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Inverse Trigonometric Functions

These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively.

double asin (double x) Function
float asinf (float x) Function
long double asinl (long double x) Function
These functions compute the arc sine of x--that is, the value whose sine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

The arc sine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, asin signals a domain error.

double acos (double x) Function
float acosf (float x) Function
long double acosl (long double x) Function
These functions compute the arc cosine of x--that is, the value whose cosine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between 0 and pi (inclusive).

The arc cosine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, acos signals a domain error.

double atan (double x) Function
float atanf (float x) Function
long double atanl (long double x) Function
These functions compute the arc tangent of x--that is, the value whose tangent is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

double atan2 (double y, double x) Function
float atan2f (float y, float x) Function
long double atan2l (long double y, long double x) Function
This function computes the arc tangent of y/x, but the signs of both arguments are used to determine the quadrant of the result, and x is permitted to be zero. The return value is given in radians and is in the range -pi to pi, inclusive.

If x and y are coordinates of a point in the plane, atan2 returns the signed angle between the line from the origin to that point and the x-axis. Thus, atan2 is useful for converting Cartesian coordinates to polar coordinates. (To compute the radial coordinate, use hypot; see Exponents and Logarithms.)

If both x and y are zero, atan2 returns zero.

ISO C99 defines complex versions of the inverse trig functions.

complex double casin (complex double z) Function
complex float casinf (complex float z) Function
complex long double casinl (complex long double z) Function
These functions compute the complex arc sine of z--that is, the value whose sine is z. The value returned is in radians.

Unlike the real-valued functions, casin is defined for all values of z.

complex double cacos (complex double z) Function
complex float cacosf (complex float z) Function
complex long double cacosl (complex long double z) Function
These functions compute the complex arc cosine of z--that is, the value whose cosine is z. The value returned is in radians.

Unlike the real-valued functions, cacos is defined for all values of z.

complex double catan (complex double z) Function
complex float catanf (complex float z) Function
complex long double catanl (complex long double z) Function
These functions compute the complex arc tangent of z--that is, the value whose tangent is z. The value is in units of radians.