Книга: Programming with POSIX® Threads

5.4.2 Using thread-specific data

5.4.2 Using thread-specific data

int pthread_setspecific (pthread_key_t key, const void *value);

void *pthread_getspecific (pthread_key_t key);

You can use the pthread_getspecific function to determine the thread's current value for a key, or pthread_setspecific to change the current value. Take a look at Section 7.3.1 for ideas on using thread-specific data to adapt old libraries that rely on static data to be thread-safe.

A thread-specific data value of NULL means something special to Pthreads—do not set a thread-specific data value of NULL unless you really mean it.

The initial value for any new key (in all threads) is NULL. Also, Pthreads sets the thread-specific data value for a key to NULL before calling that key's destructor (passing the previous value of the key) when a thread terminates[4]. If your thread-specific data value is the address of heap storage, for example, and you want to free that storage in your destructor, you must use the argument passed to the destructor rather than calling pthread_getspecific.

Pthreads will not call the destructor for a thread-specific data key if the terminating thread has a value of NULL for that key. NULL is special, meaning "this key has no value." If you ever use pthread_setspecific to set the value ofa thread-specific data key to NULL, you need to remember that you are not setting the value NULL, but rather stating that the key no longer has a value in the current thread.

Destructor functions are called only when the thread terminates, not when the value of a thread-specific data key is changed.

Another important thing to remember is that thread-specific data key destructor functions are not called when you replace an existing value for that key. That is, if you allocate a structure in heap and assign a pointer to that structure as the value of a thread-specific data key, and then later allocate a new structure and assign a pointer to that new structure to the same thread-speciflc data key, in the same thread, you are responsible for freeing the old structure. Pthreads will not free the old structure, nor will it call your destructor function with a pointer to the old structure.

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