源代码：
- (void)sumer{
@synchronized(_arr){
[_arr addObject:@"1"];
}
}
用clang 重写后输出：
static void _I_Testdd_sumer(Testdd * self, SEL _cmd) {
{ id _rethrow = 0; id _sync_obj = (id)(*(NSMutableArray **)((char *)self + OBJC_IVAR_$_Testdd$_arr));
//enter
objc_sync_enter(_sync_obj);
try {
struct _SYNC_EXIT { _SYNC_EXIT(id arg) : sync_exit(arg) {}
~_SYNC_EXIT() {objc_sync_exit(sync_exit);}
id sync_exit;
} _sync_exit(_sync_obj);
((void (*)(id, SEL, ObjectType))(void *)objc_msgSend)((id)(*(NSMutableArray **)((char *)self + OBJC_IVAR_$_Testdd$_arr)), sel_registerName("addObject:"), (id)(NSString *)&__NSConstantStringImpl__var_folders_5m_jf1yfr0s54x2_6rf51qlcjp00000gp_T_Testdd_043a95_mi_0);
} catch (id e) {_rethrow = e;}
{ struct _FIN { _FIN(id reth) : rethrow(reth) {}
~_FIN() { if (rethrow) objc_exception_throw(rethrow); }
id rethrow;
} _fin_force_rethow(_rethrow);}
}
}
简化代码为：
id _rethrow = 0;
id _sync_obj = (id)(*(NSMutableArray **)((char *)self + OBJC_IVAR_$_Testdd$_arr));
objc_sync_enter(_sync_obj);
try{
objc_sync_exit(sync_exit);
}catch (id e) { _rethrow = e;}
@synchronized不仅可以实现锁，还可以输出意外。
再查看objc_sync_enter的文档
/**
* Begin synchronizing on 'obj'.
* Allocates recursive pthread_mutex associated with 'obj' if needed.
*
* @param obj The object to begin synchronizing on.
*
* @return OBJC_SYNC_SUCCESS once lock is acquired.
*/
OBJC_EXPORT int
objc_sync_enter(id _Nonnull obj)
得知其使用了递归的互斥锁。
再看这里找到 objc-sync 的全部源码，部分如下：
typedef struct SyncData {
struct SyncData* nextData;
id object;
int threadCount; // number of THREADS using this block
recursive_mutex_t mutex;
} SyncData;
typedef struct {
SyncData *data;
spinlock_t lock;
char align[64 - sizeof (spinlock_t) - sizeof (SyncData *)];
} SyncList
// Begin synchronizing on 'obj'.
// Allocates recursive mutex associated with 'obj' if needed.
// Returns OBJC_SYNC_SUCCESS once lock is acquired.
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
}
done:
if ( result == RECURSIVE_MUTEX_NOT_LOCKED )
result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
return result;
}
以上可以看到加锁，解锁操作。同时Spinlock prevents multiple threads from creating multiple locks for the same new object.
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