semaphore.h

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// Copyright 2013 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_BASE_PLATFORM_SEMAPHORE_H_
#define V8_BASE_PLATFORM_SEMAPHORE_H_
 
#include "src/base/base-export.h"
#include "src/base/lazy-instance.h"
#if V8_OS_WIN
#include "src/base/win32-headers.h"
#endif
 
#if V8_OS_DARWIN
#include <dispatch/dispatch.h>
#elif V8_OS_ZOS
#include "zos-semaphore.h"
#elif V8_OS_POSIX
#include <semaphore.h>
#endif
 
#if V8_OS_STARBOARD
#include "starboard/common/semaphore.h"
#endif
 
namespace v8 {
namespace base {
 
// Forward declarations.
class TimeDelta;
 
// ----------------------------------------------------------------------------
// Semaphore
//
// A semaphore object is a synchronization object that maintains a count. The
// count is decremented each time a thread completes a wait for the semaphore
// object and incremented each time a thread signals the semaphore. When the
// count reaches zero,  threads waiting for the semaphore blocks until the
// count becomes non-zero.
 
class V8_BASE_EXPORT Semaphore {
 public:
  explicit Semaphore(int count);
  Semaphore(const Semaphore&) = delete;
  Semaphore& operator=(const Semaphore&) = delete;
  ~Semaphore();
 
  // Increments the semaphore counter.
  void Signal();
 
  // Decrements the semaphore counter if it is positive, or blocks until it
  // becomes positive and then decrements the counter.
  void Wait();
 
  // Like Wait() but returns after rel_time time has passed. If the timeout
  // happens the return value is false and the counter is unchanged. Otherwise
  // the semaphore counter is decremented and true is returned.
  bool WaitFor(const TimeDelta& rel_time) V8_WARN_UNUSED_RESULT;
 
#if V8_OS_DARWIN
  using NativeHandle = dispatch_semaphore_t;
#elif V8_OS_POSIX
  using NativeHandle = sem_t;
#elif V8_OS_WIN
  using NativeHandle = HANDLE;
#elif V8_OS_STARBOARD
  using NativeHandle = starboard::Semaphore;
#endif
 
  NativeHandle& native_handle() {
    return native_handle_;
  }
  const NativeHandle& native_handle() const {
    return native_handle_;
  }
 
 private:
  NativeHandle native_handle_;
};
 
// POD Semaphore initialized lazily (i.e. the first time Pointer() is called).
// Usage:
//   // The following semaphore starts at 0.
//   static LazySemaphore<0>::type my_semaphore = LAZY_SEMAPHORE_INITIALIZER;
//
//   void my_function() {
//     // Do something with my_semaphore.Pointer().
//   }
//
 
template <int N>
struct CreateSemaphoreTrait {
  static Semaphore* Create() {
    return new Semaphore(N);
  }
};
 
template <int N>
struct LazySemaphore {
  using typename LazyDynamicInstance<Semaphore, CreateSemaphoreTrait<N>,
                                     ThreadSafeInitOnceTrait>::type;
};
 
#define LAZY_SEMAPHORE_INITIALIZER LAZY_DYNAMIC_INSTANCE_INITIALIZER
 
}  // namespace base
}  // namespace v8
 
#endif  // V8_BASE_PLATFORM_SEMAPHORE_H_