logbl(3) — Linux manual page


LOGB(3)                   Linux Programmer's Manual                  LOGB(3)

NAME         top

       logb, logbf, logbl - get exponent of a floating-point value

SYNOPSIS         top

       #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)):

           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
               || _XOPEN_SOURCE >= 500
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
       logbf(), logbl():
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION         top

       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 normalized.

RETURN VALUE         top

       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         top

       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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                Attribute     Value   │
       │logb(), logbf(), logbl() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       C99, POSIX.1-2001, POSIX.1-2008.

SEE ALSO         top

       ilogb(3), log(3)

COLOPHON         top

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       latest version of this page, can be found at

                                 2017-09-15                          LOGB(3)