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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 .