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**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.1‐2008 defers to the ISO C standard. These functions shall compute the principal value of the arc tangent of their argumentx. An application wishing to check for error situations should seterrnoto zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrnois non- zero orfetestexcept(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 ofxin the range [−π/2,π/2] radians. Ifxis NaN, a NaN shall be returned. Ifxis ±0,xshall be returned. Ifxis ±Inf, ±π/2 shall be returned. Ifxis subnormal, a range error may occur andxshould be returned. Ifxis not returned,atan(),atanf(), andatanl() 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 ofxis subnormal. If the integer expression (math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall be set to[ERANGE]. If the integer expression (math_errhandling& MATH_ERREXCEPT) is non-zero, then the underflow floating-point exception shall be raised.Thefollowingsectionsareinformative.

**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.1‐2008,Section4.19,TreatmentofErrorConditionsforMathematicalFunctions,<math.h>

**COPYRIGHT**

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .