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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>clock</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="clock"></a> <a name="tag_03_74"></a><!-- clock --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_74_01"></a>NAME</h4><blockquote>clock - report CPU time used</blockquote><h4><a name="tag_03_74_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/time.h.html">time.h</a>><br><br> clock_t clock(void);<br></tt></code></p></blockquote><h4><a name="tag_03_74_03"></a>DESCRIPTION</h4><blockquote><div class="box"><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"border="0"> The functionality described on this reference page is aligned with the ISO C standard. Any conflict between therequirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers tothe ISO C standard. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></div><p>The <i>clock</i>() function shall return the implementation's best approximation to the processor time used by the process sincethe beginning of an implementation-defined era related only to the process invocation.</p></blockquote><h4><a name="tag_03_74_04"></a>RETURN VALUE</h4><blockquote><p>To determine the time in seconds, the value returned by <i>clock</i>() should be divided by the value of the macroCLOCKS_PER_SEC. <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"border="0"> CLOCKS_PER_SEC is defined to be one million in <a href="../basedefs/time.h.html"><i><time.h></i></a>. <imgsrc="../images/opt-end.gif" alt="[Option End]" border="0"> If the processor time used is not available or its value cannot berepresented, the function shall return the value ( <b>clock_t</b>)-1.</p></blockquote><h4><a name="tag_03_74_05"></a>ERRORS</h4><blockquote><p>No errors are defined.</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_74_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_74_07"></a>APPLICATION USAGE</h4><blockquote><p>In order to measure the time spent in a program, <i>clock</i>() should be called at the start of the program and its returnvalue subtracted from the value returned by subsequent calls. The value returned by <i>clock</i>() is defined for compatibilityacross systems that have clocks with different resolutions. The resolution on any particular system need not be to microsecondaccuracy.</p><p>The value returned by <i>clock</i>() may wrap around on some implementations. For example, on a machine with 32-bit values for<b>clock_t</b>, it wraps after 2147 seconds or 36 minutes.</p></blockquote><h4><a name="tag_03_74_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_74_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_74_10"></a>SEE ALSO</h4><blockquote><p><a href="asctime.html"><i>asctime</i>()</a> , <a href="ctime.html"><i>ctime</i>()</a> , <a href="difftime.html"><i>difftime</i>()</a> , <a href="gmtime.html"><i>gmtime</i>()</a> , <a href="localtime.html"><i>localtime</i>()</a>, <a href="mktime.html"><i>mktime</i>()</a> , <a href="strftime.html"><i>strftime</i>()</a> , <a href="strptime.html"><i>strptime</i>()</a> , <a href="time.html"><i>time</i>()</a> , <a href="utime.html"><i>utime</i>()</a> , the BaseDefinitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/time.h.html"><i><time.h></i></a></p></blockquote><h4><a name="tag_03_74_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 1. Derived from Issue 1 of the SVID.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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