logb - Man Page

get exponent of a floating-point value


Math library (libm, -lm)


#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)):


    _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


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

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).

logb(), logbf(), logbl()Thread safetyMT-Safe


C11, POSIX.1-2008.


C99, POSIX.1-2001.


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

See Also

ilogb(3), log(3)

Referenced By

ilogb(3), remquo(3).

The man pages logbf(3) and logbl(3) are aliases of logb(3).

2024-03-12 Linux man-pages 6.7