man3/log1p.3

.\" Copyright 1995 Jim Van Zandt <jrv@vanzandt.mv.com>
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" Modified 2002-07-27 by Walter Harms
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.TH LOG1P 3   2010-09-20 "" "Linux Programmer's Manual"
.SH NAME
log1p, log1pf, log1pl \-  logarithm of 1 plus argument
.SH SYNOPSIS
.nf
.B #include <math.h>
.sp
.BI "double log1p(double " x );
.br
.BI "float log1pf(float " x );
.br
.BI "long double log1pl(long double " x );
.sp
.fi
Link with \fI\-lm\fP.
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR log1p ():
.RS 4
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE ||
_POSIX_C_SOURCE\ >=\ 200112L;
.br
or
.I cc\ -std=c99
.RE
.br
.BR log1pf (),
.BR log1pl ():
.RS 4
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE\ >=\ 200112L;
.br
or
.I cc\ -std=c99
.RE
.ad b
.SH DESCRIPTION
.I log1p(x)
returns a value equivalent to
.nf

    log (1 + \fIx\fP)

.fi
It is computed in a way
that is accurate even if the value of \fIx\fP is near zero.
.SH RETURN VALUE
On success, these functions return the natural logarithm of
.IR "(1\ +\ x)" .

If
.I x
is a NaN,
a NaN is returned.

If
.I x
is positive infinity, positive infinity is returned.

If
.I x
is \-1, a pole error occurs,
and the functions return
.RB - HUGE_VAL ,
.RB - HUGE_VALF ,
or
.RB - HUGE_VALL ,
respectively.

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 .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.TP
Pole error: \fIx\fP is \-1
.\" .I errno
.\" is set to
.\" .BR ERANGE .
A divide-by-zero floating-point exception
.RB ( FE_DIVBYZERO )
is raised.
.PP
These functions do not set
.IR errno .
.\" FIXME . Is it intentional that these functions do not set errno?
.\" log(), log2(), log10() do set errno
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6792
.SH "CONFORMING TO"
C99, POSIX.1-2001.
.\" BSD
.SH "SEE ALSO"
.BR exp (3),
.BR expm1 (3),
.BR log (3)