file descriptor sets specified a file descriptor that is not a valid open file descriptor.</dd>

<dt>[EINTR]</dt>

<dd>The function was interrupted before any of the selected events occurred and before the timeout interval expired. 

<p><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
If SA_RESTART has been set for the interrupting signal, it is implementation-defined whether the function restarts or returns with
[EINTR]. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
</dd>

<dt>[EINVAL]</dt>

<dd>An invalid timeout interval was specified.</dd>

<dt>[EINVAL]</dt>

<dd>The <i>nfds</i> argument is less than 0 or greater than FD_SETSIZE.</dd>

<dt>[EINVAL]</dt>

<dd><sup>[<a href="javascript:open_code('XSR')">XSR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
One of the specified file descriptors refers to a STREAM or multiplexer that is linked (directly or indirectly) downstream from a
multiplexer. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
</dl>
</blockquote>

<hr>
<div class="box"><em>The following sections are informative.</em></div>

<h4><a name="tag_03_485_06"></a>EXAMPLES</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_03_485_07"></a>APPLICATION USAGE</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_03_485_08"></a>RATIONALE</h4>

<blockquote>
<p>In previous versions of the Single UNIX Specification, the <i>select</i>() function was defined in the <a href=
"../basedefs/sys/time.h.html"><i>&lt;sys/time.h&gt;</i></a> header. This is now changed to <a href=
"../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a>. The rationale for this change was as follows: the introduction of
the <i>pselect</i>() function included the <a href="../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a> header and the
<a href="../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a> header defines all the related definitions for the
<i>pselect</i>() and <i>select</i>() functions. Backwards-compatibility to existing XSI implementations is handled by allowing <a
href="../basedefs/sys/time.h.html"><i>&lt;sys/time.h&gt;</i></a> to include <a href=
"../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a>.</p>
</blockquote>

<h4><a name="tag_03_485_09"></a>FUTURE DIRECTIONS</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_03_485_10"></a>SEE ALSO</h4>

<blockquote>
<p><a href="accept.html"><i>accept</i>()</a>, <a href="alarm.html"><i>alarm</i>()</a>, <a href=
"connect.html"><i>connect</i>()</a>, <a href="fcntl.html"><i>fcntl</i>()</a>, <a href="poll.html"><i>poll</i>()</a>, <a href=
"read.html"><i>read</i>()</a>, <a href="recvmsg.html"><i>recvmsg</i>()</a>, <a href="sendmsg.html"><i>sendmsg</i>()</a>, <a
href="setitimer.html"><i>setitimer</i>()</a>, <a href="ualarm.html"><i>ualarm</i>()</a>, <a href="write.html"><i>write</i>()</a>
, the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a>, <a href=
"../basedefs/sys/time.h.html"><i>&lt;sys/time.h&gt;</i></a></p>
</blockquote>

<h4><a name="tag_03_485_11"></a>CHANGE HISTORY</h4>

<blockquote>
<p>First released in Issue 4, Version 2.</p>
</blockquote>

<h4><a name="tag_03_485_12"></a>Issue 5</h4>

<blockquote>
<p>Moved from X/OPEN UNIX extension to BASE.</p>

<p>In the ERRORS section, the text has been changed to indicate that [EINVAL] is returned when <i>nfds</i> is less than 0 or
greater than FD_SETSIZE. It previously stated less than 0, or greater than or equal to FD_SETSIZE.</p>

<p>Text about <i>timeout</i> is moved from the APPLICATION USAGE section to the DESCRIPTION.</p>
</blockquote>

<h4><a name="tag_03_485_13"></a>Issue 6</h4>

<blockquote>
<p>The Open Group Corrigendum U026/6 is applied, changing the occurrences of <i>readfs</i> and <i>writefs</i> in the
<i>select</i>() DESCRIPTION to be <i>readfds</i> and <i>writefds</i>.</p>

<p>Text referring to sockets is added to the DESCRIPTION.</p>

<p>The DESCRIPTION and ERRORS sections are updated so that references to STREAMS are marked as part of the XSI STREAMS Option
Group.</p>

<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>

<ul>
<li>
<p>These functions are now mandatory.</p>
</li>
</ul>

<p>The <i>pselect</i>() function is added for alignment with IEEE&nbsp;Std&nbsp;1003.1g-2000 and additional detail related to
sockets semantics is added to the DESCRIPTION.</p>

<p>The <i>select</i>() function now requires inclusion of <a href=
"../basedefs/sys/select.h.html"><i>&lt;sys/select.h&gt;</i></a>.</p>

<p>The <b>restrict</b> keyword is added to the <i>select</i>() prototype for alignment with the ISO/IEC&nbsp;9899:1999
standard.</p>

<p>IEEE&nbsp;Std&nbsp;1003.1-2001/Cor&nbsp;2-2004, item XSH/TC2/D6/70 is applied, updating the DESCRIPTION to reference the signal
mask in terms of the calling thread rather than the process.</p>
</blockquote>

<div class="box"><em>End of informative text.</em></div>

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