Version 0.0.5.

[ocaml-ancient.git] / ancient.mli

diff --git a/ancient.mli b/ancient.mli
index 9f574db..191447e 100644 (file)
--- a/ancient.mli
+++ b/ancient.mli
@@ -1,5 +1,5 @@
 (** Mark objects as 'ancient' so they are taken out of the OCaml heap.
-  * $Id: ancient.mli,v 1.1 2006-09-27 12:07:07 rich Exp $
+  * $Id: ancient.mli,v 1.4 2006-09-28 12:40:07 rich Exp $
   *)
 
 type 'a ancient
@@ -29,3 +29,81 @@ val delete : 'a ancient -> unit
     *
     * Forgetting to delete an ancient object results in a memory leak.
     *)
+
+(** {6 Shared memory mappings} *)
+
+type md
+  (** Memory descriptor handle. *)
+
+val attach : Unix.file_descr -> nativeint -> md
+  (** [attach fd baseaddr] attaches to a new or existing file
+    * which may contain shared objects.
+    *
+    * Initially [fd] should be a read/writable, zero-length file
+    * (for example you could create this using {!Unix.openfile} and
+    * passing the flags [O_RDWR], [O_TRUNC], [O_CREAT]).
+    * One or more objects can then be shared in this file
+    * using {!Unix.share}.
+    *
+    * For new files, [baseaddr] specifies the virtual address to
+    * map the file.  Specifying [Nativeint.zero] ([0n]) here lets [mmap(2)]
+    * choose this, but on some platforms (notably Linux/AMD64)
+    * [mmap] chooses very unwisely, tending to map the memory
+    * just before [libc] with hardly any headroom to grow.  If
+    * you encounter this sort of problem (usually a segfault or
+    * illegal instruction inside libc), then look at [/proc/PID/maps]
+    * and choose a more suitable address.
+    *
+    * If the file was created previously, then the [baseaddr] is
+    * ignored.  The underlying [mmalloc] library will map the
+    * file in at the same place as before.
+    *)
+
+val detach : md -> unit
+  (** [detach md] detaches from an existing file, and closes it.
+    *)
+
+val share : md -> int -> 'a -> 'a ancient
+  (** [share md key obj] does the same as {!Ancient.mark} except
+    * that instead of copying the object into local memory, it
+    * writes it into memory which is backed by the attached file.
+    *
+    * Shared mappings created this way may be shared between
+    * other OCaml processes which can access the underlying
+    * file.  See {!Ancient.attach}, {!Ancient.detach}.
+    *
+    * More than one object can be stored in a file.  They are
+    * indexed using integers in the range [0..1023] (the limit
+    * is hard-coded in [mmalloc/mmprivate.h]).  The [key] parameter
+    * controls which object is written/overwritten by [share].
+    * If you do not wish to use this feature, just pass [0]
+    * as the key.
+    *
+    * Do not call {!Ancient.delete} on a mapping created like this.
+    * Instead, call {!Ancient.detach} and, if necessary, delete the
+    * underlying file.
+    *
+    * Caution when sharing files/objects between processes:
+    * The underlying [mmalloc] library does not do any sort of
+    * locking, so all calls to [share] must ensure that they have
+    * exclusive access to the underlying file while in progress.
+    * (Other processes should not even call {!Ancient.get} while
+    * this is happening, but it seems safe to be just reading an
+    * ancient object from the file).
+    *)
+
+val get : md -> int -> 'a ancient
+  (** [get md key] returns the object indexed by [key] in the
+    * attached file.
+    *
+    * The key is in the range [0..1023] (the limit is hard-coded in
+    * [mmalloc/mmprivate.h]).  If you do not wish to use this feature,
+    * just pass [0] as the key when sharing / getting.
+    *
+    * You need to annotate the returned object with the correct
+    * type.  As with the Marshal module, there is no type checking,
+    * and setting the wrong type will likely cause a segfault
+    * or undefined behaviour.
+    *
+    * @raises [Not_found] if no object is associated with the key.
+    *)