VS rand () problem with pthread-win32

I am entering a strange problem in pthread programming I have compiled the following code in vs2005 with pthread-w32

#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <pthread.h>
#include <windows.h>

pthread_mutex_t lock;

void* thread1(void *) {
  int r1;
  while(true) {
    pthread_mutex_lock(&lock); // rand is maybe a CS
    r1 = rand() % 1500;
    pthread_mutex_unlock(&lock);
    Sleep(r1); printf("1:%d\n", r1);
  }
  return NULL;
}

void* thread2(void *) {
  int r2;
  while(true) {
    pthread_mutex_lock(&lock);
    r2 = rand() % 1500;
    pthread_mutex_unlock(&lock);
    Sleep(r2); printf("2:%d\n", r2);
  }
  return NULL;
}

int main() {
  srand((int)time(NULL));
  pthread_mutex_init(&lock, NULL);

  pthread_t tc_p, tc_v;
  pthread_create(&tc_p, NULL, thread1, NULL);
  pthread_create(&tc_v, NULL, thread2, NULL);

  pthread_join(tc_p, NULL);
  pthread_join(tc_v, NULL);

  pthread_mutex_destroy(&lock);

    return 0;
}

      

        
        
        
      

    

and the output looks like this

2:41
1:41
1:467
2:467
1:334
2:334
1:1000
2:1000

      

        
        
        
      

    

it is exactly like rand (), returns the same result in each of the two calls and I have srand (), but the result does not change every time I run the program

I am very new to multithreading programming and I have heard of rand () which is not thread safe. but I still can't figure out if the program above is wrong or if the rand () function has some problems in it.