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



SYNOPSIS
       #include <&lt;math.h>&gt;
       #include <&lt;errno.h>&gt;
       #include <&lt;fenv.h>&gt;

NAME
       math_error - detecting errors from mathematical functions

DESCRIPTION
       On  error,  many  of  the  mathematical  functions declared in &lt;math.h&gt;
       return a NaN (not a number).   However,  rather  than  looking  at  the
       return  value (which is not always possible) one can also check whether
       an error was signaled.  There are two signaling mechanisms:  the  older
       one  sets errno; the newer one uses the floating-point exception mecha-
       nism (the use of  feclearexcept(3)  and  fetestexcept(3),  as  outlined
       below) described in fenv(3).

       C99  and  POSIX.1-2001  specify a math_errhandling identifier, which is
       supposed to indicate which of these two mechanisms is in use; the stan-
       dards require that at least one be in use, but permit both to be avail-
       able.  Although glibc does not support this identifier, in practice  it
       supports both mechanisms.

       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,  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.
       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 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
       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  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.

       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 &lt;complex.h&gt;), 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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-21                     MATH_ERROR(7)