Manpages - logb.3

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*():

      _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

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.

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

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

*ilogb*(3), *log*(3)

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