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.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Inspired by a page by Walter Harms created 2002-08-10
.\"
.TH LOGB 3 2004-10-31 "" "Linux Programmer's Manual"
.SH NAME
logb, logbf, logbl \- get exponent of a floating point value
.SH SYNOPSIS
.B #include <math.h>
.sp
.BI "double logb(double " x );
.br
.BI "float logbf(float " x );
.br
.BI "long double logbl(long double " x );
.sp
Link with \-lm.
.SH DESCRIPTION
These functions extract the exponent of
.I x
and return it as a floating-point value. If
.B FLT_RADIX
is two,
.BI logb( x )
is equal to
.BI floor(log2( x ))\fR,
except it's probably faster.
.LP
If
.I x
is de-normalized,
.B logb()
returns the exponent
.I x
would have if it were normalized.
.LP
If
.I x
is zero, -HUGE_VAL (resp. -HUGE_VALF, -HUGE_VALL)
is returned, and a pole error occurs.
If
.I x
is infinite, plus infinity is returned.
If
.I x
is NaN, NaN is returned.
.\" .SH "RETURN VALUE"
.\" These functions return the exponent part of their argument.
.SH ERRORS
In order to check for errors, set
.I errno
to zero and call
.I feclearexcept(FE_ALL_EXCEPT)
before calling these functions. On return, if
.I errno
is non-zero or
.I fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an error has occurred.
.LP
If an error occurs and
.I "(math_errhandling & MATH_ERRNO)"
is non-zero, then
.I errno is set to
.BR ERANGE .
If an error occurs and
.I "(math_errhandling & MATH_ERREXCEPT)"
is non-zero, then the divide-by-zero floating-point exception is raised.
.LP
A pole error occurs when
.I x
is zero.
.SH HISTORY
The
.B logb
function occurs in BSD 4.3.
.\" see IEEE.3 in the BSD 4.3 manual
.SH "CONFORMING TO"
ISO C99
.SH "SEE ALSO"
.BR log (3),
.BR ilogb (3)
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