How can I generate threads deadlock on MacOs?

Question

I'm learning to handle threads deadlock, but by executing the C code on my terminal, execution does not generate deadlocks even if it should. Can anyone tell me why and possibly how to force the deadlock?

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

void *func(void *p);

pthread_mutex_t m1,m2;

int main(){

  pthread_t tid[2];
  pthread_attr_t attr;
  pthread_attr_init(&attr);

  int id[2];

  for(int i=0;i<2;i++){
    id[i]=i;
    pthread_create(&tid[i],&attr,func,(&id[i]));
  }

  pause();

}

void *func(void *p){

  int *i=p;
  while(1){
    if(*i==0){
      pthread_mutex_lock(&m1);
      pthread_mutex_lock(&m2);
      printf("I'm Thread %d\n", *i);
      pthread_mutex_unlock(&m1);
      pthread_mutex_unlock(&m2);
    }
    else{
      pthread_mutex_lock(&m2);
      pthread_mutex_lock(&m1);
      printf("I'm Thread %d\n", *i);
      pthread_mutex_unlock(&m1);
      pthread_mutex_unlock(&m2);
    }
  }
  pthread_exit(0);

}

Create two threads (`T1` and `T2`) and two mutexes (`M1` and `M2`). In thread `T1` lock `M1` and then `M2`. In thread `T2` lock `M2` and then `M1`. — Sean Bright, Jun 04 '19 at 20:16
Or even better, show the code and ask why it does not deadlock. — klutt, Jun 04 '19 at 20:17

score 0 · Answer 1 · answered Jun 07 '19 at 08:48

0

Change the assignment of the variable in your thread: int *i=(int *)p;

You need to point to the argument else both threads run the same lock unlock sequence. Where you call pthread_create there is a implicit cast.

answered Jun 07 '19 at 08:48

Gerhard

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score 0 · Answer 2 · answered Jun 09 '19 at 23:49

I could not figure out how to get your code to deadlock. I was able to get the following example to deadlock on macOS:

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

typedef struct {
    const char *mutex_name;
    pthread_mutex_t *pmutex;
} mutex_data_t;

typedef struct {
    const char *thread_name;
    mutex_data_t mutex_list[2];
} thread_data_t;

void *deadlock(void *data) {
    thread_data_t *pthread_data = data;
    const char *name = pthread_data->thread_name;
    printf("%s: Starting...\n", name);
    printf("%s: Locking %s...\n", name, pthread_data->mutex_list[0].mutex_name);
    pthread_mutex_lock(pthread_data->mutex_list[0].pmutex);
    printf("%s: Sleeping...\n", name);
    sleep(1);
    printf("%s: Locking %s...\n", name, pthread_data->mutex_list[1].mutex_name);
    pthread_mutex_lock(pthread_data->mutex_list[1].pmutex);
    printf("Done\n");
    return NULL;
}

int main()
{
    pthread_mutex_t mutex1;
    pthread_mutex_t mutex2;
    mutex_data_t mutex1_data = { "mutex1", &mutex1 };
    mutex_data_t mutex2_data = { "mutex2", &mutex2 };
    thread_data_t thread1_data = { "thread1", { mutex1_data, mutex2_data }};
    thread_data_t thread2_data = { "thread2", { mutex2_data, mutex1_data }};
    pthread_t thread1;
    pthread_t thread2;

    pthread_mutex_init(&mutex1, NULL);
    pthread_mutex_init(&mutex2, NULL);
    pthread_create(&thread1, NULL, deadlock, &thread1_data);
    pthread_create(&thread2, NULL, deadlock, &thread2_data);

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    return 0;
}

Output

thread1: Starting...
thread2: Starting...
thread1: Locking mutex1...
thread2: Locking mutex2...
thread1: Sleeping...
thread2: Sleeping...
thread1: Locking mutex2...
thread2: Locking mutex1...

How can I generate threads deadlock on MacOs?

2 Answers2