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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><fenv.h></title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="<fenv.h>"></a> <a name="tag_13_12"></a><!-- <fenv.h> --> <!--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_13_12_01"></a>NAME</h4><blockquote>fenv.h - floating-point environment</blockquote><h4><a name="tag_13_12_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><tt>#include <fenv.h></tt></p></blockquote><h4><a name="tag_13_12_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><fenv.h></i> header shall define the following data types through <b>typedef</b>:</p><dl compact><dt><b>fenv_t</b></dt><dd>Represents the entire floating-point environment. The floating-point environment refers collectively to any floating-pointstatus flags and control modes supported by the implementation.</dd><dt><b>fexcept_t</b></dt><dd>Represents the floating-point status flags collectively, including any status the implementation associates with the flags. Afloating-point status flag is a system variable whose value is set (but never cleared) when a floating-point exception is raised,which occurs as a side effect of exceptional floating-point arithmetic to provide auxiliary information. A floating-point controlmode is a system variable whose value may be set by the user to affect the subsequent behavior of floating-point arithmetic.</dd></dl><p>The <i><fenv.h></i> header shall define the following constants if and only if the implementation supports thefloating-point exception by means of the floating-point functions <a href="../functions/feclearexcept.html"><i>feclearexcept</i>()</a>, <a href="../functions/fegetexceptflag.html"><i>fegetexceptflag</i>()</a>, <a href="../functions/feraiseexcept.html"><i>feraiseexcept</i>()</a>, <a href="../functions/fesetexceptflag.html"><i>fesetexceptflag</i>()</a>, and <a href="../functions/fetestexcept.html"><i>fetestexcept</i>()</a>. Each expands to an integer constant expression with values such thatbitwise-inclusive ORs of all combinations of the constants result in distinct values.</p><blockquote><pre>FE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW</pre></blockquote><p>The <i><fenv.h></i> header shall define the following constant, which is simply the bitwise-inclusive OR of allfloating-point exception constants defined above:</p><blockquote><pre>FE_ALL_EXCEPT</pre></blockquote><p>The <i><fenv.h></i> header shall define the following constants if and only if the implementation supports getting andsetting the represented rounding direction by means of the <a href="../functions/fegetround.html"><i>fegetround</i>()</a> and <ahref="../functions/fesetround.html"><i>fesetround</i>()</a> functions. Each expands to an integer constant expression whose valuesare distinct non-negative vales.</p><blockquote><pre>FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD</pre></blockquote><p>The <i><fenv.h></i> header shall define the following constant, which represents the default floating-point environment(that is, the one installed at program startup) and has type pointer to const-qualified <b>fenv_t</b>. It can be used as anargument to the functions within the <i><fenv.h></i> header that manage the floating-point environment.</p><blockquote><pre>FE_DFL_ENV</pre></blockquote><p>The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.</p><pre><tt>int feclearexcept(int);int fegetexceptflag(fexcept_t *, int);int feraiseexcept(int);int fesetexceptflag(const fexcept_t *, int);int fetestexcept(int);int fegetround(void);int fesetround(int);int fegetenv(fenv_t *);int feholdexcept(fenv_t *);int fesetenv(const fenv_t *);int feupdateenv(const fenv_t *);</tt></pre><p>The FENV_ACCESS pragma provides a means to inform the implementation when an application might access the floating-pointenvironment to test floating-point status flags or run under non-default floating-point control modes. The pragma shall occureither outside external declarations or preceding all explicit declarations and statements inside a compound statement. Whenoutside external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered, oruntil the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until anotherFENV_ACCESS pragma is encountered (including within a nested compound statement), or until the end of the compound statement; atthe end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If thispragma is used in any other context, the behavior is undefined. If part of an application tests floating-point status flags, setsfloating-point control modes, or runs under non-default mode settings, but was translated with the state for the FENV_ACCESS pragmaoff, the behavior is undefined. The default state (on or off) for the pragma is implementation-defined. (When execution passes froma part of the application translated with FENV_ACCESS off to a part translated with FENV_ACCESS on, the state of the floating-pointstatus flags is unspecified and the floating-point control modes have their default settings.)