--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libobjc/THREADS	Fri Oct 27 22:46:09 2017 +0900
@@ -0,0 +1,339 @@
+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 <scottc@net-community.com>
+
+* 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.  For example, objc_mutex_lock().  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 currently GCC's gthread code (gthr.h and related).  For
+example, __gthread_objc_mutex_lock().  The thread system is
+automatically configured when GCC is configured.  On most platforms
+this thread backend is able to automatically switch to non-multi-threaded
+mode if the threading library is not linked in.
+
+If you want to compile libobjc standalone, then you would need to modify
+the configure.ac and makefiles for it and you need to import the
+gthread code from GCC.
+
+******************************************************************************
+* 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.
+
+__gthr_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.
+
+__gthr_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.
+
+__gthr_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.
+
+__gthr_objc_thread_yield(void), void
+	Yields the processor, called by objc_thread_yield.
+
+__gthr_objc_thread_exit(void), int
+	Terminates the thread, called by objc_thread_exit.
+	Return -1 if error otherwise function does not return.
+
+__gthr_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.
+
+__gthr_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.
+
+__gthr_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.
+
+__gthr_objc_mutex_allocate(objc_mutex_t mutex), int
+	Allocates a new mutex, called by objc_mutex_allocate.
+	Return -1 if error otherwise return 0.
+
+__gthr_objc_mutex_deallocate(objc_mutex_t mutex), int
+	Free a mutex, called by objc_mutex_deallocate.
+	Return -1 if error otherwise return 0.
+
+__gthr_objc_mutex_lock(objc_mutex_t mutex), int
+	Locks a mutex, called by objc_mutex_lock.
+	Return -1 if error otherwise return 0.
+	
+__gthr_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.
+
+__gthr_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.
+*
+
+__gthr_objc_condition_allocate(objc_condition_t condition), int
+	Allocate a condition mutex, called by objc_condition_allocate.
+	Return -1 if error otherwise return 0.
+
+__gthr_objc_condition_deallocate(objc_condition_t condition), int
+	Deallocate a condition, called by objc_condition_deallocate.
+	Return -1 if error otherwise return 0.
+
+__gthr_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.
+	
+__gthr_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.
+
+__gthr_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.