double logb(doublex);
float logbf(floatx);
long double logbl(long doublex);

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 Conditions
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 .