< c‎ | numeric‎ | fenv
Revision as of 07:51, 3 September 2013 by Nate (Talk | contribs)

Defined in header <<fenv.h>>
int feclearexcept( int excepts );
(since C99)

Attempts to clear the floating-point exceptions that are listed in the bitmask argument excepts, which is a bitwise OR of the floating point exception macros.



excepts - bitmask listing the exception flags to clear

Return value

0 if all indicated exceptions were successfully cleared or if excepts is zero. Returns a non-zero value on error.


#include <fenv.h>
#include <stdio.h>
#include <math.h>
#include <float.h>
 * A possible implementation of hypot which makes use of many advanced
 * floating point features.
double hypot_demo(double a, double b) {
  const int range_problem = FE_OVERFLOW | FE_UNDERFLOW;
  // try a fast algorithm
  double result = sqrt(a * a + b * b);
  if (!fetestexcept(range_problem))  // no overflow or underflow
    return result;                   // return the fast result
  // do a more complicated calculation to avoid overflow or underflow
  int a_exponent,b_exponent;
  frexp(a, &a_exponent);
  frexp(b, &b_exponent);
  if (a_exponent - b_exponent > DBL_MAX_EXP)
    return fabs(a) + fabs(b);        // we can ignore the smaller value
  // scale so that fabs(a) is near 1
  double a_scaled = scalbn(a, -a_exponent);
  double b_scaled = scalbn(b, -a_exponent);
  // overflow and underflow is now impossible 
  result = sqrt(a_scaled * a_scaled + b_scaled * b_scaled);
  // undo scaling
  return scalbn(result, a_exponent);
int main (int argc, char** argv) {
  // Normal case takes the fast route
  printf("hypot(%f, %f) = %f\n", 3.0, 4.0, hypot_demo(3.0, 4.0));
  // Extreme case takes the slow but more accurate route
  printf("hypot(%e, %e) = %e\n", DBL_MAX / 2.0, 
                                DBL_MAX / 2.0, 
                                hypot_demo(DBL_MAX / 2.0, DBL_MAX / 2.0));


hypot(3.000000, 4.000000) = 5.000000
hypot(8.988466e+307, 8.988466e+307) = 1.271161e+308

See also

determines which of the specified floating-point status flags are set
(function) [edit]
C++ documentation for feclearexcept