Question

I’m trying to write some code using posix threads but I’m stuck from the first step and to be honest I can’t realise why. I know that my problem has probably to do with some sort of synchronisation but I can’t figure it out.

What I have in main is the following:

while(1){
  int x = getX();
  pthread_t t;
  printf("Main: %d\n",x);
  pthread_create(&t, NULL, process_x, &x);
}

When I try to print the x value from the main and also the process_x function what I get is something like this:

Main: 1

Main: 2

Main: 3

Main: 4

Main: 5

Process_x: 5

What do I miss here?

—- EDIT —–

OK, maybe I need to provide some more info. The getX function receives data from a socket and returns an identifier while for our needs now the process_x just prints the argument it receives.

Answer 1

Apparently using malloc doesn’t work because it sets x to 0 so the process always reads 0. But thanks to everyones ideas I finally made it work using a condition variable and now everything is ok. Here is the code.

int main{
  pthread_t t;
  int x;
  while(1){
    pthread_mutex_lock(&receive);
    x = getX();
    printf("Main: %d\n",x);
    pthread_create(&t, NULL, process_x, &x);
    pthread_cond_wait(&goOn, &receive);
    pthread_mutex_unlock(&receive);
  }
}

void *process_x(void* arg){
  pthread_mutex_lock(&receive);
  int x = *(int *) arg;
  pthread_cond_signal(&goOn);
  pthread_mutex_unlock(&receive);
}