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.\" Copyright 1995 Jim Van Zandt <jrv@vanzandt.mv.com>
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
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.\" Modified 2002-07-27 Walter Harms
.\" 	(walter.harms@informatik.uni-oldenburg.de)
.\"
.TH EXPM1 3  2015-03-02 "" "Linux Programmer's Manual"
.SH NAME
expm1, expm1f, expm1l \- exponential minus 1
.SH SYNOPSIS
.nf
.B #include <math.h>
.sp
.BI "double expm1(double " x );
.br
.BI "float expm1f(float " x );
.br
.BI "long double expm1l(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 expm1 ():
.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 expm1f (),
.BR expm1l ():
.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
These functions return a value equivalent to
.nf

    exp(x) \- 1

.fi
The result is computed in a way that is accurate even if the value of
.I x
is near
zero\(ema case where
.I "exp(x) \- 1"
would be inaccurate due to
subtraction of two numbers that are nearly equal.
.SH RETURN VALUE
On success, these functions return
.IR "exp(x)\ \-\ 1" .

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

If
.I x
is +0 (\-0),
+0 (\-0) is returned.

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

If
.I x
is negative infinity, \-1 is returned.

If the result overflows, a range error occurs,
and the functions return
.RB - HUGE_VAL ,
.RB - HUGE_VALF ,
or
.RB - HUGE_VALL ,
respectively.
.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
Range error, overflow
.I errno
is set to
.BR ERANGE
(but see BUGS).
An overflow floating-point exception
.RB ( FE_OVERFLOW )
is raised.
.\"
.\" POSIX.1 specifies an optional range error (underflow) if
.\" x is subnormal.  Glibc does not implement this.
.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 expm1 (),
.BR expm1f (),
.BR expm1l ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
.\" BSD.
.SH BUGS
For some large negative
.I x
values (where the function result approaches \-1),
.BR expm1 ()
raises a bogus underflow floating-point exception.
.\" FIXME .
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6778

For some large positive
.I x
values,
.BR expm1 ()
raises a bogus invalid floating-point exception in addition to the expected
overflow exception, and returns a NaN instead of positive infinity.
.\" FIXME .
.\" Bug raised: http://sources.redhat.com/bugzilla/show_bug.cgi?id=6814
.\" e.g., expm1(1e5) through expm1(1.00199970127e5),
.\" but not expm1(1.00199970128e5) and beyond.

Before version 2.11,
.\" It looks like the fix was in 2.11, or possibly 2.12.
.\" I have no test system for 2.11, but 2.12 passes.
.\" From the source (sysdeps/i386/fpu/s_expm1.S) it looks
.\" like the changes were in 2.11.
the glibc implementation did not set
.\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=6788
.I errno
to
.B ERANGE
when a range error occurred.
.SH SEE ALSO
.BR exp (3),
.BR log (3),
.BR log1p (3)