| Pole error: x is 0 | A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised. |
| logb() | 4.3BSD (see IEEE.3 in the 4.3BSD manual). |
NAME
logb, logbf, logbl - get exponent of a floating-point value
LIBRARY
Math library (libm, -lm)
SYNOPSIS
#include <math.h>double logb(double \nx\n);\n
\nfloat logbf(float \nx\n);\n
\nlong double logbl(long double \nx\n);logb():
\n
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
\n
|| _XOPEN_SOURCE >= 500
\n
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
\n
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCElogbf(), logbl():
\n
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
\n
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
\n
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCEDESCRIPTION
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.
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 |
STANDARDS
C11, POSIX.1-2008.
HISTORY
C99, POSIX.1-2001.
- logb()
4.3BSD (see IEEE.3 in the 4.3BSD manual).
SEE ALSO
ilogb(3), log(3)