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'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH atan 3 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
atan, atanf, atanl \- arc tangent function
.SH LIBRARY
Math library
.RI ( libm ,\~ \-lm )
.SH SYNOPSIS
.nf
.B #include <math.h>
.P
.BI "double atan(double " x );
.BI "float atanf(float " x );
.BI "long double atanl(long double " x );
.P
.fi
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR atanf (),
.BR atanl ():
.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 calculate the principal value of the arc tangent of
.IR x ;
that is the value whose tangent is
.IR x .
.SH RETURN VALUE
On success, these functions return the principal value of the arc tangent of
.I x
in radians; the return value is in the range [\-pi/2,\ pi/2].
.P
If
.I x
is a NaN, a NaN is returned.
.P
If
.I x
is +0 (\-0),
+0 (\-0) is returned.
.P
If
.I x
is positive infinity (negative infinity), +pi/2 (\-pi/2) is returned.
.\"
.\" POSIX.1-2001 documents an optional range error for subnormal x;
.\" glibc 2.8 does not do this.
.SH ERRORS
No errors occur.
.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 atan (),
.BR atanf (),
.BR atanl ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C99, POSIX.1-2001.
.P
The variant returning
.I double
also conforms to
SVr4, 4.3BSD, C89.
.SH SEE ALSO
.BR acos (3),
.BR asin (3),
.BR atan2 (3),
.BR carg (3),
.BR catan (3),
.BR cos (3),
.BR sin (3),
.BR tan (3)
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