LOGB(P) POSIX Programmer's Manual LOGB(P)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 ALSOfeclearexcept() , fetestexcept() , ilogb() , scalb() , the Base Defini‐
tions 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 .
IEEE/The Open Group 2003 LOGB(P)
