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

**#include <math.h>**

**double logb(double
**`x`

**);**

**float logbf(float **`x`

**);**

**long double logbl(long double
**`x`

**);**

Link with `-lm`

.

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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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 equal to
**floor(log2(**`x`

**)),** except that
it is probably faster.

If `x`

is subnormal, logb() returns the
exponent `x`

would have if it were normalized.

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.

See math_error(7) 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`

.

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |

logb(), logbf(), logbl() | Thread safety | MT-Safe |

C99, POSIX.1-2001, POSIX.1-2008.

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