OpenH323的基础线程实现

1. PThread_H323

OpenH323的基础线程类为PThread_H323。在PThread_H323中定义了一系列关于线程运行参数，例如线程运行优先级，线程栈大小等。需要注意的一点是，PThread_H323依赖于Ptlib库的实现，而PTLib库支持多种操作系统，所以PThread_H323定义了多个实现文件，对gs_phone来说其使用的实现文件为libraries/ptlib-2.12.8/src/ptlib/unix/tlibthrd.cxx，在查看代码时需要注意这一点（仅示例Linux实现）。
相关的构造函数如下所示：

//////////////////////////////////////////////////////////////////////////////

//
//  Called to construct a PThread_H323 for either:
//
//       a) The primordial PProcesss thread
//       b) A non-PTLib thread that needs to use PTLib routines, such as PTRACE
//
//  This is always called in the context of the running thread, so naturally, the thread
//  is not paused
//

PThread_H323::PThread_H323(bool isProcess)
  : m_type(isProcess ? e_IsProcess : e_IsExternal)
  , m_originalStackSize(0) // 0 indicates external thread
  , m_threadId(pthread_self())
  , PX_priority(NormalPriority)
#if defined(P_LINUX)
  , PX_linuxId(syscall(SYS_gettid))
#endif
  , PX_suspendMutex(MutexInitialiser)
  , PX_suspendCount(0)
  , PX_state(PX_running)
#ifndef P_HAS_SEMAPHORES
  , PX_waitingSemaphore(NULL)
  , PX_WaitSemMutex(MutexInitialiser)
#endif
{
#ifdef P_RTEMS
  PAssertOS(socketpair(AF_INET,SOCK_STREAM,0,unblockPipe) == 0);
#else
  PAssertOS(::pipe(unblockPipe) == 0);
#endif

  if (isProcess)
    return;

  PProcess::Current().InternalThreadStarted(this);
}


//
//  Called to construct a PThread_H323 for a normal PTLib thread.
//
//  This is always called in the context of some other thread, and
//  the PThread_H323 is always created in the paused state
//
PThread_H323::PThread_H323(PINDEX stackSize,
                 AutoDeleteFlag deletion,
                 Priority priorityLevel,
                 const PString & name)
  : m_type(deletion == AutoDeleteThread ? e_IsAutoDelete : e_IsManualDelete)
//change by Jacky at 20150624 .  c++ do not supprot max with long unsigned int and int. 
//  , m_originalStackSize(std::max(stackSize, 16*PTHREAD_STACK_MIN)) // Set a decent (256K) stack size that won't eat all virtual memory
  , m_originalStackSize(std::max(stackSize, 16*PTHREAD_STACK_MIN)) // Set a decent (256K) stack size that won't eat all virtual memory
  , m_threadName(name)
  , m_threadId(PNullThreadIdentifier)  // indicates thread has not started
  , PX_priority(priorityLevel)
#if defined(P_LINUX)
  , PX_linuxId(0)
#endif
  , PX_suspendMutex(MutexInitialiser)
  , PX_suspendCount(1)
  , PX_state(PX_firstResume) // new thread is actually started the first time Resume() is called.
#ifndef P_HAS_SEMAPHORES
  , PX_waitingSemaphore(NULL)
  , PX_WaitSemMutex(MutexInitialiser)
#endif
{
  PAssert(m_originalStackSize > 0, PInvalidParameter);

#ifdef P_RTEMS
  PAssertOS(socketpair(AF_INET,SOCK_STREAM,0,unblockPipe) == 0);
#else
  PAssertOS(::pipe(unblockPipe) == 0);
#endif
  PX_NewHandle("Thread unblock pipe", PMAX(unblockPipe[0], unblockPipe[1]));

  // If need to be deleted automatically, make sure thread that does it runs.
  if (m_type == e_IsAutoDelete)
    PProcess::Current().SignalTimerChange();

  PTRACE(5, "PTLib\tCreated thread " << this << ' ' << m_threadName);
}

初始化完毕之后，PThread_H323类对外提供了统一的接口对线程进行启动该接口如下所示：

void PThread_H323::Resume()
{
  Suspend(false);
}

其中Resume接口又调用了Suspend接口，Suspend接口如下所示：

void PThread_H323::Suspend(PBoolean susp)
{
  PAssertPTHREAD(pthread_mutex_lock, (&PX_suspendMutex));

