HP-UX Reference (11i v1 00/12) - 3 Library Functions A-M (vol 6)

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atan2(3M) atan2(3M)

NAME

atan2(), atan2f() - arctangent-and-quadrant functions

SYNOPSIS

#include <math.h>

double atan2(double y, double x);

float atan2f(float y, float x);

DESCRIPTION

atan2() returns the arctangent of y/x, in the range −π to π, using the signs of both arguments to deter-

mine the quadrant of the return value.

atan2f() is a float version of atan2(); it takes float arguments and returns a float result. To

use this function, compile either with the default -Ae option or with the -Aa and -D_HPUX_SOURCE

options.

atan2f() is not speciﬁed by any standard, but it is named in accordance with the conventions speciﬁed in

the "Future Library Directions" section of the ANSI C standard.

To use these functions, make sure your program includes <

math.h>, and link in the math library by speci-

fying

-lm on the compiler or linker command line.

Millicode versions of the atan2() function are available. Millicode versions of math library functions are

usually faster than their counterparts in the standard library. To use these versions, compile your program

with the +Olibcalls or the +Oaggressive optimization option.

For special cases, the millicode versions return the same values as their standard library counterparts (see

the RETURN VALUE section).

For more information, see the HP-UX Floating-Point Guide.

RETURN VALUE

If y and x are +INFINITY,

atan2() returns π/4.

If y is +INFINITY and x is −INFINITY,

atan2()

returns 3∗π/4.

If y is −INFINITY and x is +INFINITY,

atan2() returns −π/4.

If y and x are −INFINITY,

atan2() returns −3∗π/4.

If y is zero and x is greater than zero,

atan2() returns zero.

If y is zero and x is less than zero, atan2() returns π.

If y is −zero and x is less than zero, atan2() returns −π.

If y is greater than zero and x is zero,

atan2() returns π/2.

If y is less than zero and x is zero, atan2() returns −π/2.

If y/x would overﬂow, atan2() returns ±π/2. The result will be π/2 if y is greater than zero and −π/2 if y is

less than zero.

If y/x after rounding would be smaller in magnitude than MINDOUBLE , atan2() returns ±π or zero. The

result is zero if x is greater than zero, π if x is less than zero and y is greater than zero, and −π if x and y

are both less than zero.

If both x and y are zero,

atan2() returns zero.

If x or y is NaN, atan2() returns NaN.

If the correct value after rounding would be smaller in magnitude than MINDOUBLE, atan2() returns

zero.

ERRORS

No errors are deﬁned.