acosl (3) manual page

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acos, acosf, acosl - arc cosine function


#include <math.h>

double acos(double x);
float acosf(float x);
long double acosl(long double x);

Link with -lm.


The acos() function calculates the arc cosine of x; that is the value whose cosine is x. If x falls outside the range -1 to 1, acos() fails and errno is set.

Return Value

The acos() function returns the arc cosine in radians and the value is mathematically defined to be between 0 and PI (inclusive).


x is out of range.

Conforming to

SVr4, POSIX.1-2001, 4.3BSD, C89. The float and long double variants are C99 requirements.

See Also

asin(3) , atan(3) , atan2(3) , cacos(3) , cos(3) , sin(3) , tan(3)

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