objective-c - dispatch_semaphore_t 重用-我在这里缺少什么？

Question

我有一些代码在其中使用 dispatch_semaphore_t 来指示操作完成。当信号量是成员变量时，它的行为似乎不正确。我将展示有效的示例代码和一个似乎无效的示例:

@implementation someClass  
{  
  dispatch_semaphore_t memberSem;  
  dispatch_semaphore_t* semPtr;  
  NSThread* worker;
  BOOL taskDone;  
}  

- (id)init  
{  
  // Set up the worker thread and launch it - not shown here.  
  memberSem= dispatch_semaphore_create(0); 
  semPtr= NULL;  
  taskDone= FALSE;  
}  

- (void)dealloc  
{  
  // Clean up the worker thread as needed - not shown here.  
  if((NULL != semPtr) && (NULL != *semPtr))  
    disptatch_release(*semPtr);  

  dispatch_release(memberSem);  
}  

- (void)doSomethingArduous  
{  
  while([self notDone])  // Does something like check a limit.  
    [self doIt];  // Does something like process data and increment a counter.  

  taskDone= TRUE;  // I know this should be protected, but keeping the example simple for now.  

  if((NULL != semPtr) && (NULL != *semPtr))  
    dispatch_semaphore_signal(*semPtr);  // I will put a breakpoint here, call it  "SIGNAL"  
}  

- (BOOL)getSomethingDoneUseLocalSemaphore  
{  
  taskDone= FALSE;  // I know this should be protected, but keeping the example simple for now.  
  dispatch_semaphore_t localSem= dispatch_semaphore_create(0);  
  semPtr= &localSem;  
  [self performSelector:doSomethingArduous onThread:worker withObject:nil waitUntilDone:NO];  

  dispatch_time_t timeUp= dispatch_time(DISPATCH_TIME_NOW, (uint64_t)(2.5 * NSEC_PER_SEC));  
  dispatch_semaphore_wait(localSem, timeUp);  

  semPtr= NULL;  
  dispatch_release(localSem);  

  // I know I could just return taskDone. The example is this way to show what the problem is.  
  if(taskDone)  // Again with thread safety.  
    return TRUE;    

  return FALSE;  
}  

- (BOOL)getSomethingDoneUseMemberSemaphore  
{  
  taskDone= FALSE;  // I know this should be protected, but keeping the example simple for now.  

  semPtr= &memberSem;  // I will put a breakpoint here, call it "START"  
  [self performSelector:doSomethingArduous onThread:worker withObject:nil waitUntilDone:NO];  

  dispatch_time_t timeUp= dispatch_time(DISPATCH_TIME_NOW, (uint64_t)(2.5 * NSEC_PER_SEC));  
  dispatch_semaphore_wait(memberSem, timeUp);  

  semPtr= NULL;  

  // I know I could just return taskDone. The example is this way to show what the problem is.  
  if(taskDone)  // Again with thread safety.  
    return TRUE;  // I will put a breakpoint here, call it "TASK_DONE"   

  return FALSE;  // I will put a breakpoint here, call it "TASK_NOT_DONE"  
}  

- (void)hereIsWhereWeBringItTogether  
{  
  BOOL gotItDoneLocal= [self getSomethingDoneUseLocalSemaphore];  // Will return TRUE.  
  gotItDoneLocal= [self getSomethingDoneUseLocalSemaphore];  // Will return TRUE.  
  gotItDoneLocal= [self getSomethingDoneUseLocalSemaphore];  // Will return TRUE.  

  BOOL gotItDoneMember= [self getSomethingDoneUseMemberSemaphore];  // Will return TRUE. I will put a breakpoint here, call it "RUN_TEST"  
  gotItDoneMember= [self getSomethingDoneUseMemberSemaphore];  // Will return FALSE.  
}  

因此，鉴于该代码和我得到/得到的结果，我按照我的真实代码中的描述放置了断点:一个在主函数中，一个在工作函数中开始，一个在成员信号量发出信号的地方，以及等待后两个。

我发现在我使用成员信号量的情况下，在第一轮我停在断点“RUN_TEST”，运行并命中断点“START”，运行然后命中断点“SIGNAL”，运行然后命中断点“TASK_DONE”——一切如预期。

当我继续运行时，我遇到断点“START”，运行然后遇到断点“TASK_NOT_DONE”，运行然后遇到断点“SIGNAL”

也就是说，当我使用作为成员的信号量运行序列，并执行看起来正确的信号/等待时，我第二次尝试等待那个信号量时，我似乎错过了它，它在我发出信号后收到信号已经退出等待。

我似乎要么没有管理计数权(信号/等待配对)，要么该成员信号量不会返回到未发出信号的状态。

我觉得这里缺少一些基本的东西。任何输入将不胜感激。

编辑:最终我似乎遗漏了什么是因为我的实际代码有点复杂。不是从艰巨的任务中干净利落地返回，而是涉及多个线程和一个 postNotification。我用通知处理程序中的代码替换了 postNotification - 它设置了一个标志并向信号量发出信号。这样就消除了通知处理程序可能引入的任何延迟。

Answer 1

是的，这是预期的行为。如果您等待信号超时，当信号到来时，next 调用 dispatch_semaphore_wait 将捕获该信号量。考虑以下示例:

例如:

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
dispatch_time_t timeout;

// in 5 seconds, issue signal

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    sleep(5);
    NSLog(@"Signal 1");
    dispatch_semaphore_signal(semaphore);
});

// wait four seconds for signal (i.e. we're going to time out before the signal)

NSLog(@"Waiting 1");
timeout = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC));
if (dispatch_semaphore_wait(semaphore, timeout))
    NSLog(@"Waiting for 1: timed out");
else
    NSLog(@"Waiting for 1: caught signal");

// now, let's issue a second signal in another five seconds

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    sleep(5);
    NSLog(@"Signal 2");
    dispatch_semaphore_signal(semaphore);
});

// wait another four seconds for signal

// this time we're not going to time out waiting for the second signal, 
// because we'll actually catch that first signal, "signal 1")

NSLog(@"Waiting 2");
timeout = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC));
if (dispatch_semaphore_wait(semaphore, timeout))
    NSLog(@"Waiting for 2: timed out");
else
    NSLog(@"Waiting for 2: caught signal");

// note, "signal 2" is still forthcoming and the above code's 
// signals and waits are unbalanced 

因此，当您使用类实例变量时，您的 getSomethingDoneUseMemberSemaphore 的行为与上面类似，其中对 dispatch_semaphore_wait 的第二次调用将捕获发出的第一个信号，因为 (a) 它是同一个信号量； (b) 如果第一次调用 dispatch_semaphore_signal 超时。

但是如果你每次都使用唯一的信号量，那么第二次调用dispatch_semaphore_wait将不会响应第一个信号量的dispatch_semaphore_signal。