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Re: malloc/free problems within a thread

On Jul 29, 2006, at 6:12 PM, Ken Tozier wrote: 
I'm scanning a large folder hierarchy on a server that takes quite
awhile to execute (avg 12 minutes) and I want the machine to be
usable while this is going on in the background. I'm adding and
removing objects to a mutable array allocated in the init method
from the thread.

It seems to me that the correct way to approach this problem is to spawn a *single* thread that scans the folder hierarchy in question, not one every 5 seconds. So long as the scanning thread is the only one that manipulates your mutable array (until the scan process is done), there's no need to synchronize its access by means of locks. When the scan is done, the thread then should communicate to the application (more precisely, to the application's main thread) that the array is now built and ready for consumption, and then proceed to exit gracefully. Meanwhile, the application's main thread can go on doing its own business.

If you want, you can have the scanning thread set the value of a "progress variable" (a counter of some sort, a percentage, or something along those lines) which is then periodically (think timer!) read by the application's main thread and displayed in the application's user interface, thereby providing the user with some feedback on the scanning progress. Since that progress variable *is* being accessed by more than one thread, its access does need to be synchronized, which you accomplish by bracketing its setting (in the scanning thread) and its reading (in the main thread's timer code) with a lock/unlock pair.

This way, you have at most 2 threads running at any time while the application is active. A point to keep in mind is who is responsible for allocating and releasing the mutable array. The sensible thing to do is to have the application main thread allocate it, pass it to the scanning thread at the time the thread is created, and forget about it until the scanning is done, at which point the main application thread (which should again be the only thread running) can safely use the array and dispose of it after being done with it.

Wagner
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