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table of contents

ATAN(3P) | POSIX Programmer's Manual | ATAN(3P) |

# 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(doublex); float atanf(floatx); long double atanl(long doublex);

# 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.1‐2017 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 non-zero or
*fetestexcept*(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, 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 implementation-defined 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 non-zero, then *errno* shall be set to **[ERANGE]**.
If the integer expression (*math_errhandling* & MATH_ERREXCEPT) is
non-zero, then the underflow floating-point 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 non-zero.

# RATIONALE¶

None.

# FUTURE DIRECTIONS¶

None.

# SEE ALSO¶

* atan2()*,

*,*

*feclearexcept*()*,*

*fetestexcept*()*,*

*isnan*()

*tan*()The Base Definitions volume of POSIX.1‐2017, *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 1003.1-2017, 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/man-pages/reporting_bugs.html .

2017 | IEEE/The Open Group |