On Fri, 23 Feb 2007 23:39:03 +0100, Troy Melhase <[EMAIL PROTECTED]> wrote:
>> The first time A starts, it should open a B process and start >> communicating with it. All other times an A instance starts it should >> simply talk with the B that already is open. > > B should write its process id to a location known by both > applications. When A starts, it should read that PID from the file > and attempt to communicate with the process having that PID. > > When B starts, it should also check for the file. If it's found and > if the PID in it is present in the process table, then B should exit. > Otherwise, it should start normally and write its own PID to the file. You have a race condition here: if another instance of B is created between the check and the creation of the file, you'll have two instances running. And remember Murphy's law: yes, it will happen, and quicker than you can imagine ;-) To prevent this from happening, I usually create a special directory for the PID file, and I delete both the file and the directory when the process exits. The advantage is that os.mkdir is basically a "test and set" in a single operation: if two of these are attempted at "the same time", only one can succeed. It is also cross-platform and even works on shared file systems. So the code would be something like: ## Try to create directory try: os.mkdir(os.path.join(common_directory, "B_lock")) except OSError: ## Failed: another instance is running sys.exit() ## Create the PID file # (...) try: ## Application code # (...) finally: ## Remove the PID file # (...) ## Delete directory os.rmdir(os.path.join(common_directory, "B_lock")) HTH -- python -c "print ''.join([chr(154 - ord(c)) for c in 'U(17zX(%,5.zmz5(17l8(%,5.Z*(93-965$l7+-'])" -- http://mail.python.org/mailman/listinfo/python-list
