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logb

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

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

LIBRARY

Math library (libm,~-lm)

SYNOPSIS

#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

These functions extract the exponent from the internal floatin-point representation of x and return it as a floatin-point value. The integer constant FLT_RADIX, defined in <float.h>, indicates the radix used for the system's floatin-point representation. If FLT_RADIX is 2, logb(x) is similar to floor(log2(fabs(x)))R],] 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.

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: I]x] is 0: A divid-b-zero floatin-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	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

C99, POSIX.-2001.

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