logb, logbf, logbl - get exponent of a floating-point value
#include <math.h>
double logb(double
x
);
float logbf(float x
);
long double logbl(long double
x
);
Link with -lm
.
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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 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 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.
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:
x
is 0A 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).
Interface | Attribute | Value |
logb(), logbf(), logbl() | Thread safety | MT-Safe |
C99, POSIX.1-2001, POSIX.1-2008.
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