核心是：将标示对象与锁建立关联。
线程
锁
标识；
异常；
NSString *test = @"test";
@try { // Allocates a lock for test and locks it objc_sync_enter(test); test = nil;} @finally { // Passed `nil`, so the lock allocated in `objc_sync_enter` // above is never unlocked or deallocated objc_sync_exit(test); }int objc_sync_enter(id obj)
{
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = id2data(obj, ACQUIRE);
require_action_string(data != NULL, done, result = OBJC_SYNC_NOT_INITIALIZED, "id2data failed");
result = recursive_mutex_lock(&data->mutex);
require_noerr_string(result, done, "mutex_lock failed");
} else {
// @synchronized(nil) does nothing
if (DebugNilSync) {
_objc_inform("NIL SYNC DEBUG: @synchronized(nil); set a breakpoint on objc_sync_nil to debug");
}
objc_sync_nil();
}
done:
return result;
}
// End synchronizing on 'obj'.
// Returns OBJC_SYNC_SUCCESS or OBJC_SYNC_NOT_OWNING_THREAD_ERROR
int objc_sync_exit(id obj)
{
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = id2data(obj, RELEASE);
require_action_string(data != NULL, done, result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR, "id2data failed");
result = recursive_mutex_unlock(&data->mutex);
require_noerr_string(result, done, "mutex_unlock failed");
} else {
// @synchronized(nil) does nothing
}