+++ /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 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. 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.
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