NAME

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

LIBRARY

       Math library (libm, -lm)

SYNOPSIS

       #include <math.h>

       double logb(double x);
       float logbf(float x);
       long double logbl(long double x);

   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 similar to floor(log2(fabs(x))), except  that  the  latter  may
       give an incorrect integer due to intermediate rounding.

       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 │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       C99, POSIX.1-2001.

       logb() 4.3BSD (see IEEE.3 in the 4.3BSD manual).

SEE ALSO

       ilogb(3), log(3)