重新执行多线程代码后，为什么输出不一样？

Question

When I execute the following code, after every re-compile and re-execute, the answers(outputs) are not the same. What is a reason for it?

 #include <iostream>
#include <cstdlib>
//#include <pthread.h>

using namespace std;

#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   cout << "Hello World! Thread ID, " << tid << endl;
   pthread_exit(NULL);
}

int main ()
{
   pthread_t threads[NUM_THREADS];
   int rc;
   int i;
   for( i=0; i < NUM_THREADS; i++ ){
      cout << "main() : creating thread, " << i << endl;
      rc = pthread_create(&threads[i], NULL, 
                          PrintHello, (void *)i);
      if (rc){
         cout << "Error:unable to create thread," << rc << endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

g++ test.cpp -o test -lpthread ./test

Output1:

main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
Hello World! Thread ID, 0
main() : creating thread, 3
Hello World! Thread ID, 1
Hello World! Thread ID, 2
main() : creating thread, 4
Hello World! Thread ID, 3
Hello World! Thread ID, 4

g++ test.cpp -o test -lpthread ./test

Output2:

main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
Hello World! Thread ID, 0
main() : creating thread, 4
Hello World! Thread ID, 3
Hello World! Thread ID, 2
Hello World! Thread ID, 4
Hello World! Thread ID, 1

Answer 1

由于线程未同步，因此执行顺序无法精确确定，并且取决于您的执行上下文，例如，同时正在运行其他哪些进程等，每次运行程序时可能会有所不同。

Answer 2

yes. I have used of
pthread_mutex_lock(&mutex); and pthread_mutex_unlock(&mutex); as follows

#include <iostream>
#include <cstdlib>
#include <pthread.h>
#include <unistd.h>

using namespace std;

#define NUM_THREADS     5
pthread_mutex_t   mutex = PTHREAD_MUTEX_INITIALIZER;
void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   //sleep(1);

 cout << "Hello World! Thread ID, " << tid << endl;

pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
}

int main ()
{
   pthread_t threads[NUM_THREADS];
   int rc;
   int i;
//pthread_attr_t attr;
//pthread_attr_init(&attr);
   //pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
   for( i=0; i < NUM_THREADS; i++ ){
pthread_mutex_lock(&mutex);
      cout << "main() : creating thread, " << i << endl;

      rc = pthread_create(&threads[i], NULL, PrintHello, (void *)i);

//sleep(0.5);
      if (rc){
         cout << "Error:unable to create thread," << rc << endl;
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

and my outputs is always:

main() : creating thread, 0
Hello World! Thread ID, 0
main() : creating thread, 1
Hello World! Thread ID, 1
main() : creating thread, 2
Hello World! Thread ID, 2
main() : creating thread, 3
Hello World! Thread ID, 3
main() : creating thread, 4
Hello World! Thread ID, 4