## table of contents

LOGB(3) | Linux Programmer's Manual | LOGB(3) |

# NAME¶

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

# SYNOPSIS¶

#include <math.h>

double logb(doublex);float logbf(floatx);long double logbl(long doublex);

Link with *-lm*.

**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¶

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.

# RETURN VALUE¶

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¶

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*.

# ATTRIBUTES¶

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

Interface |
Attribute |
Value |

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

# CONFORMING TO¶

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

# SEE ALSO¶

# COLOPHON¶

This page is part of release 5.13 of the Linux *man-pages*
project. A description of the project, information about reporting bugs, and
the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.

2021-03-22 |