mutex-nostarve.c

#include "common_threads.h"
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

//
// Here, you have to write (almost) ALL the code. Oh no!
// How can you show that a thread does not starve
// when attempting to acquire this mutex you build?
//

char *mutexSring = "/mutex";
char *t1Sring = "/t1";
char *t2String = "/t2";

typedef struct __ns_mutex_t {
  int room1; // number of threads in the waiting room one.
  int room2;
  sem_t *mutex; // lock counters
  sem_t *t1;    // turnstile
  sem_t *t2;
} ns_mutex_t;

void ns_mutex_init(ns_mutex_t *m) {
  m->room1 = 0;
  m->room2 = 0;
  m->mutex = Sem_open(mutexSring, 1);
  m->t1 = Sem_open(t1Sring, 1);
  m->t2 = Sem_open(t2String, 0);
}

void ns_mutex_acquire(ns_mutex_t *m) {
  Sem_wait(m->mutex);
  m->room1++;
  Sem_post(m->mutex);

  Sem_wait(m->t1);
  m->room2++;
  Sem_wait(m->mutex);
  m->room1--;

  if (m->room1 == 0) {
    Sem_post(m->mutex);
    Sem_post(m->t2);
  } else {
    Sem_post(m->mutex);
    Sem_post(m->t1);
  }

  Sem_wait(m->t2);
  m->room2--;
}

void ns_mutex_release(ns_mutex_t *m) {
  if (m->room2 == 0)
    Sem_post(m->t1);
  else
    Sem_post(m->t2);
}

ns_mutex_t m;
int counter = 0;

void *worker(void *arg) {
  ns_mutex_acquire(&m);
  counter++;
  ns_mutex_release(&m);
  return NULL;
}

int main(int argc, char *argv[]) {
  assert(argc == 2);
  int num_threads = atoi(argv[1]);
  ns_mutex_init(&m);
  pthread_t p[num_threads];
  printf("parent: begin\n");

  int i;
  for (i = 0; i < num_threads; i++)
    Pthread_create(&p[i], NULL, worker, NULL);

  for (i = 0; i < num_threads; i++)
    Pthread_join(p[i], NULL);

  printf("counter: %d\n", counter);
  printf("parent: end\n");
  Sem_close(m.mutex);
  Sem_close(m.t1);
  Sem_close(m.t2);
  Sem_unlink(mutexSring);
  Sem_unlink(t1Sring);
  Sem_unlink(t2String);
  return 0;
}