semaphore

public class Semaphore implements java.io.Serializable { private static final long serialVersionUID = -3222578661600680210L; /** All mechanics via AbstractQueuedSynchronizer subclass */ private final Sync sync; /** * 继承AQS以实现信号量机制，使用AQS的state属性代表允许使用的信号量的数量，该Sync类有两个 * 派生类，其中一个是公平模式的，另一个是非公平模式的 */ abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 1192457210091910933L; Sync(int permits) { setState(permits); } final int getPermits() { return getState(); } //非公平模式的获取信号量 final int nonfairTryAcquireShared(int acquires) { for (;;) { int

通过前面对 线程互斥锁lock / 线程事件event / 线程条件变量condition / 线程定时器timer 的讲解，相信你对线程threading模块已经有了一定的了解，同时执行多个线程的确可以提高程序的效率，但是并非线程的数量越多越好，可能对于计算机而言，你直接运行20~30线程可能没太大影响，如果同时运行上千个甚至上万个呢？我相信你电脑会直接瘫痪…… 一.semaphore信号量原理 多线程同时运行，能提高程序的运行效率，但是并非线程越多越好，而semaphore信号量可以通过内置计数器来控制同时运行线程的数量，启动线程(消耗信号量)内置计数器会自动减一，线程结束(释放信号量)内置计数器会自动加一；内置计数器为零，启动线程会阻塞，直到有本线程结束或者其他线程结束为止; 二.semaphore信号量相关函数介绍 acquire() — 消耗信号量，内置计数器减一; release() — 释放信号量，内置计数器加一; 在semaphore信号量有一个内置计数器，控制线程的数量，acquire()会消耗信号量，计数器会自动减一；release()会释放信号量，计数器会自动加一；当计数器为零时，acquire()调用被阻塞，直到release()释放信号量为止。 三.semaphore信号量使用 创建多个线程，限制同一时间最多运行5个线程，示例代码如下： # !usr

With reference to my previous question about GDB not pinpointing the SIGSEGV point , My thread code is as follows: void *runner(void *unused) { do { sem_wait(&x); ... if(/*condition 1 check*/) { sem_post(&x); sleep(5); sem_wait(&x); if(/*repeat condition 1 check; after atleast 5 seconds*/) { printf("LEAVING...\n"); sem_post(&x); // putting exit(0); here resolves the dilemma return(NULL); } } sem_post(&x); }while(1); } Main code: sem_t x; int main(void) { sem_init(&x,0,1); ... pthread_t thrId; pthread_create(&thrId,NULL,runner,NULL); ... pthread_join(thrId,NULL); return(0); } Edit: Having an

1、Semaphore可以控同时访问的线程个数 2、Semaphore类位于java.util.concurrent包下，它提供了2个构造器： //参数permits表示许可数目，即同时可以允许多少线程进行访问 public Semaphore(int permits) { sync = new NonfairSync(permits); } //这个多了一个参数fair表示是否是公平的，即等待时间越久的越先获取许可 public Semaphore(int permits, boolean fair) { sync = (fair)? new FairSync(permits) : new NonfairSync(permits); } 3、重要方法，acquire()、release()方法： acquire()用来获取一个许可，若无许可能够获得，则会一直等待，直到获得许可 release()用来释放许可。注意，在释放许可之前，必须先获获得许可 这4个方法都会被阻塞 public void acquire() throws InterruptedException { } //获取一个许可 public void acquire(int permits) throws InterruptedException { } //获取permits个许可 public void

this code has been posted before, and been used as well, from what i could gather. i'm in a situation where i need the code to NOT continue until i know if i have access to the contacts. on Xcode 5.0.2 and iOS 6, this works just fine. on iOS 7, it hangs forever, and then when i kill the app the dialog box comes up asking to allow access to the contacts. ABAddressBookRef addressBook = ABAddressBookCreate(); __block BOOL accessGranted = NO; if (ABAddressBookRequestAccessWithCompletion != NULL) { // we're on iOS 6 dispatch_semaphore_t sema = dispatch_semaphore_create(0);

Looking at some Apple code sample, I found this: func metadataOutput(_ output: AVCaptureMetadataOutput, didOutput metadataObjects: [AVMetadataObject], from connection: AVCaptureConnection) { // wait() is used to drop new notifications if old ones are still processing, to avoid queueing up a bunch of stale data. if metadataObjectsOverlayLayersDrawingSemaphore.wait(timeout: .now()) == .success { DispatchQueue.main.async { // Some processing... self.metadataObjectsOverlayLayersDrawingSemaphore.signal() } } } Context of the code : This is the delegate method when using video capture to detect a QR