multi-threads synchronization use conditional mutex

#include <pthread.h>
int thread_flag;
pthread_cond_t thread_flag_cv;
pthread_mutex_t thread_flag_mutex;
void initialize_flag ()
{
/* Initialize the mutex and condition variable.
pthread_mutex_init (&thread_flag_mutex, NULL);
pthread_cond_init (&thread_flag_cv, NULL);
/* Initialize the flag value. */
thread_flag = 0;
}
*/
/* Calls do_work repeatedly while the thread flag is set; blocks if
the flag is clear. */
void* thread_function (void* thread_arg)
{
/* Loop infinitely. */
while (1) {
/* Lock the mutex before accessing the flag value. */
pthread_mutex_lock (&thread_flag_mutex);
while (!thread_flag)
/* The flag is clear. Wait for a signal on the condition
variable, indicating that the flag value has changed. When the
signal arrives and this thread unblocks, loop and check the
flag again. */
pthread_cond_wait (&thread_flag_cv, &thread_flag_mutex);
/* When we’ve gotten here, we know the flag must be set. Unlock
the mutex. */
pthread_mutex_unlock (&thread_flag_mutex);
/* Do some work. */
do_work ();
}
return NULL;
}
/* Sets the value of the thread flag to FLAG_VALUE.
*/
void set_thread_flag (int flag_value)
{
/* Lock the mutex before accessing the flag value. */
pthread_mutex_lock (&thread_flag_mutex);
/* Set the flag value, and then signal in case thread_function is
blocked, waiting for the flag to become set. However,
thread_function can’t actually check the flag until the mutex is
unlocked. */
thread_flag = flag_value;
pthread_cond_signal (&thread_flag_cv);
/* Unlock the mutex. */
pthread_mutex_unlock (&thread_flag_mutex);
}