interprocess mutex

interprocess mutex

[Janne Hellsten]

Hello,

I need to sequentialize two processes accessing the same GDBM database 
(OCaml's dbm library).  Since GDBM allows for only one writer at a time, 
I need a way to block until the previous writer has finished.  I think 
there is a way to do this with the original GDBM library since it claims 
to handle the file lockings properly -- I could block based on the 
returned error codes.  However, this functionality does not appear to be 
exposed through the Dbm module.

Of course I can implement this blocking myself with an interprocess 
mutex.  But how can I implement such a mutex in OCaml?  I couldn't find 
anything from the standard library that would resemble my problem.

Is it easily doable or do I need to go and start hacking C code?  I've 
never done it on Unix but I know it's easily done on Windows.

Thanks in advance,
Janne

P.S.  I'm using GDBM merely as a cache between process invocations, so 
using a more heavyweight DB is out of the question.

Re: [Caml-list] interprocess mutex

[Alex Baretta]

Janne Hellsten wrote:
> Hello,
> 
> I need to sequentialize two processes accessing the same GDBM database 
> (OCaml's dbm library).  Since GDBM allows for only one writer at a time, 
> I need a way to block until the previous writer has finished.  I think 
> there is a way to do this with the original GDBM library since it claims 
> to handle the file lockings properly -- I could block based on the 
> returned error codes.  However, this functionality does not appear to be 
> exposed through the Dbm module.
> 
> Of course I can implement this blocking myself with an interprocess 
> mutex.  But how can I implement such a mutex in OCaml?  I couldn't find 
> anything from the standard library that would resemble my problem.

Unix.lockf is the way to go.

Alex

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Re: [Caml-list] interprocess mutex

[Janne Hellsten]

OK, this is what I came up with (uses ExtLib's Std.finally):

---
let create_lock name =
  let chnl = open_out name in
  output_string chnl "Lockfile - don't delete";
  close_out chnl

let with_write_block name f =
  if not (Sys.file_exists name) then
    create_lock name;
  let fd = Unix.openfile name [Unix.O_RDWR] 0o660 in
  Std.finally
    (fun () -> Unix.close fd)
    (fun _ ->
       Unix.lockf fd Unix.F_LOCK 0;
       f ()) ()

let test =
  with_write_block "lock_name" (fun () -> Printf.printf "this code 
blocks if other process already acquired lock!")
---

I hope I'm using lockf correctly.

Thanks for the help!

Best regards,
Janne

Alex Baretta wrote:

> Janne Hellsten wrote:
>
>> Hello,
>>
>> I need to sequentialize two processes accessing the same GDBM 
>> database (OCaml's dbm library).  Since GDBM allows for only one 
>> writer at a time, I need a way to block until the previous writer has 
>> finished.  I think there is a way to do this with the original GDBM 
>> library since it claims to handle the file lockings properly -- I 
>> could block based on the returned error codes.  However, this 
>> functionality does not appear to be exposed through the Dbm module.
>>
>> Of course I can implement this blocking myself with an interprocess 
>> mutex.  But how can I implement such a mutex in OCaml?  I couldn't 
>> find anything from the standard library that would resemble my problem.
>
>
> Unix.lockf is the way to go.
>
> Alex
>

Re: [Caml-list] interprocess mutex

[Frédéric Gava]

Hi,

you could also used threads. It is easier.
http://caml.inria.fr/ocaml/htmlman/manual038.html

After you could used "Mutex", "Condition" and "Event" to synchronise your
threads...

To synnchronise true "process", you could used "waitpid".

Regards,
Frédéric Gava



> Hello,
>
> I need to sequentialize two processes accessing the same GDBM database
> (OCaml's dbm library).  Since GDBM allows for only one writer at a time,
> I need a way to block until the previous writer has finished.  I think
> there is a way to do this with the original GDBM library since it claims
> to handle the file lockings properly -- I could block based on the
> returned error codes.  However, this functionality does not appear to be
> exposed through the Dbm module.
>
> Of course I can implement this blocking myself with an interprocess
> mutex.  But how can I implement such a mutex in OCaml?  I couldn't find
> anything from the standard library that would resemble my problem.
>
> Is it easily doable or do I need to go and start hacking C code?  I've
> never done it on Unix but I know it's easily done on Windows.
>
> Thanks in advance,
> Janne
>
> P.S.  I'm using GDBM merely as a cache between process invocations, so
> using a more heavyweight DB is out of the question.
>
>
>
>
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