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(double x);

float logbf(float x);

long double logbl(long double x);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
logb():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or cc -std=c99

logbf(), logbl():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;

or cc -std=c99

# 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¶

The logb(), logbf(), and logbl() functions are thread-safe.

# CONFORMING TO¶

C99, POSIX.1-2001.