# logb(3p) [posix man page]

```LOGB(3P)						     POSIX Programmer's Manual							  LOGB(3P)

PROLOG
This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
logb, logbf, logbl -- radix-independent exponent

SYNOPSIS
#include <math.h>

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

DESCRIPTION
The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard.

These  functions shall compute the exponent of x, which is the integral part of logr |x|, as a signed floating-point value, for non-zero x,
where r is the radix of the machine's floating-point arithmetic, which is the value of FLT_RADIX defined in the <float.h> header.

If x is subnormal it is treated as though it were normalized; thus for finite positive x:

An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT)  before  calling  these
functions.  On  return,	if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error
has occurred.

RETURN VALUE
Upon successful completion, these functions shall return the exponent of x.

If x is +-0, logb(), logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

On systems that support the IEC 60559 Floating-Point option, a pole error shall occur;
otherwise, a pole error may occur.

If x is NaN, a NaN shall be returned.

If x is +-Inf, +Inf shall be returned.

ERRORS
These functions shall fail if:

Pole Error  The value of x is +-0.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to  [ERANGE].   If  the  integer
expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.

These functions may fail if:

Pole Error  The value of x is 0.

If  the  integer  expression  (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE].  If the integer
expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.

The following sections are informative.

EXAMPLES
None.

APPLICATION USAGE
On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of  each  other,  but	at
least one of them must be non-zero.

RATIONALE
None.

FUTURE DIRECTIONS
None.

feclearexcept(), fetestexcept(), ilogb(), scalbln()

The Base Definitions volume of POSIX.1-2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>