[m-users.] Foreign language interface

[Volker Wysk]

Hi!

This is the first time I've done something non-trivial with the FLI. It
looks good, but I'm not sure if I've done it right. My foreign predicate
reads the process' environment and returns it as a list of strings:


:- pragma foreign_decl("C", "#include <unistd.h>").
:- pragma foreign_decl("C", "#include <string.h>").

:- pragma foreign_proc("C",
    get_environment_0(List::out, In::di, Out::uo),
    [ will_not_call_mercury,
      promise_pure
    ],
    "
    List = MR_list_empty();
    for (char ** str = __environ;
         *str;
         str++) {
       void * copy = MR_GC_malloc(strlen(*str) + 1);
       strcpy(copy, *str);
       List = MR_list_cons(copy, List);
    }
    Out = In;
    ").

:- pred get_environment_0(list(string)::out, io::di, io::uo) is det.


I assume:
- The foreign predicate is pure, because it is an IO action.
- The strings in the returned list will be garbage collected.
- It is necessary to copy them over to Mercury land, like I did. If
  pointers into the process' environment were handed over to Mercury, they 
  would possibly get garbage collected, what can't work.


Happy hacking,
Volker