mirror of https://github.com/mkerrisk/man-pages
162 lines
3.9 KiB
Groff
162 lines
3.9 KiB
Groff
.\" Copyright 2004 Andries Brouwer <aeb@cwi.nl>.
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.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Inspired by a page by Walter Harms created 2002-08-10
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.\"
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.TH LOGB 3 2021-03-22 "" "Linux Programmer's Manual"
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.SH NAME
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logb, logbf, logbl \- get exponent of a floating-point value
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.SH SYNOPSIS
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.nf
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.B #include <math.h>
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.PP
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.BI "double logb(double " x );
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.BI "float logbf(float " x );
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.BI "long double logbl(long double " x );
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.fi
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.PP
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Link with \fI\-lm\fP.
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR logb ():
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.nf
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_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
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|| _XOPEN_SOURCE >= 500
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.\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
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|| /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.fi
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.PP
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.BR logbf (),
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.BR logbl ():
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.nf
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_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
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|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
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|| /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.fi
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.SH DESCRIPTION
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These functions extract the exponent from the
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internal floating-point representation of
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.I x
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and return it as a floating-point value.
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The integer constant
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.BR FLT_RADIX ,
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defined in
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.IR <float.h> ,
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indicates the radix used for the system's floating-point representation.
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If
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.B FLT_RADIX
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is 2,
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.BI logb( x )
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is equal to
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.BI floor(log2( x ))\fR,
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except that it is probably faster.
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.PP
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If
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.I x
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is subnormal,
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.BR logb ()
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returns the exponent
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.I x
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would have if it were normalized.
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.SH RETURN VALUE
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On success, these functions return the exponent of
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.IR x .
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.PP
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If
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.I x
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is a NaN,
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a NaN is returned.
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.PP
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If
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.I x
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is zero, then a pole error occurs, and the functions return
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.RB \- HUGE_VAL ,
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.RB \- HUGE_VALF ,
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or
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.RB \- HUGE_VALL ,
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respectively.
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.PP
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If
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.I x
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is negative infinity or positive infinity, then
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positive infinity is returned.
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.SH ERRORS
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See
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.BR math_error (7)
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for information on how to determine whether an error has occurred
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when calling these functions.
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.PP
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The following errors can occur:
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.TP
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Pole error: \fIx\fP is 0
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.\" .I errno
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.\" is set to
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.\" .BR ERANGE .
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A divide-by-zero floating-point exception
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.RB ( FE_DIVBYZERO )
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is raised.
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.PP
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These functions do not set
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.IR errno .
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.\" FIXME . Is it intentional that these functions do not set errno?
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.\" log(), log2(), log10() do set errno
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.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6793
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.\"
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.\" .SH HISTORY
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.\" The
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.\" .BR logb ()
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.\" function occurs in 4.3BSD.
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.\" see IEEE.3 in the 4.3BSD manual
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.ad l
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.nh
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.TS
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allbox;
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lbx lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR logb (),
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.BR logbf (),
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.BR logbl ()
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T} Thread safety MT-Safe
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.TE
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.hy
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.ad
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.sp 1
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.SH CONFORMING TO
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C99, POSIX.1-2001, POSIX.1-2008.
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.SH SEE ALSO
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.BR ilogb (3),
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.BR log (3)
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