ATAN
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
atan,
atanf,
atanl
 arc tangent function
SYNOPSIS
#include <math.h>
double atan(double x);
float atanf(float x);
long double atanl(long double x);
DESCRIPTION
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here and the
ISO C standard is unintentional. This volume of POSIX.12017 defers to the ISO C standard.
These functions shall compute the principal value of the arc tangent of
their argument
x.
An application wishing to check for error situations should set
errno
to zero and call
feclearexcept(FE_ALL_EXCEPT)
before calling these functions. On return, if
errno
is nonzero or fetestexcept(FE_INVALID  FE_DIVBYZERO 
FE_OVERFLOW  FE_UNDERFLOW) is nonzero, an error has occurred.
RETURN VALUE
Upon successful completion, these functions shall return the arc
tangent of
x
in the range [π/2,π/2] radians.
If
x
is NaN, a NaN shall be returned.
If
x
is ±0,
x
shall be returned.
If
x
is ±Inf, ±π/2 shall be returned.
If
x
is subnormal, a range error may occur
and
x
should be returned.
If
x
is not returned,
atan(),
atanf(),
and
atanl()
shall return an implementationdefined value no greater in magnitude
than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.
ERRORS
These functions may fail if:
 Range Error

The value of
x
is subnormal.

If the integer expression (math_errhandling & MATH_ERRNO) is
nonzero, then
errno
shall be set to
[ERANGE].
If the integer expression (math_errhandling & MATH_ERREXCEPT) is
nonzero, then the underflow floatingpoint exception shall be raised.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
On error, the expressions (
math_errhandling & MATH_ERRNO) and
(
math_errhandling & MATH_ERREXCEPT) are independent of each
other, but at least one of them must be nonzero.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
atan2(),
feclearexcept(),
fetestexcept(),
isnan(),
tan()
The Base Definitions volume of POSIX.12017,
Section 4.20, Treatment of Error Conditions for Mathematical Functions,
<math.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.12017, Standard for Information Technology
 Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/manpages/reporting_bugs.html .