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.TH logb 3M "12 Jul 2006" "SunOS 5.11" "Mathematical Library Functions"
.SH NAME
logb, logbf, logbl \- radix-independent exponent
.SH SYNOPSIS
.LP
.nf
c99 [ \fIflag\fR... ] \fIfile\fR... \fB-lm\fR [ \fIlibrary\fR... ]
#include <math.h>

\fBdouble\fR \fBlogb\fR(\fBdouble\fR \fIx\fR);
.fi

.LP
.nf
\fBfloat\fR \fBlogbf\fR(\fBfloat\fR \fIx\fR);
.fi

.LP
.nf
\fBlong double\fR \fBlogbl\fR(\fBlong double\fR \fIx\fR);
.fi

.LP
.nf
cc [ \fIflag\fR... ] \fIfile\fR... \fB-lm\fR [ \fIlibrary\fR... ]
#include <math.h>

\fBdouble\fR \fBlogb\fR(\fBdouble\fR \fIx\fR);
.fi

.LP
.nf
\fBfloat\fR \fBlogbf\fR(\fBfloat\fR \fIx\fR);
.fi

.LP
.nf
\fBlong double\fR \fBlogbl\fR(\fBlong double\fR \fIx\fR);
.fi

.SH DESCRIPTION
.sp
.LP
These functions compute the exponent of \fIx\fR, which is the integral part of
log(\fIr\fR) |\fIx\fR|, as a signed floating point value, for non-zero \fIx\fR,
where \fIr\fR is the radix of the machine's floating-point arithmetic, which is
the value of \fBFLT_RADIX\fR defined in the <\fBfloat.h\fR> header.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, these functions return the exponent of \fIx\fR.
.sp
.LP
If \fIx\fR is subnormal:
.RS +4
.TP
.ie t \(bu
.el o
For SUSv3-conforming applications compiled with the \fBc99\fR compiler driver
(see \fBstandards\fR(5)), the exponent of \fIx\fR as if \fIx\fR were normalized
is returned.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Otherwise, if compiled with the \fBcc\fR compiler driver, \(mi1022, \(mi126,
and \(mi16382 are returned for \fBlogb()\fR, \fBlogbf()\fR, and \fBlogbl()\fR,
respectively.
.RE
.sp
.LP
If \fIx\fR is \(+-0, a pole error occurs and \fBlogb()\fR, \fBlogbf()\fR, and
\fBlogbl()\fR return \(mi\fBHUGE_VAL\fR, \(mi\fBHUGE_VALF\fR, and
\(mi\fBHUGE_VALL\fR, respectively.
.sp
.LP
If \fIx\fR is NaN, a NaN is returned.
.sp
.LP
If \fIx\fR is \(+-Inf, +Inf is returned.
.SH ERRORS
.sp
.LP
These functions will fail if:
.sp
.ne 2
.mk
.na
\fBPole Error\fR
.ad
.RS 14n
.rt  
The value of \fIx\fR is \(+-0.
.sp
If the integer expression (\fBmath_errhandling\fR & \fBMATH_ERREXCEPT\fR) is
non-zero, the divide-by-zero floating-point exception is raised.
.sp
The \fBlogb()\fR function sets \fBerrno\fR to \fBEDOM\fR if the value of
\fIx\fR is 0.
.RE

.SH USAGE
.sp
.LP
An application wanting to check for exceptions should call
\fBfeclearexcept\fR(\fBFE_ALL_EXCEPT\fR) before calling these functions. On
return, if \fBfetestexcept\fR(\fBFE_INVALID\fR | \fBFE_DIVBYZERO\fR |
\fBFE_OVERFLOW\fR | \fBFE_UNDERFLOW\fR) is non-zero, an exception has been
raised. An application should either examine the return value or check the
floating point exception flags to detect exceptions.
.sp
.LP
An application can also set \fBerrno\fR to 0 before calling \fBlogb()\fR. On
return, if \fBerrno\fR is non-zero, an error has occurred. The \fBlogbf()\fR
and \fBlogbl()\fR functions do not set \fBerrno\fR.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
tab() box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPEATTRIBUTE VALUE
_
Interface StabilityStandard
_
MT-LevelMT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBfeclearexcept\fR(3M), \fBfetestexcept\fR(3M), \fBilogb\fR(3M),
\fBmath.h\fR(3HEAD), \fBmatherr\fR(3M), \fBscalb\fR(3M), \fBattributes\fR(5),
\fBstandards\fR(5)