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+'\" t
+.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
+.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
+.\"     <mtk.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.\" Inspired by a page by Walter Harms created 2002-08-10
+.\"
+.TH logb 3 (date) "Linux man-pages (unreleased)"
+.SH NAME
+logb, logbf, logbl \- get exponent of a floating-point value
+.SH LIBRARY
+Math library
+.RI ( libm ", " \-lm )
+.SH SYNOPSIS
+.nf
+.B #include <math.h>
+.P
+.BI "double logb(double " x );
+.BI "float logbf(float " x );
+.BI "long double logbl(long double " x );
+.fi
+.P
+.RS -4
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.RE
+.P
+.BR logb ():
+.nf
+    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
+        || _XOPEN_SOURCE >= 500
+.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
+        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
+        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
+.fi
+.P
+.BR logbf (),
+.BR logbl ():
+.nf
+    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
+        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
+        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
+.fi
+.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 similar to
+.BI floor(log2(fabs( x )))\f[R],\f[]
+except that the latter may give an incorrect integer
+due to intermediate rounding.
+.P
+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 .
+.P
+If
+.I x
+is a NaN,
+a NaN is returned.
+.P
+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.
+.P
+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.
+.P
+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.
+.P
+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: https://www.sourceware.org/bugzilla/show_bug.cgi?id=6793
+.\"
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbx lb lb
+l l l.
+Interface	Attribute	Value
+T{
+.na
+.nh
+.BR logb (),
+.BR logbf (),
+.BR logbl ()
+T}	Thread safety	MT-Safe
+.TE
+.SH STANDARDS
+C11, POSIX.1-2008.
+.SH HISTORY
+C99, POSIX.1-2001.
+.TP
+.BR logb ()
+4.3BSD
+(see IEEE.3 in the 4.3BSD manual).
+.SH SEE ALSO
+.BR ilogb (3),
+.BR log (3)