  // Check for start up condition, first time Resume() is called
  if (PX_state == PX_firstResume) {
    if (susp)
      PX_suspendCount++;
    else {
      if (PX_suspendCount > 0)
        PX_suspendCount--;
      if (PX_suspendCount == 0)
        PX_StartThread();
    }

    PAssertPTHREAD(pthread_mutex_unlock, (&PX_suspendMutex));
    return;
  }

#if defined(P_MACOSX) && (P_MACOSX <= 55)
  // Suspend - warn the user with an Assertion
  PAssertAlways("Cannot suspend threads on Mac OS X due to lack of pthread_kill()");
#else
  if (!IsTerminated()) {

    // if suspending, then see if already suspended
    if (susp) {
      PX_suspendCount++;
      if (PX_suspendCount == 1) {
        if (m_threadId != pthread_self()) {
          signal(SUSPEND_SIG, PX_SuspendSignalHandler);
          pthread_kill(m_threadId, SUSPEND_SIG);
        }
        else {
          PAssertPTHREAD(pthread_mutex_unlock, (&PX_suspendMutex));
          PX_SuspendSignalHandler(SUSPEND_SIG);
          return;  // Mutex already unlocked
        }
      }
    }

    // if resuming, then see if to really resume
    else if (PX_suspendCount > 0) {
      PX_suspendCount--;
      if (PX_suspendCount == 0) 
        PXAbortBlock();
    }
  }

  PAssertPTHREAD(pthread_mutex_unlock, (&PX_suspendMutex));
#endif // P_MACOSX
}

Suspend函数中调用了PX_StartThread，这个函数负责真正线程的建立，设置线程的相关属性等并将线程投入运行状态，如下所示：

void PThread_H323::PX_StartThread()
{
  // This should be executed inside the PX_suspendMutex to avoid races
  // with the thread starting.
  PX_state = PX_starting;

  pthread_attr_t threadAttr;
  pthread_attr_init(&threadAttr);
  PAssertPTHREAD(pthread_attr_setdetachstate, (&threadAttr, PTHREAD_CREATE_DETACHED));

#if defined(P_LINUX)

  pthread_attr_setstacksize(&threadAttr, m_originalStackSize);

  struct sched_param sched_params;
  PAssertPTHREAD(pthread_attr_setschedpolicy, (&threadAttr, GetSchedParam(PX_priority, sched_params)));
  PAssertPTHREAD(pthread_attr_setschedparam,  (&threadAttr, &sched_params));

#elif defined(P_RTEMS)
  pthread_attr_setstacksize(&threadAttr, 2*PTHREAD_MINIMUM_STACK_SIZE);
  pthread_attr_setinheritsched(&threadAttr, PTHREAD_EXPLICIT_SCHED);
  pthread_attr_setschedpolicy(&threadAttr, SCHED_OTHER);
  struct sched_param sched_param;
  sched_param.sched_priority = 125; /* set medium priority */
  pthread_attr_setschedparam(&threadAttr, &sched_param);
#endif

  PProcess & process = PProcess::Current();

  // create the thread
  PAssertPTHREAD(pthread_create, (&m_threadId, &threadAttr, &PThread_H323::PX_ThreadMain, this));

  // put the thread into the thread list
  process.InternalThreadStarted(this);

  pthread_attr_destroy(&threadAttr);

#ifdef P_MACOSX
  if (PX_priority == HighestPriority) {
    PTRACE(1, "set thread to have the highest priority (MACOSX)");
    SetPriority(HighestPriority);
  }
#endif
}

其中线程中执行的函数为PX_ThreadMain，该函数的实现如下：

void * PThread_H323::PX_ThreadMain(void * arg)
{
  PThread_H323 * thread = reinterpret_cast(arg);

#if P_USE_THREAD_CANCEL
  // make sure the cleanup routine is called when the threade cancelled
  pthread_cleanup_push(&PThread_H323::PX_ThreadEnd, arg);
#endif

  thread->PX_ThreadBegin();

  // now call the the thread main routine
  thread->Main();

#if P_USE_THREAD_CANCEL
  pthread_cleanup_pop(1); // execute the cleanup routine
#else
  PX_ThreadEnd(arg);
#endif

  return NULL;
}

可以看到该函数实际执行了Main函数，该函数为虚函数，供外部调用，从而实现相关的功能。PX_ThreadBegin仅仅执行了一些校验，并且做了一些线程投入运行前的准备工作。