math_error

MATH_ERROR(7)              Linux Programmer's Manual             MATH_ERROR(7)

NAME
       math_error - detecting errors from mathematical functions

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

DESCRIPTION
       When  an error occurs, most library functions indicate this fact by re-
       turning a special value (e.g., -1 or NULL).  Because they typically re-
       turn  a  floating-point  number, the mathematical functions declared in
       <math.h> indicate an error using other mechanisms.  There are  two  er-
       ror-reporting  mechanisms: the older one sets errno; the newer one uses
       the floating-point exception mechanism (the use of feclearexcept(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 de-
       scribed 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
       return value of the function depends on whether the range error was  an
       overflow or an underflow.

       A  floating  result overflows if the result is finite, but is too large
       to represented in the result type.  When an overflow occurs, the  func-
       tion  returns the value HUGE_VAL, HUGE_VALF, or HUGE_VALL, depending on
       whether the function result type is double, float, or long double.  er-
       rno  is  set  to ERANGE, and an "overflow" (FE_OVERFLOW) floating-point
       exception is raised.

       A floating result underflows if the result is too small  to  be  repre-
       sented  in  the  result  type.   If an underflow occurs, a mathematical
       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 ex-
       ception 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 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 ar-
       gument  values  before  each call.  For example, the following code en-
       sures that log(3)'s argument is not a NaN and is not zero (a  pole  er-
       ror) 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.

       The  gcc(1)  -fno-math-errno option causes the executable to employ im-
       plementations 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

COLOPHON
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       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                     MATH_ERROR(7)