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math_error(7)                     Miscellaneous Information 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 (glibc 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

Linux man-pages 6.7                          2023-10-31                                math_error(7)