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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "LOGB" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" logb
.SH NAME
logb, logbf, logbl \- radix-independent exponent
.SH SYNOPSIS
.LP
\fB#include <math.h>
.br
.sp
double logb(double\fP \fIx\fP\fB);
.br
float logbf(float\fP \fIx\fP\fB);
.br
long double logbl(long double\fP \fIx\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
These functions shall compute the exponent of \fIx\fP, which is the
integral part of log\fI_r\fP
|\fIx\fP|, as a signed floating-point value, for non-zero \fIx\fP,
where \fIr\fP is the radix of the machine's floating-point
arithmetic, which is the value of FLT_RADIX defined in the \fI<float.h>\fP
header.
.LP
If \fIx\fP is subnormal it is treated as though it were normalized;
thus for finite positive \fIx\fP:
.sp
.RS
.nf
\fB1 <=\fP \fIx\fP \fB* FLT_RADIX**-logb(x) < FLT_RADIX
\fP
.fi
.RE
.LP
An application wishing to check for error situations should set \fIerrno\fP
to zero and call
\fIfeclearexcept\fP(FE_ALL_EXCEPT) before calling these functions.
On return, if \fIerrno\fP is non-zero or
\fIfetestexcept\fP(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an error has occurred.
.SH RETURN VALUE
.LP
Upon successful completion, these functions shall return the exponent
of \fIx\fP.
.LP
If \fIx\fP is \(+-0, a pole error shall occur and \fIlogb\fP(), \fIlogbf\fP(),
and \fIlogbl\fP() shall return -HUGE_VAL,
-HUGE_VALF, and -HUGE_VALL, respectively.
.LP
If
\fIx\fP is NaN, a NaN shall be returned.
.LP
If \fIx\fP is \(+-Inf, +Inf shall be returned.
.SH ERRORS
.LP
These functions shall fail if:
.TP 7
Pole\ Error
The value of \fIx\fP is \(+-0.
.LP
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then \fIerrno\fP 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.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
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.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfeclearexcept\fP() , \fIfetestexcept\fP() , \fIilogb\fP() , \fIscalb\fP()
, the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 4.18, Treatment of Error Conditions
for
Mathematical Functions, \fI<float.h>\fP, \fI<math.h>\fP
.SH 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 .
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