[Caml-list] Best way to synchronize OS processes?

[Caml-list] Best way to synchronize OS processes?

[Ranjan Bagchi]

Hi --

I'm writing some code which will end up executing concurrently on 
several OS processes.  I'd like to serialize access to some specific OS 
resources (for instance, writing to a single file).  The Unix module 
doesn't appear to offer anything like a critical section or an OS 
mutex.  Is there a preferred way to do this? 

Thanks,

Ranjan

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Re: [Caml-list] Best way to synchronize OS processes?

[Shawn Wagner]

On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
> Hi --
> 
> I'm writing some code which will end up executing concurrently on 
> several OS processes.  I'd like to serialize access to some specific OS 
> resources (for instance, writing to a single file).  The Unix module 
> doesn't appear to offer anything like a critical section or an OS 
> mutex.  Is there a preferred way to do this?

File locking with Unix.lockf?


-- 
Shawn Wagner
shawnw@speakeasy.org

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Re: [Caml-list] Best way to synchronize OS processes?

[Richard Jones]

On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
> Hi --
> 
> I'm writing some code which will end up executing concurrently on 
> several OS processes.  I'd like to serialize access to some specific OS 
> resources (for instance, writing to a single file).  The Unix module 
> doesn't appear to offer anything like a critical section or an OS 
> mutex.  Is there a preferred way to do this? 

What about Unix.lockf?

Rich.

val lockf : file_descr -> lock_command -> int -> unit
(** [lockf fd cmd size] puts a lock on a region of the file opened
   as [fd]. The region starts at the current read/write position for
   [fd] (as set by {!Unix.lseek}), and extends [size] bytes forward if
   [size] is positive, [size] bytes backwards if [size] is negative,
   or to the end of the file if [size] is zero.
   A write lock (set with [F_LOCK] or [F_TLOCK]) prevents any other
   process from acquiring a read or write lock on the region.
   A read lock (set with [F_RLOCK] or [F_TRLOCK]) prevents any other
   process from acquiring a write lock on the region, but lets
   other processes acquire read locks on it. *)

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
If I have not seen as far as others, it is because I have been
standing in the footprints of giants.  -- from Usenet

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Re: [Caml-list] Best way to synchronize OS processes?

[Ranjan Bagchi]

Cool - I was playing with that call today, although I'm getting 
occasional EDEADLK exceptions thrown though.  Is there an example of 
correct use? 

What I'm doing (code isn't handy right now) is before writing to the 
file, I get a write lock, write my bit, and then unlock the region.   
Reading bits try to get a Read lock, and then unlock when they're done.

I'm using | in_channel_of_descr| and out_channel_of_descr to do the i/o 
-- is this a problem?

Ranjan


Shawn Wagner wrote:

>On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
>  
>
>>Hi --
>>
>>I'm writing some code which will end up executing concurrently on 
>>several OS processes.  I'd like to serialize access to some specific OS 
>>resources (for instance, writing to a single file).  The Unix module 
>>doesn't appear to offer anything like a critical section or an OS 
>>mutex.  Is there a preferred way to do this?
>>    
>>
>
>File locking with Unix.lockf?
>
>
>  
>

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Re: [Caml-list] Best way to synchronize OS processes?

[Jean-Christophe Filliatre]

Ranjan Bagchi writes:
 > Cool - I was playing with that call today, although I'm getting 
 > occasional EDEADLK exceptions thrown though.  Is there an example of 
 > correct use? 

I use it like this (bd_lock is the lock file) :

======================================================================
...

(* START OF CRITICAL SECTION *)
let fd_lock =
  try
  let fd = openfile bd_lock [O_WRONLY; O_CREAT] 0o666 in
  lockf fd F_LOCK 0;
  fd
  with e -> printf "%s" (Printexc.to_string e); exit 0

...CRITICAL SECTION GOES HERE...

let _ = lockf fd_lock F_ULOCK 0; close fd_lock
(* END OF CRITICAL SECTION *)

...
======================================================================

Hope this helps,
-- 
Jean-Christophe Filliâtre (http://www.lri.fr/~filliatr)

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Re: [Caml-list] Best way to synchronize OS processes?

[Alex Baretta]

Shawn Wagner wrote:
> On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
> 
>>Hi --
>>
>>I'm writing some code which will end up executing concurrently on 
>>several OS processes.  I'd like to serialize access to some specific OS 
>>resources (for instance, writing to a single file).  The Unix module 
>>doesn't appear to offer anything like a critical section or an OS 
>>mutex.  Is there a preferred way to do this?
> 
> 
> File locking with Unix.lockf?

I disagree. I faced the same problem and found that it cannot be easily 
solved with lockf. Unix.lockf locks specific regions in a file, whereas 
Ranjan seems to need the equivalent of a mutex, which should be 
implemented as a lock over the entire file. This is done by the flock 
primitive in Posix systems. Unluckily, flock has not been included in 
the Unix module.

I have written my own Flock module, which might be useful to Rajan. It 
supports locking Pervasive channels as well as locking the underlying 
file descriptors. I have sent it to Caml team as a contribution to the 
Unix module, but apparently it was not accepted.

I'm willing to send it to anyone requesting it. Note that it is released 
under the GPL--not the LGPL--so licensing issues might arise.

Alex

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Re: [Caml-list] Best way to synchronize OS processes?

