C callbacks for GNU readline command completion

[vlaviron]

@nojb

OCaml strings consist of arbitrary sequences of bytes and may contain embedded NULLs. In particular they are not terminated with NULL, so you will need to add those by hand on the C side.

This is slightly misleading: the actual representation of OCaml strings in memory ensures that there is a NULL character just after the last character that is part of the string.
So as long as your string doesn’t contain other NULL characters (which you can check with caml_string_is_c_safe), you can cast it to a C string with the String_val macro.