I’m trying to understand what happens at the C level when I call a function in R. For instance, print("Hello, World!") will call UseMethod("print"). From the “table” in names.c, line 832, I know that UseMethod() in turn will call do_usemethod:

{"UseMethod",   do_usemethod,   0,     200, -1, {PP_FUNCALL, PREC_FN,   0}},

I have a basic understanding of what the columns mean, but when I look at the code for do_usemethod, in objects.c on line 538, I get confused:

/* This is a primitive SPECIALSXP */
attribute_hidden NORET
SEXP do_usemethod(SEXP call, SEXP op, SEXP args, SEXP env)
{ 
    ...
}

Can someone explain how and where do_usemethod picks up the input arguments? I guess op could be the arity column in names.c, but what about call, args and env? The answer may be obvious, but I find it very difficult to follow the breadcrumbs here, and I have been at it for weeks.

I have read the help for UseMethod() and also started reading Appendix A of Statistical Models in S (Chambers & Hastie, 1992), but did not find an answer to my question. I don’t know of any books that go more into details.