C function from R libraries, acf function

Question

I am new to R. I noticed that the acf function R makes this call:

acf <- .Call(C_acf, x, lag.max, type == "correlation")

I want to find the C_acf function in the c library and possibly change it. Does anyone know how I can find the C_acf function? What C code is it written? I can even call its modified version locally, but I want to know what version C does under the hood.