Question

I was trying to test the pthread by using mutex:

#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <stdlib.h>
#include <unistd.h>

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int global = 0;
void thread_set();
void thread_read();



int main(void){
  pthread_t thread1, thread2;
  int re_value1, re_value2;
  int i;
  for(i = 0; i < 5; i++){
    re_value1 = pthread_create(&thread1,NULL, (void*)&thread_set,NULL);
    re_value2 = pthread_create(&thread2,NULL,(void*)&thread_read,NULL);
  }

  pthread_join(thread1,NULL);
  pthread_join(thread2,NULL);
  /* sleep(2); */ // without it the 5 iteration couldn't finish
  printf("exsiting\n");
  exit(0);
}

void thread_set(){
  pthread_mutex_lock(&mutex1);
  printf("Setting data\t");
  global = rand();
  pthread_mutex_unlock(&mutex1);
}
void thread_read(){
  int data;
  pthread_mutex_lock(&mutex1);
  data = global;
  printf("the value is: %d\n",data);
  pthread_mutex_unlock(&mutex1);
}

without the sleep(), the code won’t finish the 5 iteration:

Setting data the value is: 1804289383
the value is: 1804289383
Setting data the value is: 846930886
exsiting

Setting data the value is: 1804289383
Setting data the value is: 846930886
the value is: 846930886
exsiting

It works only add the sleep() to the main thread, I think it should work without the sleep(), because the join() function wait for each child thread terminate

Any one can tell me why is it?

Answer 1

Your use of mutex objects looks fine, but this loop

for(i = 0; i < 5; i++) {
     re_value1 = pthread_create(&thread1,NULL, (void*)&thread_set,NULL);
     re_value2 = pthread_create(&thread2,NULL,(void*)&thread_read,NULL);
}

is asking for trouble as you are reusing the same thread instances thread1 and thread2 for each iteration of your loop. Internally this must be causing problems although I do not know exactly how it would manifest itself. You should really use a separate instance of thread object per thread you want to ensure reliable running. I do not know what would happen if you called pthread_create using an instance of a thread object that is already running but I suspect it is not a wise thing to do. I would suspect that at best it will block until the thread function has exited.

Also you are not cheking the return values from pthread_create() either which might be a good idea. In summary I would use a separate instance of thread objects, or add the pthread_join calls to the inside of your loop so that you are certain that the threads are finished running before the next call to pthread_create().

Finally the function signature for functions passed to pthread_create() are of the type

 void* thread_function(void*);

and not

 void thead_function()

like you have in your code.