Output for sample code for an upcoming exam concerning pthread - pthreads

Question

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
int token = 2;
int value = 3;
void * red ( void *arg ) {
int myid = * ((int *) arg);
pthread_mutex_lock( &mutex );
while ( myid != token) {
pthread_cond_wait( &cond, &mutex );
}
value = value + (myid + 3);
printf( "RED: id is %d \n", value);
token = (token + 1) % 3;
pthread_cond_broadcast( &cond );
pthread_mutex_unlock( &mutex );
}
void * blue ( void *arg ) {
int myid = * ((int *) arg);
pthread_mutex_lock( &mutex );
while ( myid != token) {
pthread_cond_wait( &cond, &mutex );
}
value = value * (myid + 2);
printf( "BLUE: id is %d \n", value);
token = (token + 1) % 3;
pthread_cond_broadcast( &cond );
pthread_mutex_unlock( &mutex );
}
void * white ( void *arg ) {
int myid = * ((int *) arg);
pthread_mutex_lock( &mutex );
while ( myid != token) {
pthread_cond_wait( &cond, &mutex );
}
value = value * (myid + 1);
printf( "WHITE: id is %d \n", value);
token = (token + 1) % 3;
pthread_cond_broadcast( &cond );
pthread_mutex_unlock( &mutex );
}
main( int argc, char *argv[] ) {
pthread_t tid;
int count = 0;
int id1, id2, id3;
id1 = count;
n = pthread_create( &tid, NULL, red, &id1);
id2 = ++count;
n = pthread_create( &tid, NULL, blue, &id2);
id3 = ++count;
n = pthread_create( &tid, NULL, white, &id3);
if ( n = pthread_join( tid, NULL ) ) {
fprintf( stderr, "pthread_join: %s\n", strerror( n ) );
exit( 1 );
}
}
I am just looking for comments and or notes to what the output would be. THIS IS FOR AN EXAM AND WAS OFFERED AS AN EXAMPLE. THIS IS NOT HOMEWORK OR GOING TO BE USED FOR ANY TYPE OF SUBMISSION. I am looking to understand what is going on. Any help is greatly appreciated.

Answer 1

I'm going to assume that you know the function of the locks, condition variables, and the waits. Basically you have three threads that each call Red, Blue, and White. Token is originally 2, and value is originally 3.
Red is called when id1 = 0, but it will stay in the while block calling wait() until the token = 0.
Blue is called when id3 = 1, and will stay in the while block called wait() until the token is 1.
White is called when id2 = 2, and will stay in the while block calling wait() until the token is 2.
So White will enter the critical section first, since it's the only one that won't enter the while loop. So value = 3 * ( 3 ) = 9; token = ( 3 ) % 3 = 0;
Broadcast wakes every waiting thread, but the only one that will enter the critical section is Red. It adds 3 to value for 12; token = ( 1 ) % 3 = 1; Broadcast wakes Blue.
Blue enters the critical section. value = 12 * 3; token = 2 ( but it doesn't matter anymore ).
This would be the order of the threads would execute, which is what I assume the test is really asking. However, what should really come out is just:
White is 9
This is because there is only one pthread_t tid. So after pthread_join( tid, NULL ), it can immediately exit. If you put different pthread_t in each of the pthread_create() then all of them would print.