math_error(7) - Linux manual page (original) (raw)

matherror(7) Miscellaneous Information Manual matherror(7)

NAME top

   math_error - detecting errors from mathematical functions

SYNOPSIS top

   **#include <math.h>**
   **#include <errno.h>**
   **#include <fenv.h>**

DESCRIPTION top

   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](../man3/errno.3.html)_; the newer one uses the floating-point exception
   mechanism (the use of [feclearexcept(3)](../man3/feclearexcept.3.html) and [fetestexcept(3)](../man3/fetestexcept.3.html), as
   outlined below) described in [fenv(3)](../man3/fenv.3.html).

   A portable program that needs to check for an error from a
   mathematical function should set _[errno](../man3/errno.3.html)_ to zero, and make the
   following call

       feclearexcept(FE_ALL_EXCEPT);

   before calling a mathematical function.

   Upon return from the mathematical function, if _[errno](../man3/errno.3.html)_ is nonzero,
   or the following call (see [fenv(3)](../man3/fenv.3.html)) 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](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ may be set to **ERANGE**, and an "underflow"
   (**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 top

   The _matherrhandling_ 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](../man3/errno.3.html)_, exceptions
   retrievable via [fetestexcept(3)](../man3/fetestexcept.3.html)) 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](../man3/errno.3.html)_.
   A few functions set _[errno](../man3/errno.3.html)_, 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](../man3/errno.3.html)_ and [fetestexcept(3)](../man3/fetestexcept.3.html) 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)](../man3/log.3.html)'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)](../man1/gcc.1.html) _-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](../man3/errno.3.html)_ on error.
   (The [gcc(1)](../man1/gcc.1.html) _-ffast-math_ option also enables _-fno-math-errno_.)  An
   error can still be tested for using [fetestexcept(3)](../man3/fetestexcept.3.html).

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Linux man-pages 6.10 2024-05-02 matherror(7)