logb(3) - Linux manual page (original) (raw)

logb(3) Library Functions Manual logb(3)

NAME top

   logb, logbf, logbl - get exponent of a floating-point value

LIBRARY top

   Math library (_libm_, _-lm_)

SYNOPSIS top

   **#include <math.h>**

   **double logb(double** _x_**);**
   **float logbf(float** _x_**);**
   **long double logbl(long double** _x_**);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   **logb**():
       _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
           || _XOPEN_SOURCE >= 500
           || /* Since glibc 2.19: */ _DEFAULT_SOURCE
           || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

   **logbf**(), **logbl**():
       _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
           || /* Since glibc 2.19: */ _DEFAULT_SOURCE
           || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION top

   These functions extract the exponent from the internal floating-
   point representation of _x_ and return it as a floating-point value.
   The integer constant **FLT_RADIX**, defined in _<float.h>_, indicates
   the radix used for the system's floating-point representation.  If
   **FLT_RADIX** is 2, **logb(**_x_**)** is similar to **floor(log2(fabs(**_x_**)))**, except
   that the latter may give an incorrect integer due to intermediate
   rounding.

   If _x_ is subnormal, **logb**() returns the exponent _x_ would have if it
   were normalized.

RETURN VALUE top

   On success, these functions return the exponent of _x_.

   If _x_ is a NaN, a NaN is returned.

   If _x_ is zero, then a pole error occurs, and the functions return
   -**HUGE_VAL**, -**HUGE_VALF**, or -**HUGE_VALL**, respectively.

   If _x_ is negative infinity or positive infinity, then positive
   infinity is returned.

ERRORS top

   See [math_error(7)](../man7/math%5Ferror.7.html) for information on how to determine whether an
   error has occurred when calling these functions.

   The following errors can occur:

   Pole error: _x_ is 0
          A divide-by-zero floating-point exception (**FE_DIVBYZERO**) is
          raised.

   These functions do not set _[errno](../man3/errno.3.html)_.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **logb**(), **logbf**(), **logbl**()             │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   C99, POSIX.1-2001.

   **logb**() 4.3BSD (see IEEE.3 in the 4.3BSD manual).

COLOPHON top

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Linux man-pages 6.10 2024-07-23 logb(3)

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