--- /dev/null
+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 graditude 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@ocbi.com>
+
+******************************************************************************
+* Simple test program:
+
+A simple test program can be found in the thread-test directory.
+
+******************************************************************************
+* 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.
+
+******************************************************************************
+* Linking:
+
+On Solaris, you must link with -lthread to include the system
+thread library. We use its low level thread and mutex implementations.
+
+On OSF/1, you must link with -lpthreads to include the pthreads library.
+
+On WIN32, thread support is built-in to the WIN32 API; refer to your
+compiler documentation for the appropriate library.
+
+******************************************************************************
+* 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.
+
+__objc_init_thread_system(void), int
+ Initialize the thread subsystem. Call once by __objc_exec_class.
+
+__objc_fini_thread_system(void), int
+ Closes the thread subsystem.
+
+objc_thread_detach(SEL selector, id object, id argument), int
+ 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 threads 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 threads 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.
+
+******************************************************************************
+* Mutexs:
+
+Mutexs can be locked recursively. Each mutex 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.
+
+objc_mutex_allocate(void), Mutex_t
+ Allocates a new mutex. Mutex is initially unlocked.
+
+objc_mutex_deallocate(Mutex_t mutex), int
+ Free a mutex. Before freeing the mutex, makes sure that no
+ one else is using it.
+
+objc_mutex_lock(Mutex_t mutex), int
+ Locks a mutex. As mentioned earlier, the same thread may call
+ this routine repeatedly.
+
+objc_mutex_trylock(Mutex_t mutex), int
+ Attempts to lock a mutex. Returns -1 if failed. If lock on
+ mutex can be acquired then function operates exactly as
+ objc_mutex_lock.
+
+objc_mutex_unlock(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.
+
+******************************************************************************
+* Sample run of thread-test/checks/test01.m
+
+<< program started >> -- Program started
+__objc_exec_class(Object.m) -- Initialize once
+__objc_init_mutex_system
+__objc_init_thread_system
+__objc_init_selector_tables()
+__objc_init_class_tables()
+__objc_init_dispatch_tables()
+__objc_exec_class(Protocol.m) -- Called repeatedly
+__objc_init_protocols(0x000746d4) -- Called repeatedly
+class_add_method_list(0x74718, 0x74208) -- Called repeatedly
+<< main called >> -- Main called
+__objc_init_install_dtable(0x6d980, 0x6d5c0) -- Called repeatedly
+<< delegatePool filled, count=10 >> -- Code in secondary function
+__objc_init_install_dtable(0x76268, 0x70614) -- Called repeatedly
+Array: count=1 -- More secondary code.
+EltNodeCollector: count=1
+<< end of program >> -- End of program
+
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