[Ville-Pertti Keinonen]

On May 12, 2004, at 2:27 PM, Alex Baretta wrote:

> I disagree. I faced the same problem and found that it cannot be 
> easily solved with lockf. Unix.lockf locks specific regions in a file, 
> whereas Ranjan seems to need the equivalent of a mutex, which should 
> be implemented as a lock over the entire file. This is done by the 
> flock primitive in Posix systems. Unluckily, flock has not been 
> included in the Unix module.

Actually flock is non-POSIX (it's BSD) and is missing from some 
reasonably modern systems.

It's entirely possible to lock the entire file using the fcntl used by 
Unix.lockf, making it equivalent to flock.  The problem is that the 
OCaml Unix module doesn't allow the l_whence parameter to be set, which 
is a serious omission.

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Re: [Caml-list] Best way to synchronize OS processes?

[Gerd Stolpmann]

Am Mit, 2004-05-12 um 13.27 schrieb Alex Baretta:
> Shawn Wagner wrote:
> > On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
> > 
> >>Hi --
> >>
> >>I'm writing some code which will end up executing concurrently on 
> >>several OS processes.  I'd like to serialize access to some specific OS 
> >>resources (for instance, writing to a single file).  The Unix module 
> >>doesn't appear to offer anything like a critical section or an OS 
> >>mutex.  Is there a preferred way to do this?
> > 
> > 
> > File locking with Unix.lockf?
> 
> I disagree. I faced the same problem and found that it cannot be easily 
> solved with lockf. Unix.lockf locks specific regions in a file, whereas 
> Ranjan seems to need the equivalent of a mutex, which should be 
> implemented as a lock over the entire file. This is done by the flock 
> primitive in Posix systems. Unluckily, flock has not been included in 
> the Unix module.

And this is good. flock is not as portable as lockf. flock usually does
not work over NFS, whereas lockf works. Normally, you need flock only to
synchronize with third-party BSD programs such as sendmail.

lockf does not lock regions of a file. It just locks abstract
byteranges, whether you interpret them as file regions is up to you.
That means, you can lock byteranges that do not exist in the file. If
you like, you can view an empty lockf-locked file as 2^32 potential
mutexes (or whatever the maximum file size is).

Gerd 
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
------------------------------------------------------------

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Re: [Caml-list] Best way to synchronize OS processes?

[Olivier Andrieu]

 Ville-Pertti Keinonen [Wed, 12 May 2004]:
 > 
 > On May 12, 2004, at 2:27 PM, Alex Baretta wrote:
 > 
 > > I disagree. I faced the same problem and found that it cannot be 
 > > easily solved with lockf. Unix.lockf locks specific regions in a file, 
 > > whereas Ranjan seems to need the equivalent of a mutex, which should 
 > > be implemented as a lock over the entire file. This is done by the 
 > > flock primitive in Posix systems. Unluckily, flock has not been 
 > > included in the Unix module.
 > 
 > Actually flock is non-POSIX (it's BSD) and is missing from some 
 > reasonably modern systems.
 > 
 > It's entirely possible to lock the entire file using the fcntl used by 
 > Unix.lockf, making it equivalent to flock.  

They are not *exactly* equivalent: flock does not work on NFS
filesystems whereas fnctl does, and they have different behaviour with
exec or dup I believe.

 > The problem is that the OCaml Unix module doesn't allow the
 > l_whence parameter to be set, which is a serious omission.

Well, it follows the lockf API, which starts the lock at the current
file position. You could use lseek to reposition it but this may confuse
a Pervasives channel buffering this file.

By the way, lockf can block, waiting for a lock. Shouldn't there be
some enter_blocking_section()/leave_blocking_section() somewhere ?

-- 
   Olivier

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Re: [Caml-list] Best way to synchronize OS processes?

[Ranjan Bagchi]

Incidentally -- I found my problem -- basic Unix system programming, I 
needed to seek back to the start of the area before attempting to unlock 
it.  D'oh!

Ranjan

Ranjan Bagchi wrote:

> Cool - I was playing with that call today, although I'm getting 
> occasional EDEADLK exceptions thrown though.  Is there an example of 
> correct use?
> What I'm doing (code isn't handy right now) is before writing to the 
> file, I get a write lock, write my bit, and then unlock the region.   
> Reading bits try to get a Read lock, and then unlock when they're done.
>
> I'm using | in_channel_of_descr| and out_channel_of_descr to do the 
> i/o -- is this a problem?
>
> Ranjan
>
>
> Shawn Wagner wrote:
>
>> On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
>>  
>>
>>> Hi --
>>>
>>> I'm writing some code which will end up executing concurrently on 
>>> several OS processes.  I'd like to serialize access to some specific 
>>> OS resources (for instance, writing to a single file).  The Unix 
>>> module doesn't appear to offer anything like a critical section or 
>>> an OS mutex.  Is there a preferred way to do this?
>>>   
>>
>>
>> File locking with Unix.lockf?
>>
>>
>>  
>>
>
>

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Re: [Caml-list] Best way to synchronize OS processes?

[Shawn Wagner]

On Tue, May 11, 2004 at 11:05:23AM -0700, Ranjan Bagchi wrote:
> Hi --
> 
> I'm writing some code which will end up executing concurrently on 
> several OS processes.  I'd like to serialize access to some specific OS 
> resources (for instance, writing to a single file).  The Unix module 
> doesn't appear to offer anything like a critical section or an OS 
> mutex.  Is there a preferred way to do this? 
> 

A related note that I thought of last night: How about when you need
something more complicated than a simple mutex? A quick look at the hump
didn't find a library for working with SysV semaphores in ocaml. Would that
be useful for anyone?

-- 
Shawn Wagner
shawnw@speakeasy.org

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