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| <h4 class="subsection">20.5.3 Examining the FPU status word</h4> |
| |
| <p>ISO C99<!-- /@w --> defines functions to query and manipulate the |
| floating-point status word. You can use these functions to check for |
| untrapped exceptions when it's convenient, rather than worrying about |
| them in the middle of a calculation. |
| |
| <p>These constants represent the various IEEE 754<!-- /@w --> exceptions. Not all |
| FPUs report all the different exceptions. Each constant is defined if |
| and only if the FPU you are compiling for supports that exception, so |
| you can test for FPU support with ‘<samp><span class="samp">#ifdef</span></samp>’. They are defined in |
| <samp><span class="file">fenv.h</span></samp>. |
| |
| <dl> |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <dt><code>FE_INEXACT</code><a name="index-FE_005fINEXACT-2373"></a><dd> The inexact exception. |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <br><dt><code>FE_DIVBYZERO</code><a name="index-FE_005fDIVBYZERO-2374"></a><dd> The divide by zero exception. |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <br><dt><code>FE_UNDERFLOW</code><a name="index-FE_005fUNDERFLOW-2375"></a><dd> The underflow exception. |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <br><dt><code>FE_OVERFLOW</code><a name="index-FE_005fOVERFLOW-2376"></a><dd> The overflow exception. |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <br><dt><code>FE_INVALID</code><a name="index-FE_005fINVALID-2377"></a><dd> The invalid exception. |
| </dl> |
| |
| <p>The macro <code>FE_ALL_EXCEPT</code> is the bitwise OR of all exception macros |
| which are supported by the FP implementation. |
| |
| <p>These functions allow you to clear exception flags, test for exceptions, |
| and save and restore the set of exceptions flagged. |
| |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>feclearexcept</b> (<var>int excepts</var>)<var><a name="index-feclearexcept-2378"></a></var><br> |
| <blockquote><p>This function clears all of the supported exception flags indicated by |
| <var>excepts</var>. |
| |
| <p>The function returns zero in case the operation was successful, a |
| non-zero value otherwise. |
| </p></blockquote></div> |
| |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>feraiseexcept</b> (<var>int excepts</var>)<var><a name="index-feraiseexcept-2379"></a></var><br> |
| <blockquote><p>This function raises the supported exceptions indicated by |
| <var>excepts</var>. If more than one exception bit in <var>excepts</var> is set |
| the order in which the exceptions are raised is undefined except that |
| overflow (<code>FE_OVERFLOW</code>) or underflow (<code>FE_UNDERFLOW</code>) are |
| raised before inexact (<code>FE_INEXACT</code>). Whether for overflow or |
| underflow the inexact exception is also raised is also implementation |
| dependent. |
| |
| <p>The function returns zero in case the operation was successful, a |
| non-zero value otherwise. |
| </p></blockquote></div> |
| |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>fetestexcept</b> (<var>int excepts</var>)<var><a name="index-fetestexcept-2380"></a></var><br> |
| <blockquote><p>Test whether the exception flags indicated by the parameter <var>except</var> |
| are currently set. If any of them are, a nonzero value is returned |
| which specifies which exceptions are set. Otherwise the result is zero. |
| </p></blockquote></div> |
| |
| <p>To understand these functions, imagine that the status word is an |
| integer variable named <var>status</var>. <code>feclearexcept</code> is then |
| equivalent to ‘<samp><span class="samp">status &= ~excepts</span></samp>’ and <code>fetestexcept</code> is |
| equivalent to ‘<samp><span class="samp">(status & excepts)</span></samp>’. The actual implementation may |
| be very different, of course. |
| |
| <p>Exception flags are only cleared when the program explicitly requests it, |
| by calling <code>feclearexcept</code>. If you want to check for exceptions |
| from a set of calculations, you should clear all the flags first. Here |
| is a simple example of the way to use <code>fetestexcept</code>: |
| |
| <pre class="smallexample"> { |
| double f; |
| int raised; |
| feclearexcept (FE_ALL_EXCEPT); |
| f = compute (); |
| raised = fetestexcept (FE_OVERFLOW | FE_INVALID); |
| if (raised & FE_OVERFLOW) { /* ... */ } |
| if (raised & FE_INVALID) { /* ... */ } |
| /* ... */ |
| } |
| </pre> |
| <p>You cannot explicitly set bits in the status word. You can, however, |
| save the entire status word and restore it later. This is done with the |
| following functions: |
| |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>fegetexceptflag</b> (<var>fexcept_t *flagp, int excepts</var>)<var><a name="index-fegetexceptflag-2381"></a></var><br> |
| <blockquote><p>This function stores in the variable pointed to by <var>flagp</var> an |
| implementation-defined value representing the current setting of the |
| exception flags indicated by <var>excepts</var>. |
| |
| <p>The function returns zero in case the operation was successful, a |
| non-zero value otherwise. |
| </p></blockquote></div> |
| |
| <!-- fenv.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>fesetexceptflag</b> (<var>const fexcept_t *flagp, int excepts</var>)<var><a name="index-fesetexceptflag-2382"></a></var><br> |
| <blockquote><p>This function restores the flags for the exceptions indicated by |
| <var>excepts</var> to the values stored in the variable pointed to by |
| <var>flagp</var>. |
| |
| <p>The function returns zero in case the operation was successful, a |
| non-zero value otherwise. |
| </p></blockquote></div> |
| |
| <p>Note that the value stored in <code>fexcept_t</code> bears no resemblance to |
| the bit mask returned by <code>fetestexcept</code>. The type may not even be |
| an integer. Do not attempt to modify an <code>fexcept_t</code> variable. |
| |
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