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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  corresponding
       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
       These functions shall compute the exponent of x, which is the  integral
       part  of  log_r  |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:

              1 <= x * FLT_RADIX**-logb(x) < FLT_RADIX
       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, a pole error shall occur and logb(), logbf(), and logbl()
       shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
       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.

       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(), ilogb(), scalb(), the Base Definitions
       volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Condi-
       tions for Mathematical Functions, <float.h>, <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                             LOGB(3P)