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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(): _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
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: Pole error: x is 0 A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised. These functions do not set errno.
This page is part of release 3.35 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man- pages/. 2010-09-20 LOGB(3)