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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 (consult 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  supported),  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  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.
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, Copyright (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.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                             LOG2(3P)