.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" Inspired by a page by Walter Harms created 2002-08-10
.\"
.TH LOGB 3 2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
logb, logbf, logbl \- get exponent of a floating-point value
.SH SYNOPSIS
.B #include <math.h>
.PP
.BI "double logb(double " x );
.br
.BI "float logbf(float " x );
.br
.BI "long double logbl(long double " x );
.PP
Link with \fI\-lm\fP.
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.ad l
.BR logb ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
    || _XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.br
.BR logbf (),
.BR logbl ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.ad b
.SH DESCRIPTION
These functions extract the exponent from the
internal floating-point representation of
.I x
and return it as a floating-point value.
The integer constant
.BR FLT_RADIX ,
defined in
.IR <float.h> ,
indicates the radix used for the system's floating-point representation.
If
.B FLT_RADIX
is 2,
.BI logb( x )
is equal to
.BI floor(log2( x ))\fR,
except that it is probably faster.
.PP
If
.I x
is subnormal,
.BR logb ()
returns the exponent
.I x
would have if it were normalized.
.SH RETURN VALUE
On success, these functions return the exponent of
.IR x .
.PP
If
.I x
is a NaN,
a NaN is returned.
.PP
If
.I x
is zero, then a pole error occurs, and the functions return
.RB - HUGE_VAL ,
.RB - HUGE_VALF ,
or
.RB - HUGE_VALL ,
respectively.
.PP
If
.I x
is negative infinity or positive infinity, then
positive infinity is returned.
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.PP
The following errors can occur:
.TP
Pole error: \fIx\fP is 0
.\" .I errno
.\" is set to
.\" .BR ERANGE .
A divide-by-zero floating-point exception
.RB ( FE_DIVBYZERO )
is raised.
.PP
These functions do not set
.IR errno .
.\" FIXME . Is it intentional that these functions do not set errno?
.\" log(), log2(), log10() do set errno
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6793
.\"
.\" .SH HISTORY
.\" The
.\" .BR logb ()
.\" function occurs in 4.3BSD.
.\" see IEEE.3 in the 4.3BSD manual
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw24 lb lb
l l l.
Interface	Attribute	Value
T{
.BR logb (),
.BR logbf (),
.BR logbl ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
.SH SEE ALSO
.BR ilogb (3),
.BR log (3)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.