math_error(7) - man - phpman

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 er‐
       ror-reporting mechanisms: the older one sets errno; the newer one uses the floating-point ex‐
       ception  mechanism  (the  use of feclearexcept(3) and fetestexcept(3), as outlined below) de‐
       scribed 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
       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 mathemati‐
       cally correct for the function.  errno is set  to  ERANGE,  and  a  "divide-by-zero"  (FE_DI‐‐
       VBYZERO) floating-point exception is raised.

   Range error
       A range error occurs when the magnitude of the function result means that it cannot be repre‐
       sented 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 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 "un‐
       derflow" (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
       significant  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, ex‐
       ceptions retrievable via fetestexcept(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.

       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  er‐
       rno  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 5.10 of the Linux  man-pages  project.   A  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)