Thread local storage (TLC)

With Pthreads, TLS is accomplished using keys and methods to set thread-specific data:

pthread_key_t global_var_key;
    
void* worker(void* arg) {
    int *p = new int;
    *p = 1;
    pthread_setspecific(global_var_key, p);
    int* global_spec_var = (int*) pthread_getspecific(global_var_key);
    *global_spec_var += 1;
    pthread_setspecific(global_var_key, 0);
    delete p;
    pthread_exit(0);
}

In the worker thread, we allocate a new integer on the heap, and set the global key to its own value. After increasing the global variable by 1, its value will be 2, regardless of what the other threads do. We can set the global variable to 0 once we're done with it for this thread, and delete the allocated value:

int main(void) {
    pthread_t threads[5];
    
    pthread_key_create(&global_var_key, 0);
    for (int i = 0; i < 5; ++i)
        pthread_create(&threads[i],0,worker,0);
    for (int i = 0; i < 5; ++i) {
        pthread_join(threads[i], 0);
    }
    return 0;
}

A global key is set and used to reference the TLS variable, yet each of the threads we create can set its own value for this key.

While a thread can create its own keys, this method of handling TLS is fairly involved compared to the other APIs we're looking at in this chapter.