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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
.\" Modified 2002-07-25 by Walter Harms
.\" 	(walter.harms@informatik.uni-oldenburg.de)
.\"
.TH ACOS 3  2010-09-20 "" "Linux Programmer's Manual"
.SH NAME
acos, acosf, acosl \- arc cosine function
.SH SYNOPSIS
.nf
.B #include <math.h>
.sp
.BI "double acos(double " x );
.BI "float acosf(float " x );
.BI "long double acosl(long double " x );
.fi
.sp
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 acosf (),
.BR acosl ():
.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
The
.BR acos ()
function calculates the arc cosine of \fIx\fP; that is
the value whose cosine is \fIx\fP.
.SH RETURN VALUE
On success, these functions return the arc cosine of
.IR x
in radians; the return value is in the range [0,\ pi].

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

If
.I x
is +1,
+0 is returned.

If
.I x
is positive infinity or negative infinity,
a domain error occurs,
and a NaN is returned.

If
.I x
is outside the range [\-1,\ 1],
a domain error occurs,
and a NaN is returned.
.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 outside the range [\-1,\ 1]
.I errno
is set to
.BR EDOM .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.SH CONFORMING TO
C99, POSIX.1-2001.
The variant returning
.I double
also conforms to
SVr4, 4.3BSD, C89.
.SH SEE ALSO
.BR asin (3),
.BR atan (3),
.BR atan2 (3),
.BR cacos (3),
.BR cos (3),
.BR sin (3),
.BR tan (3)