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(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
_ISOC99_SOURCE; or cc -std=c99
logbf(), logbl(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600
|| _ISOC99_SOURCE; or cc -std=c99
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
If x is subnormal, logb() returns the exponent x would have if it were
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
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
These functions do not set errno.
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