This file describes in little detail the modifications to the Objective-C runtime needed to make it thread safe. First off, kudos to Galen Hunt who is the author of this great work. If you have an comments or just want to know where to send me money to express your undying gratitude for threading the Objective-C runtime you can reach Galen at: gchunt@cs.rochester.edu Any questions, comments, bug reports, etc. should send email either to the GCC bug account or to: Scott Christley * Sarray Threading: The most critical component of the Objective-C runtime is the sparse array structure (sarray). Sarrays store object selectors and implementations. Following in the tradition of the Objective-C runtime, my threading support assumes that fast message dispatching is far more important than *ANY* and *ALL* other operations. The message dispatching thus uses *NO* locks on any kind. In fact, if you look in sarray.h, you will notice that the message dispatching has not been modified. Instead, I have modified the sarray management functions so that all updates to the sarray data structure can be made in parallel will message dispatching. To support concurrent message dispatching, no dynamically allocated sarray data structures are freed while more than one thread is operational. Sarray data structures that are no longer in use are kept in a linked list of garbage and are released whenever the program is operating with a single thread. The programmer can also flush the garbage list by calling sarray_remove_garbage when the programmer can ensure that no message dispatching is taking place concurrently. The amount of un-reclaimed sarray garbage should normally be extremely small in a real program as sarray structures are freed only when using the "poseAs" functionality and early in program initialization, which normally occurs while the program is single threaded. ****************************************************************************** * Static Variables: The following variables are either statically or globally defined. This list does not include variables which are internal to implementation dependent versions of thread-*.c. The following threading designations are used: SAFE : Implicitly thread safe. SINGLE : Must only be used in single thread mode. MUTEX : Protected by single global mutex objc_runtime_mutex. UNUSED : Not used in the runtime. Variable Name: Usage: Defined: Also used in: =========================== ====== ============ ===================== __objc_class_hash MUTEX class.c __objc_class_links_resolved UNUSED class.c runtime.h __objc_class_number MUTEX class.c __objc_dangling_categories UNUSED init.c __objc_module_list MUTEX init.c __objc_selector_array MUTEX selector.c __objc_selector_hash MUTEX selector.c __objc_selector_max_index MUTEX selector.c sendmsg.c runtime.h __objc_selector_names MUTEX selector.c __objc_thread_exit_status SAFE thread.c __objc_uninstalled_dtable MUTEX sendmsg.c selector.c _objc_load_callback SAFE init.c objc-api.h _objc_lookup_class SAFE class.c objc-api.h _objc_object_alloc SINGLE objects.c objc-api.h _objc_object_copy SINGLE objects.c objc-api.h _objc_object_dispose SINGLE objects.c objc-api.h frwd_sel SAFE2 sendmsg.c idxsize MUTEX sarray.c sendmsg.c sarray.h initialize_sel SAFE2 sendmsg.c narrays MUTEX sarray.c sendmsg.c sarray.h nbuckets MUTEX sarray.c sendmsg.c sarray.h nindices MUTEX sarray.c sarray.h previous_constructors SAFE1 init.c proto_class SAFE1 init.c unclaimed_categories MUTEX init.c unclaimed_proto_list MUTEX init.c uninitialized_statics MUTEX init.c Notes: 1) Initialized once in unithread mode. 2) Initialized value will always be same, guaranteed by lock on selector hash table. ****************************************************************************** * Frontend/Backend design: The design of the Objective-C runtime thread and mutex functions utilizes a frontend/backend implementation. The frontend, as characterized by the files thr.h and thr.c, is a set of platform independent structures and functions which represent the user interface. Objective-C programs should use these structures and functions for their thread and mutex work if they wish to maintain a high degree of portability across platforms. The backend is composed of a file with the necessary code to map the ObjC thread and mutex to a platform specific implementation. For example, the file thr-solaris.c contains the implementation for Solaris. If you are compiling libobjc as part of GCC, the thr-objc.c backend is always used; this backend uses GCC's gthread code. The thread system is automatically configured when GCC is configured. Important: make sure you configure GCC using `--enable-threads' if you want threads ! If you want to compile libobjc standalone, then you would need to modify the configure.in and makefiles for it; and you need to pick an appropriate backend file for the target platform; you make this