LOGB(3) Linux Programmer's Manual LOGB(3)
logb, logbf, logbl - get exponent of a floating point value
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(), logbf(), logbl(): _BSD_SOURCE || _SVID_SOURCE ||
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or cc -std=c99
These functions extract the exponent of x and return it as a floating-
point value. If FLT_RADIX is two, logb(x) is equal to floor(log2(x)),
except that it is probably faster.
If x is de-normalized, logb() returns the exponent x would have if it
If x is zero, -HUGE_VAL (resp. -HUGE_VALF, -HUGE_VALL) is returned, and
a pole error occurs. If x is infinite, plus infinity is returned. If
x is NaN, NaN is returned.
In order to check for errors, set errno to zero and call feclearex-
cept(FE_ALL_EXCEPT) before calling these functions. On return, if
errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVER-
FLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
If an error occurs and (math_errhandling & MATH_ERRNO) is non-zero,
then errno is set to ERANGE. If an error occurs and (math_errhandling
& MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point
exception is raised.
A pole error occurs when x is zero.
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