</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_13_12_04"></a>APPLICATION USAGE</h4><blockquote><p>This header is designed to support the floating-point exception status flags and directed-rounding control modes required by theIEC 60559:1989 standard, and other similar floating-point state information. Also it is designed to facilitate codeportability among all systems.</p><p>Certain application programming conventions support the intended model of use for the floating-point environment:</p><ul><li><p>A function call does not alter its caller's floating-point control modes, clear its caller's floating-point status flags, nordepend on the state of its caller's floating-point status flags unless the function is so documented.</p></li><li><p>A function call is assumed to require default floating-point control modes, unless its documentation promises otherwise.</p></li><li><p>A function call is assumed to have the potential for raising floating-point exceptions, unless its documentation promisesotherwise.</p></li></ul><p>With these conventions, an application can safely assume default floating-point control modes (or be unaware of them). Theresponsibilities associated with accessing the floating-point environment fall on the application that does so explicitly.</p><p>Even though the rounding direction macros may expand to constants corresponding to the values of FLT_ROUNDS, they are notrequired to do so.</p><p>For example:</p><pre><tt>#include <fenv.h>void f(double x){ #pragma STDC FENV_ACCESS ON void g(double); void h(double); /* ... */ g(x + 1); h(x + 1); /* ... */}</tt></pre><p>If the function <i>g</i>() might depend on status flags set as a side effect of the first <i>x</i>+1, or if the second<i>x</i>+1 might depend on control modes set as a side effect of the call to function <i>g</i>(), then the application shallcontain an appropriately placed invocation as follows:</p><pre><tt>#pragma STDC FENV_ACCESS ON</tt></pre></blockquote><h4><a name="tag_13_12_05"></a>RATIONALE</h4><blockquote><h5><a name="tag_13_12_05_01"></a>The fexcept_t Type</h5><p><b>fexcept_t</b> does not have to be an integer type. Its values must be obtained by a call to <a href="../functions/fegetexceptflag.html"><i>fegetexceptflag</i>()</a>, and cannot be created by logical operations from the exceptionmacros. An implementation might simply implement <b>fexcept_t</b> as an <b>int</b> and use the representations reflected by theexception macros, but is not required to; other representations might contain extra information about the exceptions.<b>fexcept_t</b> might be a <b>struct</b> with a member for each exception (that might hold the address of the first or lastfloating-point instruction that caused that exception). The ISO/IEC 9899:1999 standard makes no claims about the internals ofan <b>fexcept_t</b>, and so the user cannot inspect it.</p><h5><a name="tag_13_12_05_02"></a>Exception and Rounding Macros</h5><p>Macros corresponding to unsupported modes and rounding directions are not defined by the implementation and must not be definedby the application. An application might use <b>#ifdef</b> to test for this.</p></blockquote><h4><a name="tag_13_12_06"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_13_12_07"></a>SEE ALSO</h4><blockquote><p>The System Interfaces volume of IEEE Std 1003.1-2001, <a href="../functions/feclearexcept.html"><i>feclearexcept</i>()</a>, <a href="../functions/fegetenv.html"><i>fegetenv</i>()</a>, <a href="../functions/fegetexceptflag.html"><i>fegetexceptflag</i>()</a>, <a href="../functions/fegetround.html"><i>fegetround</i>()</a>,<a href="../functions/feholdexcept.html"><i>feholdexcept</i>()</a>, <a href="../functions/feraiseexcept.html"><i>feraiseexcept</i>()</a>, <a href="../functions/fesetenv.html"><i>fesetenv</i>()</a>, <a href="../functions/fesetexceptflag.html"><i>fesetexceptflag</i>()</a>, <a href="../functions/fesetround.html"><i>fesetround</i>()</a>,<a href="../functions/fetestexcept.html"><i>fetestexcept</i>()</a>, <a href="../functions/feupdateenv.html"><i>feupdateenv</i>()</a></p></blockquote><h4><a name="tag_13_12_08"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 6. Included for alignment with the ISO/IEC 9899:1999 standard.</p><p>The return types for <a href="../functions/feclearexcept.html"><i>feclearexcept</i>()</a>, <a href="../functions/fegetexceptflag.html"><i>fegetexceptflag</i>()</a>, <a href="../functions/feraiseexcept.html"><i>feraiseexcept</i>()</a>, <a href="../functions/fesetexceptflag.html"><i>fesetexceptflag</i>()</a>, <a href="../functions/fegetenv.html"><i>fegetenv</i>()</a>, <ahref="../functions/fesetenv.html"><i>fesetenv</i>()</a>, and <a href="../functions/feupdateenv.html"><i>feupdateenv</i>()</a> arechanged from <b>void</b> to <b>int</b> for alignment with the ISO/IEC 9899:1999 standard, Defect Report 202.</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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