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
>