Re: Preventing Multiple instance of GTK application

[Liam R E Quin <liam holoweb net>]

On Sat, 2008-04-12 at 15:16 +0100, Carlos Pereira wrote:

> You can write a .lock file everytime you start and then remove it when 
> you close. To know if other instance is running, you just have to check 
> if your .lock file exists.

Please don't do this.

If you must, take into account
(1) multiple instances may create the file -- especially if it is
    on a network file system.  You can use O_CREAT|E_EXCL but it
    is not guaranteed to work over a network and neither in
    practice.

(2) after a system crash, or if your application crashes, the lock
    file will not be removed.
    A way round this is to make the lock file by a symbolic link
    to a non-existant file with your process ID in it, and then on
    startup to check if that process name exists use kill(the_proc, 0),
    but this will fail on a network drive if the program is running on
    another machine.
    A way round this is to include your machine's IP address in the
    link, but this fails on a laptop (say) that gets a new IP address
    each time it starts and which might get shut down quickly.

(3) if your application was run as root, or as another user, you might
    not be able to delete the lock file.

(4) the above 3 items don't have portable solutions between operating
systems.

It is much better to use dbus, or an X Windows property..

and even better to use a library that packages all this stuff,
as was suggested by others.

If you do end up making a lock file, consider making it a directory,
as there are generally fewer locking problems.

best,

Liam

-- 
Liam Quin - XML Activity Lead, W3C, http://www.w3.org/People/Quin/
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