The C programming asin function is defined in the math.h header file and is used to calculate the arcsine of a given value x, in radians. The arcsine function, denoted asin(x), returns the angle (in radians) whose sine is x.

The asin function takes a single argument of type double, which represents the value whose arcsine is to be calculated. The function returns a double value, which represents the calculated arcsine in radians.

Here's an example usage of the asin function to calculate the arcsine of a given input:

#include <stdio.h>
#include <math.h>

int main() {
    double x = 0.5;
    double arcsine = asin(x);
    
    printf("The arcsine of %f is %f radians\n", x, arcsine);
    
    return 0;
}

In this example, the asin function is used to calculate the arcsine of 0.5, which is stored in the x variable. The calculated arcsine in radians is then stored in the arcsine variable, and printed to the console using the printf function.

Note that the asin function returns a value between -π/2 and π/2 radians, inclusive. Also note that the input x should be within the range of -1 to 1, inclusive. If x is outside this range, the asin function returns a NaN (not a number) value.