How to pause a pthread ANY TIME I want?

Question

recently I set out to port ucos-ii to Ubuntu PC.

As we know, it\'s not possible to simulate the \"process\" in the ucos-ii by simply adding a flag in \"while\" loop

Answer 1

The Modula-3 garbage collector needs to suspend pthreads at an arbitrary time, not just when they are waiting on a condition variable or mutex. It does it by registering a (Unix) signal handler that suspends the thread and then using pthread_kill to send a signal to the target thread. I think it works (it has been reliable for others but I'm debugging an issue with it right now...) It's a bit kludgy, though....

Google for ThreadPThread.m3 and look at the routines "StopWorld" and "StartWorld". Handler itself is in ThreadPThreadC.c.

Answer 2

Here is example of thread function within a class with pause/resume functionality...

class SomeClass
{
public:
    // ... construction/destruction

    void Resume();
    void Pause();
    void Stop();

private:
    static void* ThreadFunc(void* pParam);

    pthread_t thread;
    pthread_mutex_t mutex;
    pthread_cond_t cond_var;
    int command;
};

SomeClass::SomeClass()
{
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&cond_var, NULL);

    // create thread in suspended state..
    command = 0;
    pthread_create(&thread, NULL, ThreadFunc, this);
}

SomeClass::~SomeClass()
{
    // we should stop the thread and exit ThreadFunc before calling of blocking pthread_join function
    // also it prevents the mutex staying locked..
    Stop();
    pthread_join(thread, NULL);

    pthread_cond_destroy(&cond_var);
    pthread_mutex_destroy(&mutex);
}

void* SomeClass::ThreadFunc(void* pParam)
{
    SomeClass* pThis = (SomeClass*)pParam;
    timespec time_ns = {0, 50*1000*1000};   // 50 milliseconds

    while(1)
    {
        pthread_mutex_lock(&pThis->mutex);

        if (pThis->command == 2) // command to stop thread..
        {
            // be sure to unlock mutex before exit..
            pthread_mutex_unlock(&pThis->mutex);
            return NULL;
        }
        else if (pThis->command == 0) // command to pause thread..
        {
            pthread_cond_wait(&pThis->cond_var, &pThis->mutex);
            // dont forget to unlock the mutex..
            pthread_mutex_unlock(&pThis->mutex);
            continue;
        }

        if (pThis->command == 1) // command to run..
        {
            // normal runing process..
            fprintf(stderr, "*");
        }

        pthread_mutex_unlock(&pThis->mutex);

        // it's important to give main thread few time after unlock 'this'
        pthread_yield();
        // ... or...
        //nanosleep(&time_ns, NULL);
    }
    pthread_exit(NULL);
}

void SomeClass::Stop()
{
    pthread_mutex_lock(&mutex);
    command = 2;
    pthread_cond_signal(&cond_var);
    pthread_mutex_unlock(&mutex);
}

void SomeClass::Pause()
{
    pthread_mutex_lock(&mutex);
    command = 0;
    // in pause command we dont need to signal cond_var because we not in wait state now..
    pthread_mutex_unlock(&mutex);
}

void SomeClass::Resume()
{
    pthread_mutex_lock(&mutex);
    command = 1;
    pthread_cond_signal(&cond_var);
    pthread_mutex_unlock(&mutex);
}

Answer 3

If stopping at specific points with a condition variable is insufficient, then you can't do this with pthreads. The pthread interface does not include suspend/resume functionality.

See, for example, answer E.4 here:

The POSIX standard provides no mechanism by which a thread A can suspend the execution of another thread B, without cooperation from B. The only way to implement a suspend/restart mechanism is to have B check periodically some global variable for a suspend request and then suspend itself on a condition variable, which another thread can signal later to restart B.

That FAQ answer goes on to describe a couple of non-standard ways of doing it, one in Solaris and one in LinuxThreads (which is now obsolete; do not confuse it with current threading on Linux); neither of those apply to your situation.