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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 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 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 non-zero, or the following call (see  fenv(3))  returns
       non-zero

           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 functions 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 over-
       flow 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 function returns the value HUGE_VAL, HUGE_VALF, or HUGE_VALL, depending on whether the
       function result type is double, float, or long double.  errno 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 represented 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 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 non-zero, 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 significant bit of the significand).  The  representa-
       tion 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 avail-
       able.  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 func-
       tions 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 <com-
       plex.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 func-
       tions 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
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-11                     MATH_ERROR(7)