Arc tangent function
#include <math.h> double atan(double x); float atanf(float x); long double atanl( long double x);
Link with -lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99
The atan() function calculates the principal value of the arc tangent of x; that is the value whose tangent is x.
On success, these functions return the principal value of the arc tangent of x in radians; the return value is in the range [-pi/2, pi/2].
If x is a NaN, a NaN is returned.
If x is +0 (-0), +0 (-0) is returned.
If x is positive infinity (negative infinity), +pi/2 (-pi/2) is returned.
No errors occur.
The atan(), atanf(), and atanl() functions are thread-safe.
C99, POSIX.1-2001. The variant returning double also conforms to SVr4, 4.3BSD, C89.
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