pthread_key_t vs local variable

Question

I am developing a multithreaded program in C++ with Pthread where I need to allocate local memory in each thread. After googling I found pthread_key_t type which is some kind of map which allow you to allocate memory in the TLS.

So my question is what is the difference between a local variable in the thread function and a pthread_key_t? And can you give a correct usage example of pthread_key_t?

Answer 1

The difference between thread-local storage and local variables are that thread-local storage does not need to be local to a function. Regular local variables may no longer be accessed as soon as the function that declares them returns. Thread-specific storage accessed with pthread_getspecific and pthread_setspecific may be safely used after the function that created it exits.

You can think of thread-local variables as thread-specific global variables, shared among functions running in the same thread. They are often used to make legacy code that makes heavy use of global variables multithreading-safe. As with all global variables, they should be used with care — if you can get away with only using local variables, by all means do so. If you do need global variables within a thread, use thread-local storage.

Answer 2

Since each thread has its own stack, it is much better if each thread allocates its own variables, unless you need to share data between threads. The pthread_key_* is used for data sharing.

Documentation and small examples at http://pubs.opengroup.org/onlinepubs/007904975/functions/pthread_key_create.html.