.\" Copyright 1995 Jim Van Zandt <jrv@vanzandt.mv.com>
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\"
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.\" Modified 2002-07-27 by Walter Harms
.\" 	(walter.harms@informatik.uni-oldenburg.de)
.TH LOG1P 3   2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
log1p, log1pf, log1pl \-  logarithm of 1 plus argument
.SH SYNOPSIS
.nf
.B #include <math.h>
.PP
.BI "double log1p(double " x );
.BI "float log1pf(float " x );
.BI "long double log1pl(long double " x );
.PP
.fi
Link with \fI\-lm\fP.
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.ad l
.BR log1p ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
    || _XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.br
.BR log1pf (),
.BR log1pl ():
.RS 4
_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.RE
.ad b
.SH DESCRIPTION
These functions return a value equivalent to
.PP
.nf
    log (1 + \fIx\fP)
.fi
.PP
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)" .
.PP
If
.I x
is a NaN,
a NaN is returned.
.PP
If
.I x
is positive infinity, positive infinity is returned.
.PP
If
.I x
is \-1, a pole error occurs,
and the functions return
.RB - HUGE_VAL ,
.RB - HUGE_VALF ,
or
.RB - HUGE_VALL ,
respectively.
.PP
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.
.PP
The following errors can occur:
.TP
Domain error: \fIx\fP is less than \-1
.I errno
is set to
.BR 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
.BR 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;
lbw27 lb lb
l l l.
Interface	Attribute	Value
T{
.BR log1p (),
.BR log1pf (),
.BR log1pl ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
.\" BSD
.SH BUGS
Before version 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.
.PP
Before version 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)
.SH COLOPHON
This page is part of release 5.04 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.