Am I using the mutex_trylock correctly?

Question

The racers should have an equal chance of winning. When I run the program the results seem to be correct, both racers win about half the time, but I dont think I am using the mutex_trylock correctly. Is it actually doing anything the way with how I implemented it? I am new to C so I dont know a lot about this.

Program Description: We assume two racers, at two diagonally opposite corner of a rectangular region. They have to traverse along the roads along the peripheri of the region. There are two bridges on two opposite sides of the rectangle. In order to complete one round of traversal around this, the racers have to get pass for both the bridge at a time. The conditions of the race are

1) Only one racer can get a pass at a time.

2) Before one starts one round, he has to request and get both the passes and then after finishing that round has to release the passes, and make new try to get those passes for the next round.

3) Racer1 (R1) will acquire bridge-pass B1 first, then B0. R0 will acquire B0 and then B1.

4) There is a maximum number of rounds prefixed. Whoever reaches that number first will be the winner and the race will stop.

This is how the situation looks before starting.

                 B0
        R0-------- ~  -------------
        |                          |
        |                          |
        |                          |
        |                          |
        --------- ~  ------------- R1
                  B1


#include<stdio.h>
#include<pthread.h>
#include<stdlib.h>

#define THREAD_NUM 2  
#define MAX_ROUNDS 200000
#define TRUE  1 
#define FALSE 0

/* mutex locks for each bridge */
pthread_mutex_t B0, B1;

/* racer ID */
int r[THREAD_NUM]={0,1};

/* number of rounds completed by each racer */
int numRounds[THREAD_NUM]={0,0};

void *racer(void *); /* prototype of racer routine */

int main()
{
    pthread_t tid[THREAD_NUM];
    void *status;
    int i,j;

    /* create 2 threads representing 2 racers */
    for (i = 0; i < THREAD_NUM; i++)
   {
       /*Your code here */
       pthread_create(&tid[i], NULL, racer, &r[i]);

    }

    /* wait for the join of 2 threads */
     for (i = 0; i < THREAD_NUM; i++)
    {
         /*Your code here */
         pthread_join(tid[i], &status);
    }

    printf("\n");
    for(i=0; i<THREAD_NUM; i++)
        printf("Racer %d finished %d rounds!!\n", i, numRounds[i]);

 if(numRounds[0]>=numRounds[1]) printf("\n RACER-0 WINS.\n\n");
 else  printf("\n RACER-1 WINS..\n\n");

return (0);
}


void *racer(void  *arg)
{
  int  index = *(int*)arg, NotYet;

    while( (numRounds[0] < MAX_ROUNDS) && (numRounds[1] < MAX_ROUNDS) )
 {

   NotYet = TRUE;

    /* RACER 0 tries to get both locks before she makes a round */
   if(index==0){
     /*Your code here */
     pthread_mutex_trylock(&B0);
     pthread_mutex_trylock(&B1);



   }

    /* RACER 1 tries to get both locks before she makes a round */
   if(index==1){
      /*Your code here */
     pthread_mutex_trylock(&B1);
     pthread_mutex_trylock(&B0);
     }
    numRounds[index]++;      /* Make one more round */


    /* unlock both locks */
    pthread_mutex_unlock(&B0);
    pthread_mutex_unlock(&B1);

       /* random yield to another thread */

     }

printf("racer %d made %d rounds !\n", index, numRounds[index]);

pthread_exit(0);

}

Answer 1

when first thread locks B0 and if second get scheduled to lock B1, it will cause deadlock. If first mutex is locked and second is not locked, then release first mutex and loop again. This loop can be smaller if tried with mutex_lock and not trylock.