math_error


SYNOPSIS
       #include <math.h>
       #include <errno.h>
       #include <fenv.h>

DESCRIPTION
       When  an  error  occurs,  most  library functions indicate this fact by
       returning a special value (e.g., -1 or NULL).  Because  they  typically
       return  a floating-point number, the mathematical functions declared in
       <math.h> indicate an error  using  other  mechanisms.   There  are  two
       error-reporting  mechanisms:  the  older  one sets errno; the newer one
       uses the floating-point exception  mechanism  (the  use  of  feclearex-
       cept(3) and fetestexcept(3), as outlined below) described in fenv(3).

       A portable program that needs to check for an error from a mathematical
       function should set errno to zero, and make the following call

           feclearexcept(FE_ALL_EXCEPT);

       before calling a mathematical function.

       Upon return from the mathematical function, if errno is nonzero, or the
       following call (see fenv(3)) returns nonzero

           fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
                        FE_UNDERFLOW);

       then an error occurred in the mathematical function.

       The  error  conditions  that  can  occur for mathematical functions are
       described below.

   Domain Error
       A domain error occurs when a mathematical function is supplied with  an
       argument whose value falls outside the domain for which the function is
       defined (e.g., giving a negative argument to log(3)).   When  a  domain
       error  occurs,  math functions commonly return a NaN (though some func-
       tions return a different value in this case); errno is set to EDOM, and
       an "invalid" (FE_INVALID) floating-point exception is raised.

   Pole Error
       A  pole  error  occurs when the mathematical result of a function is an
       exact infinity (e.g., the logarithm of 0 is negative infinity).  When a
       pole  error  occurs,  the function returns the (signed) value HUGE_VAL,
       HUGE_VALF, or HUGE_VALL, depending on whether the function result  type
       is double, float, or long double.  The sign of the result is that which
       is mathematically correct for the function.  errno is  set  to  ERANGE,
       and  a  "divide-by-zero"  (FE_DIVBYZERO)  floating-point  exception  is
       raised.

   Range Error
       A range error occurs when the magnitude of the  function  result  means
       that  it cannot be represented in the result type of the function.  The
       function typically returns 0.0 (C99 says a function  shall  return  "an
       implementation-defined  value  whose  magnitude  is no greater than the
       smallest normalized positive number in the specified type").  errno may
       be  set  to  ERANGE,  and  an  "overflow" (FE_UNDERFLOW) floating-point
       exception may be raised.

       Some functions deliver a range error if the supplied argument value, or
       the  correct function result, would be subnormal.  A subnormal value is
       one that is nonzero, but with a magnitude that  is  so  small  that  it
       can't  be presented in normalized form (i.e., with a 1 in the most sig-
       nificant bit of the significand).  The representation  of  a  subnormal
       number will contain one or more leading zeros in the significand.

NOTES
       The  math_errhandling  identifier  specified by C99 and POSIX.1-2001 is
       not supported by glibc.  This identifier is supposed to indicate  which
       of the two error-notification mechanisms (errno, exceptions retrievable
       via fettestexcept(3)) is in use.  The standards require that  at  least
       one  be  in use, but permit both to be available.  The current (version
       2.8) situation under glibc is messy.   Most  (but  not  all)  functions
       raise  exceptions on errors.  Some also set errno.  A few functions set
       errno, but don't raise an exception.  A very few functions do  neither.
       See the individual manual pages for details.

       To  avoid the complexities of using errno and fetestexcept(3) for error
       checking, it is often advised that one should  instead  check  for  bad
       argument  values  before  each  call.   For example, the following code
       ensures that log(3)'s argument is not a NaN and is  not  zero  (a  pole
       error) or less than zero (a domain error):

           double x, r;

           if (isnan(x) || islessequal(x, 0)) {
               /* Deal with NaN / pole error / domain error */
           }

           r = log(x);

       The  discussion on this page does not apply to the complex mathematical
       functions (i.e., those declared by <complex.h>), which in  general  are
       not required to return errors by C99 and POSIX.1-2001.

       The  gcc(1)  -fno-math-errno  option  causes  the  executable to employ
       implementations of some mathematical functions that are faster than the
       standard  implementations,  but do not set errno on error.  (The gcc(1)
       -ffast-math option also enables -fno-math-errno.)  An error  can  still
       be tested for using fetestexcept(3).

SEE ALSO
       gcc(1),  errno(3),  fenv(3),  fpclassify(3), INFINITY(3), isgreater(3),
       matherr(3), nan(3)
       info libc

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