# 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.1‐2008 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

*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*()*The Base Definitions volume of POSIX.1‐2008,*

*tan*()*Section 4.19*,

*Treatment of Error Conditions for Mathematical Functions*,

**<math.h>**