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.\" Copyright 1995 Jim Van Zandt <jrv@vanzandt.mv.com>
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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.\" manual page may be incorrect or out-of-date.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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.\" Modified 2002-07-27 by Walter Harms
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.\" (walter.harms@informatik.uni-oldenburg.de)
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.TH LOG1P 3 2017-09-15 "" "Linux Programmer's Manual"
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.SH NAME
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log1p, log1pf, log1pl \- logarithm of 1 plus argument
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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 log1p(double " x );
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.BI "float log1pf(float " x );
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.BI "long double log1pl(long double " x );
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.PP
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.fi
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Link with \fI\-lm\fP.
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.PP
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.PP
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.ad l
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.BR log1p ():
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.RS 4
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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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.RE
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.br
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.BR log1pf (),
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.BR log1pl ():
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.RS 4
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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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.RE
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.ad b
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.SH DESCRIPTION
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These functions return a value equivalent to
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.PP
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.nf
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log (1 + \fIx\fP)
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.fi
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.PP
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The result is computed in a way
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that is accurate even if the value of
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.I x
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is near zero.
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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 "(1\ +\ 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 positive infinity, 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 \-1, a pole error occurs,
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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 less than \-1 (including negative infinity),
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a domain error occurs,
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and a NaN (not a number) is returned.
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.\" POSIX.1 specifies a possible range error if x is subnormal
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.\" glibc 2.8 doesn't do this
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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 less than \-1
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.I errno
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is set to
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.BR EDOM
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(but see BUGS).
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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 \-1
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.I errno
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is set to
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.BR ERANGE
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(but see BUGS).
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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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.TS
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allbox;
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lbw27 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 log1p (),
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.BR log1pf (),
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.BR log1pl ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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C99, POSIX.1-2001, POSIX.1-2008.
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.\" BSD
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.SH BUGS
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Before version 2.22, the glibc implementation did not set
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.\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6792
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.I errno
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to
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.B EDOM
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when a domain error occurred.
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.PP
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Before version 2.22, the glibc implementation did not set
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.\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6792
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.I errno
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to
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.B ERANGE
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when a range error occurred.
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.SH SEE ALSO
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.BR exp (3),
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.BR expm1 (3),
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.BR log (3)
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.SH COLOPHON
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This page is part of release 4.15 of the Linux
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.I man-pages
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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\%https://www.kernel.org/doc/man\-pages/.
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