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HP-UX 11i Version 3: February 2007

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atan(), atanf(), atanl(), atanw(), atanq() — arctangent functions


#include <math.h>

double atan(double x);

float atanf(float x);

HP Integrity Server Only

long double atanl(long double x);

extended atanw(extended x);

quad atanq(quad x);


atan() returns the arctangent of x, in the range -π/2 to π/2.

atanf() is a float version of atan(); it takes a float argument and returns a float result.

Integrity Server Only

atanl() is a long double version of atan(); it takes a long double argument and returns a long double result.

atanw() is an extended version of atan(); it takes an extended argument and returns an extended result.

atanq() is equivalent to atanl() on HP-UX systems.


To use these functions compile either with the default -Ae option or with the -Aa and the -D_HPUX_SOURCE options.

To use (for Integrity servers) atanw() or atanq(), compile also with -fpwidetypes.

To use any of these functions, make sure your program includes <math.h>, and link in the math library by specifying -lm on the compiler or linker command line. For more information, see the HP-UX floating-point guide for HP Integrity servers at the following site: http://www.hp.com/go/fp.


Millicode versions of the atan() and atanf() functions 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).


atan(±0) returns ±0.

If x is ±INFINITY, atan() returns ±π/2 respectively.

If x is NaN, atan() returns NaN.

Whether atan() raises the inexact exception is unspecified.


No errors are defined.


atan() : SVID3, XPG4.2, ANSI C, ISO/IEC C99 (including Annex F, ``IEC 60559 floating-point arithmetic'')

atanf(), atanl() : ISO/IEC C99 (including Annex F, ``IEC 60559 floating-point arithmetic'')

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