LOG2(3P)                                            POSIX Programmer's Manual                                           LOG2(3P)



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

NAME
       log2, log2f, log2l - compute base 2 logarithm functions

SYNOPSIS
       #include <math.h>

       double log2(double x);
       float log2f(float x);
       long double log2l(long double x);


DESCRIPTION
       These functions shall compute the base 2 logarithm of their argument x, log_2(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 base 2 logarithm of x.

       If  x  is  +-0,  a  pole  error  shall  occur  and  log2(),  log2f(), and log2l() shall return -HUGE_VAL, -HUGE_VALF, and
       -HUGE_VALL, respectively.

       For finite values of x that are less than 0,  or if x is -Inf,  a domain error shall occur, and  either a  NaN  (if  sup-
       ported), or an implementation-defined value shall be returned.

       If x is NaN, a NaN shall be returned.

       If x is 1, +0 shall be returned.

       If x is +Inf, x shall be returned.

ERRORS
       These functions shall fail if:

       Domain Error
              The finite value of x is less than zero,  or x is -Inf.

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

       Pole Error
              The value of x is zero.

       If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the  inte-
       ger 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.

SEE ALSO
       feclearexcept(),  fetestexcept(),  log(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of
       Error Conditions for Mathematical Functions, <math.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                      LOG2(3P)