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

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

SYNOPSIS
       #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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

DESCRIPTION
       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.

RETURN VALUE
       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
       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.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                                                                             │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │logb(), logbf(), logbl()                                                                                                              │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

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

SEE ALSO
       ilogb(3), log(3)

                                                                               2021-03-22                                                                        LOGB(3)