multi thread launch arrangement

Question

I have 2 thread to create thread1 and thread2. When creating threads with:

pthread_create(&thread1, NULL, &function_th1, NULL);
pthread_create(&thread2, NULL, &function_th2, NULL);

The thread2 is launched before the thread1 and I'm looking for a solution to start thread1 before thread2.

Not looking for this solution:

pthread_create(&thread2, NULL, &function_th2, NULL);
pthread_create(&thread1, NULL, &function_th1, NULL);

Answer 1

There is no relation between when a thread creation call is issued and when the thread actually starts executing. It all depends on implementation, OS, etc. that's why you are seeing the two threads actually starting in a seemingly random order.

If you need thread 1 to start before thread 2 you probably have some data/logical dependency on some event. In this case you should use a condition variable to tell thread 2 when to actually begin executing.

// condition variable
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
// associated mutex
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
// state for condition variable
int shouldWait = 1;

...

void* function_th1(void* p) {
     // execute something

     // signal thread 2 to proceed
     pthread_mutex_lock(&mutex);
     shouldWait = 0;
     pthread_cond_signal(&cond);
     pthread_mutex_unlock(&mutex);

     // other stuff
}

void* function_th2(void* p) {
     // wait for signal from thread 1
     pthread_mutex_lock(&mutex);
     while(shouldWait) {
         pthread_cond_wait(&cond, &mutex);
     }
     pthread_mutex_unlock(&mutex);

     // other stuff
}    

Answer 2

Move the 'pthread_create(&thread2, NULL, &function_th2, NULL);' to the top of the 'function_th1' function.

That will certainly achieve what you ask for, no complex signalling required.

Whether what you ask for is what you actually want or need is another matter.