IEEE Std 1003.1, 2013 Edition

Copyright © 2001-2013 The IEEE and The Open Group

atan2, atan2f, atan2l - arc tangent functions

#include <math.h>

double atan2(doubley, doublex);

float atan2f(floaty, floatx);

long double atan2l(long doubley, long doublex);

^{[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 POSIX.1-2008 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

errnoto 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.

Upon successful completion, these functions shall return the arc tangent of

y/xin the range [-,] radians.If

yis ±0 andxis < 0, ± shall be returned.If

yis ±0 andxis > 0, ±0 shall be returned.If

yis < 0 andxis ±0, -/2 shall be returned.If

yis > 0 andxis ±0, /2 shall be returned.If

xis 0, a pole error shall not occur.

^{[MX]}If eitherxoryis NaN, a NaN shall be returned.If the correct value would cause underflow, a range error may occur, and

atan(),atan2f(), andatan2l() shall return an implementation-defined value no greater in magnitude than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.

^{[MXX]}If the IEC 60559 Floating-Point option is supported,y/xshould be returned.

^{[MX]}Ifyis ±0 andxis -0, ± shall be returned.If

yis ±0 andxis +0, ±0 shall be returned.For finite values of ±

y> 0, ifxis -Inf, ± shall be returned.For finite values of ±

y> 0, ifxis +Inf, ±0 shall be returned.For finite values of

x, ifyis ±Inf, ±/2 shall be returned.If

yis ±Inf andxis -Inf, ±3/4 shall be returned.If

yis ±Inf andxis +Inf, ±/4 shall be returned.If both arguments are 0, a domain error shall not occur.

These functions may fail if:

- Range Error
^{[MX]}The result underflows.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.

## Converting Cartesian to Polar Coordinates System

The function below uses

atan2() to convert a 2d vector expressed in cartesian coordinates (x,y) to the polar coordinates (rho,theta). There are other ways to compute the angletheta, usingasin()acos(), oratan(). However,atan2() presents here two advantages:

The angle's quadrant is automatically determined.

The singular cases (0,

y) are taken into account.Finally, this example uses

hypot() rather thansqrt() since it is better for special cases; seehypot() for more information.#include <math.h>

void cartesian_to_polar(const double x, const double y, double *rho, double *theta ) { *rho = hypot (x,y); /* better than sqrt(x*x+y*y) */ *theta = atan2 (y,x); }

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.

None.

None.

acos,asin,atan,feclearexcept,fetestexcept,hypot,isnan,sqrt,tanXBD

Treatment of Error Conditions for Mathematical Functions,<math.h>

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

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

The

atan2f() andatan2l() 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, and the IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/18 is applied, adding to the EXAMPLES section.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0038 [68,428] is applied.

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