feholdexcept

feholdexcept

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Saves the current floating-point environment and switches to nonstop mode

 #include <fenv.h> int feholdexcept ( fenv_t *envp  ); 

Like fegetenv( ), the feholdexcept( ) function saves the current floating-point environment in the object pointed to by the pointer argument. However, feholdexcept( ) also clears the floating-point status flags and switches the floating-point environment to a nonstop mode, meaning that after any floating-point exception, normal execution continues uninterrupted by signals or traps. The function returns 0 if it succeeds in switching to nonstop floating-point processing; otherwise, the return value is nonzero.

Example

 /*  * Compute the hypoteneuse of a right triangle, avoiding intermediate  * overflow or underflow.  *  * (This example ignores the case of one argument having great magnitude  * and the other small, causing both overflow and underflow!)  */ double hypoteneuse(double sidea, double sideb) { #pragma STDC FENV_ACCESS ON   double  sum, scale, ascaled, bscaled, invscale;   fenv_t  fpenv;   int     fpeflags;   if ( signbit( sidea ) ) sidea = fabs( sidea );   if ( signbit( sideb ) ) sideb = fabs( sideb );   feholdexcept(&fpenv);  // Save previous environment, clear exceptions,                          // switch to nonstop processing.   invscale = 1.0;   sum = sidea * sidea + sideb * sideb;   // First try whether a^2 + b^2                                          // causes any exceptions.   fpeflags = fetestexcept(FE_UNDERFLOW | FE_OVERFLOW);    // Did it?   if ( fpeflags & FE_OVERFLOW && sidea > 1.0 && sideb > 1.0 )   {     /* a^2 + b^2 caused an overflow. Scale the triangle down. */     feclearexcept(FE_OVERFLOW);     scale = scalbn(1.0, (DBL_MIN_EXP /2 ));     invscale = 1.0 / scale;     ascaled = scale * sidea;     bscaled = scale * sideb;     sum = ascaled * ascaled + bscaled * bscaled;   }   else if (fpeflags & FE_UNDERFLOW && sidea < 1.0 && sideb < 1.0 )   {     /* a^2 + b^2 caused an underflow. Scale the triangle up. */     feclearexcept(FE_UNDERFLOW);     scale = scalbn(1.0, (DBL_MAX_EXP /2 ));     invscale = 1.0 / scale;     ascaled = scale * sidea;     bscaled = scale * sideb;     sum = ascaled * ascaled + bscaled * bscaled;   }   feupdateenv(&env);        // restore the caller's environment, and                             // raise any new exceptions   /* c = (1/scale) * sqrt((a * scale)^2 + (b * scale)^2): */   return invscale * sqrt(sum); } 

See Also

fegetenv( ), fesetenv( ), feupdateenv( ), feclearexcept( ), feraisexcept( ), fegetexceptflag( ), fesetexceptflag( ), fetestexcept( )