Semaphore Techniques

[Philip Semanchuk]

On Jul 28, 2009, at 7:19 PM, Christian Heimes wrote:

> John D Giotta schrieb:
>> I'm looking to run a process with a limit of 3 instances, but each
>> execution is over a crontab interval. I've been investigating the
>> threading module and using daemons to limit active thread objects,  
>> but
>> I'm not very successful at grasping the documentation.
>>
>> Is it possible to do what I'm trying to do and if so anyone know of a
>> useful example to get started?
>
> Since you are talking about crontab I assume that you are on an os  
> that
> supports pthreads. You problem can easily be solved with a named
> semaphore (see sem_open(3) and sem_overview(7)). Unfortunately Python
> doesn't expose named semaphores. The multiprocessing library uses  
> named
> semaphores but you can't set the name yourself.
>
> You have to write your own C wrapper or search on pypi and through
> Google. If you are going to write your own semaphore I highly  
> recommend
> Cython.

My POSIX IPC extension permits manipulation of interprocess semaphores:
http://semanchuk.com/philip/posix_ipc/

There's also one for SysV IPC:
http://semanchuk.com/philip/sysv_ipc/

Enjoy
P