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'\" t
.\" Copyright 1995 Jim Van Zandt <jrv@vanzandt.mv.com>
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified 2002-07-27 by Walter Harms
.\" (walter.harms@informatik.uni-oldenburg.de)
.TH log1p 3 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
log1p, log1pf, log1pl \- logarithm of 1 plus argument
.SH LIBRARY
Math library
.RI ( libm ", " \-lm )
.SH SYNOPSIS
.nf
.B #include <math.h>
.P
.BI "double log1p(double " x );
.BI "float log1pf(float " x );
.BI "long double log1pl(long double " x );
.P
.fi
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.nf
.BR log1p ():
_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 log1pf (),
.BR log1pl ():
.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 return a value equivalent to
.P
.nf
log (1 + \fIx\fP)
.fi
.P
The result is computed in a way
that is accurate even if the value of
.I x
is near zero.
.SH RETURN VALUE
On success, these functions return the natural logarithm of
.IR "(1\ +\ x)" .
.P
If
.I x
is a NaN,
a NaN is returned.
.P
If
.I x
is positive infinity, positive infinity is returned.
.P
If
.I x
is \-1, a pole error occurs,
and the functions return
.RB \- HUGE_VAL ,
.RB \- HUGE_VALF ,
or
.RB \- HUGE_VALL ,
respectively.
.P
If
.I x
is less than \-1 (including negative infinity),
a domain error occurs,
and a NaN (not a number) is returned.
.\" POSIX.1 specifies a possible range error if x is subnormal
.\" glibc 2.8 doesn't do this
.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
Domain error: \fIx\fP is less than \-1
.I errno
is set to
.B EDOM
(but see BUGS).
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.TP
Pole error: \fIx\fP is \-1
.I errno
is set to
.B ERANGE
(but see BUGS).
A divide-by-zero floating-point exception
.RB ( FE_DIVBYZERO )
is raised.
.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 log1p (),
.BR log1pf (),
.BR log1pl ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C99, POSIX.1-2001.
.\" BSD
.SH BUGS
Before glibc 2.22, the glibc implementation did not set
.\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6792
.I errno
to
.B EDOM
when a domain error occurred.
.P
Before glibc 2.22, the glibc implementation did not set
.\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6792
.I errno
to
.B ERANGE
when a range error occurred.
.SH SEE ALSO
.BR exp (3),
.BR expm1 (3),
.BR log (3)
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