choice by assigning the OBJC_THREAD_FILE make variable to the basename of the backend file. For example, OBJC_THREAD_FILE=thr-posix would indicate that the generic posix backend file, thr-posix.c, should be compiled with the ObjC runtime library. If your platform does not support threads then you should specify the OBJC_THREAD_FILE=thr-single backend file to compile the ObjC runtime library without thread or mutex support; note that programs which rely upon the ObjC thread and mutex functions will compile and link correctly but attempting to create a thread or mutex will result in an error. It is questionable whether it is really necessary to have both a frontend and backend function for all available functionality. On the one hand, it provides a clear, consistent differentiation between what is public and what is private with the downside of having the overhead of multiple functions calls. For example, the function to have a thread yield the processor is objc_thread_yield; in the current implementation this produces a function call set: objc_thread_yield() -> __objc_thread_yield() -> system yield function This has two extra function calls over calling the platform specific function explicitly, but the issue is whether only the overhead of a single function is necessary. objc_thread_yield() -> system yield function This breaks the public/private dichotomy between the frontend/backend for the sake of efficiency. It is possible to just use a preprocessor define so as to eliminate the extra function call: #define objc_thread_yield() __objc_thread_yield() This has the undesirable effect that if objc_thread_yield is actually turned into a function based upon future need; then ObjC programs which access the thread functions would need to be recompiled versus just being relinked. ****************************************************************************** * Threads: The thread system attempts to create multiple threads using whatever operating system or library thread support is available. It does assume that all system functions are thread safe. Notably this means that the system implementation of malloc and free must be thread safe. If a system has multiple processors, the threads are configured for full parallel processing. * Backend initialization functions __objc_init_thread_system(void), int Initialize the thread subsystem. Called once by __objc_exec_class. Return -1 if error otherwise return 0. __objc_close_thread_system(void), int Closes the thread subsystem, not currently guaranteed to be called. Return -1 if error otherwise return 0. ***** * Frontend thread functions * User programs should use these functions. objc_thread_detach(SEL selector, id object, id argument), objc_thread_t Creates and detaches a new thread. The new thread starts by sending the given selector with a single argument to the given object. objc_thread_set_priority(int priority), int Sets a thread's relative priority within the program. Valid options are: OBJC_THREAD_INTERACTIVE_PRIORITY OBJC_THREAD_BACKGROUND_PRIORITY OBJC_THREAD_LOW_PRIORITY objc_thread_get_priority(void), int Query a thread's priority. objc_thread_yield(void), void Yields processor to another thread with equal or higher priority. It is up to the system scheduler to determine if the processor is taken or not. objc_thread_exit(void), int Terminates a thread. If this is the last thread executing then the program will terminate. objc_thread_id(void), int Returns the current thread's id. objc_thread_set_data(void *value), int Set a pointer to the thread's local storage. Local storage is thread specific. objc_thread_get_data(void), void * Returns the pointer to the thread's local storage. ***** * Backend thread functions * User programs should *NOT* directly call these functions. __objc_thread_detach(void (*func)(void *arg), void *arg), objc_thread_t Spawns a new thread executing func, called by objc_thread_detach. Return NULL if error otherwise return thread id. __objc_thread_set_priority(int priority), int Set the thread's priority, called by objc_thread_set_priority. Return -1 if error otherwise return 0. __objc_thread_get_priority(void), int Query a thread's priority, called by objc_thread_get_priority. Return -1 if error otherwise return the priority. __objc_thread_yield(void), void Yields the processor, called by objc_thread_yield. __objc_thread_exit(void), int Terminates the thread, called by objc_thread_exit. Return -1 if error otherwise function does not return. __objc_thread_id(void), objc_thread_t Returns the current thread's id, called by objc_thread_id. Return -1 if error otherwise return thread id. __objc_thread_set_data(void *value), int Set pointer for thread local storage, called by objc_thread_set_data. Returns -1 if error otherwise return 0. __objc_thread_get_data(void), void * Returns the pointer to the thread's local storage. Returns NULL if error, called by