FAQ-1089 Process local storage : How can I use TLS to implement writable global data which is shared between threads in the same process?


FAQ-1089 Process local storage : How can I use TLS to implement writable global data which is shared between threads in the same process?

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Classification:	C++	Category:	Development
Created:	07/15/2004	Modified:	03/01/2005
Number:	FAQ-1089
Platform:	ER5, Symbian OS v6.0, Symbian OS v6.1, Symbian OS v7.0, Symbian OS v7.0s, Symbian OS v8.0, Symbian OS v8.0a

Question:
TLS(thread local storage) allows you to associate a pointer to some global data G which will be returned each time Dll::Tls() is called.
The pointer returned is stored on a per dll, and per thread basis.
If your Dll code runs in main thread A and calls SetTls(G), then future calls to Tls() will return a pointer to G.
If another worker thread is spawned, and calls Tls() - it will return NULL since no data has been associated with this thread.

Answer:
To make G a unique instance for all threads created by the DLL, a scheme such as the following can be used:

Master thread creates a global object pointer G, and calls SetTls(G):
Each time the master thread creates a worker thread, it passes G into the thread creation function.
At the start of the worker thread function, the worker calls SetTls(G);

Each time the global data needs to be accessed from the DLL, Tls()->G->iThing is referenced. Perhaps with a macro. GLOBAL->iThing.