These functions are described in the XSI Curses standard, Issue 4. They read single-byte characters only. The standard does not define any error conditions. This implementation returns ERR if the window pointer is null, or if the lower-level wgetch call returns an ERR.
SVr3 and early SVr4 curses implementations did not reject function keys; the SVr4.0 documentation claimed that "special keys" (such as function keys, "home" key, "clear" key, etc.) are interpreted" without giving details. It lied. In fact, the character value appended to the string by those implementations was predictable but not useful (being, in fact, the low-order eight bits of the keys KEY_ value).
The functions getnstr, mvgetnstr, and mvwgetnstr were present but not documented in SVr4.