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Wed, 14 Sep 2011 20:57:58 +0200
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Subject: Re: [Caml-list] Link a .so/.dll dynamically
From: [email protected]
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Date: Wed, 14 Sep 2011 20:57:58 +0200
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References: <[email protected]> <1316016912.24759.29.camel@aurora>
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On 14 Sep 2011, at 18:15, J=E9r=E9mie Dimino wrote:
> You can also use dlopen and dlsym on unix, and=20
> LoadLibrary and GetProcAddress on windows.
Right. For truly dynamic loading, if you already have a set of .so =
files with similar API you might want to write some "loader" in C and =
compile that into your OCaml program. No need to put an extra dynlink =
layer in between. BTW Dynlink is not available on all platforms.
In this "loader" you would call dlopen() and dlsym() (or their Windows =
counterparts) to get the addresses of the functions in the .so, and make =
them available to the rest of your OCaml program. You still need to =
translate OCAml-to-C function arguments and C-to-OCaml function results. =
Note that this is somewhat insecure C plumbing, you can easily bring =
down the whole process in a number of ways.
While it is way too soon for announcements, I do have some very =
experimental code that allows to do exactly this in pure OCaml. All you =
need to do is write a C wrapper, sort of like how you do it in Python =
(see http://docs.python.org/extending/extending.html for example). The =
OCaml code looks like this:
# #load "dffi.cma";;
# open DL;;
# #install_printer Pointer.pp;;
# let lib =3D dlopen "/wherever/pgtest.dylib" [];;
val lib : DL.dllib =3D <abstr>
# let connect =3D dlsym lib "pq_connect_db" and
disconnect =3D dlsym lib "pq_finish" and
protocol_version =3D dlsym lib "pq_protocol_version";;
val connect : DL.dlfun =3D <abstr>
val disconnect : DL.dlfun =3D <abstr>
val protocol_version : DL.dlfun =3D <abstr>
# let [|conn|] =3D dlcall connect [|String "dbname=3Dname_of_database"|];;=
val conn : DL.cval =3D Ptr (PGconn*)0x0000000100100ef0
# dlcall protocol_version [|conn|];;
- : DL.cval array =3D [|Int 3|]
# dlcall disconnect [|conn|];;
- : DL.cval array =3D [||]
# dlclose lib;;
- : unit =3D ()
And pgtest.c (a PostgreSQL client lib protowrapper) looks like this:
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <libpq-fe.h>
#include "dlutil.h"
FUNC(pq_connect_db) {
ARGS; DECL;
PGconn* conn;
CHECKNUMARGS(1); CHECKSTR(0); /*1 arg, must be String*/
conn =3D PQconnectdb((const char*)GETSTR(0));
if (PQstatus(conn) =3D=3D CONNECTION_OK) {
ALLOC(1); /*return 1 value*/
SETPTR(0, PGconn*, conn); }
else {
PQfinish(conn);
ALLOC(0); }
RETURN; }
FUNC(pq_finish) {
ARGS; DECL;
CHECKNUMARGS(1); CHECKPTR(0); CHECKPTR_TYPE(0, PGconn*);
PQfinish(GETPTR(0, PGconn*));
ALLOC(0);
RETURN; }
FUNC(pq_protocol_version) {
ARGS; DECL;
CHECKNUMARGS(1); CHECKPTR(0); CHECKPTR_TYPE(0, PGconn*);
ALLOC(1);
SETINT(0, PQprotocolVersion(GETPTR(0, const PGconn*)));
RETURN; }
FUNC, ARGS, CHECKNUMARGS and the rest are nasty dlutil.h macros that do =
all the OCaml-C magic as described in the OCaml docs.
Functions all have the same signature, they receive an array of values =
amd can check the number and type of its elements. They return another =
array, like multiple value returns in Lisp. You can get pretty creative =
with this scheme.
This is very early code with plenty of missing stuff, and I haven't used =
it for anything serious yet, but if it might be of use to anyone as-is =
then just ask.
Regards.
-Daniel
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