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( )