NAME

atan2, atan2f, atan2l - arc tangent functions

SYNOPSIS

#include <math.h>

double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);


DESCRIPTION

^[CX] 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 IEEE Std 1003.1-2001 defers to the ISO C standard.

These functions shall compute the principal value of the arc tangent of y/ x, using the signs of both arguments to determine the quadrant of the return value.

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 y/ x in the range [-,] radians.

If y is ±0 and x is < 0, ± shall be returned.

If y is ±0 and x is > 0, ±0 shall be returned.

If y is < 0 and x is ±0, -/2 shall be returned.

If y is > 0 and x is ±0, /2 shall be returned.

If x is 0, a pole error shall not occur.

^[MX] If either x or y is NaN, a NaN shall be returned.

If the result underflows, a range error may occur and y/ x should be returned.

If y is ±0 and x is -0, ± shall be returned.

If y is ±0 and x is +0, ±0 shall be returned.

For finite values of ± y > 0, if x is -Inf, ± shall be returned.

For finite values of ± y > 0, if x is +Inf, ±0 shall be returned.

For finite values of x, if y is ±Inf, ±/2 shall be returned.

If y is ±Inf and x is -Inf, ±3/4 shall be returned.

If y is ±Inf and x is +Inf, ±/4 shall be returned.

If both arguments are 0, a domain error shall not occur.

ERRORS

These functions may fail if:

Range Error

^[MX] The result underflows.

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.

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

atan() , feclearexcept() , fetestexcept() , isnan() , tan() , the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <math.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in the APPLICATION USAGE section.

Issue 6

The atan2f() and atan2l() functions are added for alignment with the ISO/IEC 9899:1999 standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.