Provided by: manpages_3.24-1ubuntu1_all

#### 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 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  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
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  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.1-2001  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 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
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.1-2001.

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).

```

#### SEEALSO

```       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 3.24 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/.
```