objc_thread_get_data. ****************************************************************************** * Mutexes: Mutexes can be locked recursively. Each locked mutex remembers its owner (by thread id) and how many times it has been locked. The last unlock on a mutex removes the system lock and allows other threads to access the mutex. ***** * Frontend mutex functions * User programs should use these functions. objc_mutex_allocate(void), objc_mutex_t Allocates a new mutex. Mutex is initially unlocked. Return NULL if error otherwise return mutex pointer. objc_mutex_deallocate(objc_mutex_t mutex), int Free a mutex. Before freeing the mutex, makes sure that no one else is using it. Return -1 if error otherwise return 0. objc_mutex_lock(objc_mutex_t mutex), int Locks a mutex. As mentioned earlier, the same thread may call this routine repeatedly. Return -1 if error otherwise return 0. objc_mutex_trylock(objc_mutex_t mutex), int Attempts to lock a mutex. If lock on mutex can be acquired then function operates exactly as objc_mutex_lock. Return -1 if failed to acquire lock otherwise return 0. objc_mutex_unlock(objc_mutex_t mutex), int Unlocks the mutex by one level. Other threads may not acquire the mutex until this thread has released all locks on it. Return -1 if error otherwise return 0. ***** * Backend mutex functions * User programs should *NOT* directly call these functions. __objc_mutex_allocate(objc_mutex_t mutex), int Allocates a new mutex, called by objc_mutex_allocate. Return -1 if error otherwise return 0. __objc_mutex_deallocate(objc_mutex_t mutex), int Free a mutex, called by objc_mutex_deallocate. Return -1 if error otherwise return 0. __objc_mutex_lock(objc_mutex_t mutex), int Locks a mutex, called by objc_mutex_lock. Return -1 if error otherwise return 0. __objc_mutex_trylock(objc_mutex_t mutex), int Attempts to lock a mutex, called by objc_mutex_trylock. Return -1 if failed to acquire lock or error otherwise return 0. __objc_mutex_unlock(objc_mutex_t mutex), int Unlocks the mutex, called by objc_mutex_unlock. Return -1 if error otherwise return 0. ****************************************************************************** * Condition Mutexes: Mutexes can be locked recursively. Each locked mutex remembers its owner (by thread id) and how many times it has been locked. The last unlock on a mutex removes the system lock and allows other threads to access the mutex. * * Frontend condition mutex functions * User programs should use these functions. * objc_condition_allocate(void), objc_condition_t Allocate a condition mutex. Return NULL if error otherwise return condition pointer. objc_condition_deallocate(objc_condition_t condition), int Deallocate a condition. Note that this includes an implicit condition_broadcast to insure that waiting threads have the opportunity to wake. It is legal to dealloc a condition only if no other thread is/will be using it. Does NOT check for other threads waiting but just wakes them up. Return -1 if error otherwise return 0. objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int Wait on the condition unlocking the mutex until objc_condition_signal() or objc_condition_broadcast() are called for the same condition. The given mutex *must* have the depth 1 so that it can be unlocked here, for someone else can lock it and signal/broadcast the condition. The mutex is used to lock access to the shared data that make up the "condition" predicate. Return -1 if error otherwise return 0. objc_condition_broadcast(objc_condition_t condition), int Wake up all threads waiting on this condition. It is recommended that the called would lock the same mutex as the threads in objc_condition_wait before changing the "condition predicate" and make this call and unlock it right away after this call. Return -1 if error otherwise return 0. objc_condition_signal(objc_condition_t condition), int Wake up one thread waiting on this condition. Return -1 if error otherwise return 0. * * Backend condition mutex functions * User programs should *NOT* directly call these functions. * __objc_condition_allocate(objc_condition_t condition), int Allocate a condition mutex, called by objc_condition_allocate. Return -1 if error otherwise return 0. __objc_condition_deallocate(objc_condition_t condition), int Deallocate a condition, called by objc_condition_deallocate. Return -1 if error otherwise return 0. __objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int Wait on the condition, called by objc_condition_wait. Return -1 if error otherwise return 0 when condition is met. __objc_condition_broadcast(objc_condition_t condition), int Wake up all threads waiting on this condition. Called by objc_condition_broadcast. Return -1 if error otherwise return 0. __objc_condition_signal(objc_condition_t condition), int Wake up one thread waiting on this condition. Called by objc_condition_signal. Return -1 if error otherwise return 0.