al_join_thread - Allegro 5 API

       Wait for the thread to finish executing.  This implicitly calls al_set_thread_should_stop(3alleg5) first.

       If  ret_value  is  non-NULL,  the  value  returned  by the thread function will be stored at the location
       pointed to by ret_value.

       al_join_thread - Allegro 5 API

al_set_thread_should_stop(3alleg5), al_get_thread_should_stop(3alleg5), al_destroy_thread(3alleg5).

Allegro reference manual                                                                 al_join_thread(3alleg5)

              #include <allegro5/allegro.h>

              void al_join_thread(ALLEGRO_THREAD *thread, void **ret_value)