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double acos(double x);
float acosf(float x);
long double acosl(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 acos() function calculates the arc cosine of x; that is the value
whose cosine is x.
On success, these functions return the arc cosine of x in radians; the
return value is in the range [0, pi].
If x is a NaN, a NaN is returned.
If x is +1, +0 is returned.
If x is positive infinity or negative infinity, a domain error occurs,
and a NaN is returned.
If x is outside the range [-1, 1], a domain error occurs, and a NaN is
See math_error(7) for information on how to determine whether an error
has occurred when calling these functions.
The following errors can occur:
Domain error: x is outside the range [-1, 1]
errno is set to EDOM. An invalid floating-point exception
(FE_INVALID) is raised.
C99, POSIX.1-2001. The variant returning double also conforms to SVr4,
asin(3), atan(3), atan2(3), cacos(3), cos(3), sin(3), tan(3)