/* * tsd.c * * Description: * POSIX thread functions which implement thread-specific data (TSD). */ /* * Why we can't use Win32 TLS * -------------------------- * * In a word: Destructors * * POSIX 1003.1 1996, Section 17 allows for optional destructor functions * to be associated with each key value. * * This is my (revised) understanding of how destructors work: * * A key is created by a single thread, which then provides in every * existing thread a TSD matching the same key, but initialised * to NULL. Each new thread will also get a matching key with value NULL. * The creating thread can optionally associate a function, called a * destructor, with the key. * * When each thread exits, it calls the destructor function, which * will then perform an action on that threads key value * only. (Previously I thought that only the key creating thread ran * the destructor on the key in all threads. That proposition is * sounding scarier by the minute.) * * SOME APPROACHES TO MANAGING TSD MEMORY * * We could simply allocate enough memory on process startup to hold * all possible data for all possible threads. * * We could allocate memory for just a table to hold a single pointer * for each of POSIX_THREAD_KEYS_MAX keys. pthread_key_create() could then * allocate space for POSIX_THREADS_MAX key values in one hit and store * the location of the array in the first table. * * The standard also suggests that each thread might store key/value pairs * on its private stack. This seems like a good idea. I had concerns about * memory leaks and key re-use if a key was deleted, but the standard talks * at length on this and basically says it's up to the application to * make sure everything goes smoothly here, making sure that proper cleanup * is done before a key is deleted. (section B.17.1.3 in particular) * * One more thing to note: destructors must never be called on deleted keys. */ #include #include "pthread.h" #include "implement.h" int pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) { pthread_key_t k; int ret = 0; /* CRITICAL SECTION */ pthread_mutex_lock(&_pthread_tsd_mutex); if (_pthread_tsd_key_next >= PTHREAD_KEYS_MAX) ret = EAGAIN; k = _pthread_tsd_key_next++; _pthread_tsd_key_table[k].in_use = _PTHREAD_TSD_KEY_INUSE; _pthread_tsd_key_table[k].destructor = destructor; pthread_mutex_unlock(&_pthread_tsd_mutex); /* END CRITICAL SECTION */ *key = k; return ret; } int pthread_setspecific(pthread_key_t key, void *value) { void ** keys; int inuse; /* CRITICAL SECTION */ pthread_mutex_lock(&_pthread_tsd_mutex); inuse = (_pthread_tsd_key_table[key].in_use == _PTHREAD_TSD_KEY_INUSE); pthread_mutex_unlock(&_pthread_tsd_mutex); /* END CRITICAL SECTION */ if (! inuse) return EINVAL; keys = (void **) TlsGetValue(_pthread_TSD_keys_TlsIndex); keys[key] = value; return 0; } void * pthread_getspecific(pthread_key_t key) { void ** keys; int inuse; /* CRITICAL SECTION */ pthread_mutex_lock(&_pthread_tsd_mutex); inuse = (_pthread_tsd_key_table[key].in_use == _PTHREAD_TSD_KEY_INUSE); pthread_mutex_unlock(&_pthread_tsd_mutex); /* END CRITICAL SECTION */ if (! inuse) return (void *) NULL; keys = (void **) TlsGetValue(_pthread_TSD_keys_TlsIndex); return keys[key]; } /* pthread_key_delete: ANSI/IEEE Std 1003.1, 1996 Edition Section 17.1.3.2 This function deletes a thread-specific data key previously returned by pthread_key_create(). The thread specific data values associated with "key" need not be NULL at the time pthread_key_delete() is called. It is the responsibility of the application to free any application storage or perform any cleanup actions for data structures related to the deleted key or associated thread-specific data in any threads; this cleanup can be done either before or after pthread_key_delete() is called. Any attempt to use "key" following the call to pthread_key_delete() results in undefined behaviour. The pthread_key_delete() function shall be callable from within destructor functions. No destructor functions shall be invoked by pthread_key_delete(). Any destructor function that may have been associated with "key" shall no longer be called upon thread exit. */ int pthread_key_delete(pthread_key_t key) { int ret = 0; /* CRITICAL SECTION */ pthread_mutex_lock(&_pthread_tsd_mutex); if (_pthread_tsd_key_table[key].in_use != _PTHREAD_TSD_KEY_INUSE) { ret = EINVAL; } else { _pthread_tsd_key_table[key].in_use = _PTHREAD_TSD_KEY_DELETED; _pthread_tsd_key_table[key].destructor = NULL; } pthread_mutex_unlock(&_pthread_tsd_mutex); /* END CRITICAL SECTION */ return ret; }