Static R_MissingArg 

unsafe static R_MissingArg: SEXP

The “missing argument” sentinel value.

When an R function is called without providing a value for a formal argument, R passes R_MissingArg as a placeholder. This is different from R_NilValue (NULL) - a missing argument means “not provided”, while NULL is an explicit value.

In R: f <- function(x) missing(x); f() returns TRUE. Encapsulated by SEXP::missing_arg()