mirror of https://github.com/mkerrisk/man-pages
160 lines
3.7 KiB
Groff
160 lines
3.7 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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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.\" manual page may be incorrect or out-of-date.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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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
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.\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
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.\" Modified 2002-07-27 by Walter Harms
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.\" (walter.harms@informatik.uni-oldenburg.de)
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.\"
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.TH LOG 3 2017-09-15 "" "Linux Programmer's Manual"
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.SH NAME
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log, logf, logl \- natural logarithmic function
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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 log(double " x );
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.BI "float logf(float " x );
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.BI "long double logl(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 logf (),
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.BR logl ():
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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 return the natural logarithm of
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.IR x .
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.SH RETURN VALUE
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On success, these functions return the natural logarithm 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 1, the result is +0.
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.PP
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If
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.I x
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is positive infinity,
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positive infinity is returned.
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.PP
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If
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.I x
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is zero,
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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 (including negative infinity), then
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a domain error occurs, and a NaN (not a number) 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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Domain error: \fIx\fP is negative
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.I errno
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is set to
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.BR EDOM .
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An invalid floating-point exception
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.RB ( FE_INVALID )
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is raised.
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.TP
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Pole error: \fIx\fP is zero
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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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.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 log (),
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.BR logf (),
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.BR logl ()
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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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.PP
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The variant returning
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.I double
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also conforms to
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SVr4, 4.3BSD, C89.
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.SH BUGS
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In glibc 2.5 and earlier,
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taking the
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.BR log ()
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of a NaN produces a bogus invalid floating-point
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.RB ( FE_INVALID )
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exception.
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.SH SEE ALSO
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.BR cbrt (3),
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.BR clog (3),
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.BR log10 (3),
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.BR log1p (3),
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.BR log2 (3),
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.BR sqrt (3)
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