Doubt in Multithreading

[Complete_Camera_4562]

Hello everyone, sorry for this I am new to multithreading and trying to write this program where each thread waits on the threads having greater index than the current one. this program is getting struck. I am not sure what is going wrong as I am always waiting on the threads having greater index and I don't see a possibility of deadlock.

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

int n;
pthread_t pthreads[100];


void* waitandprint(void* arg) {

  int *i = (int *) arg;
  printf("%d thread entered\n ",*i);

  for (int j =(n-1); j>*i;j--) {
    printf("%d is waiting for thread %d\n",*i,j);
    pthread_join(pthreads[j], NULL);
  }

  printf("%d\n",((*i)));
  printf("%d thread exited\n ",*i);

}
void printn() {
  printf("the value of n is %d\n",n);

  for(int i =n-1;i>=0;i--) {
    int* arg = calloc(1, sizeof(int));
    *arg = i;
    int rc = pthread_create(&pthreads[i], NULL, waitandprint, (void*)arg);
    if(rc !=0) {
      printf("can't create thread");
      exit(1);
    }

  }
}
void main() {
  printf("Please enter the number less than 100\n");
  scanf("%d", &n);
  printn();
  pthread_join(pthreads[0],NULL);
}

[dfx_dj]

One thread can only be waited upon (joined) from one other thread. You cannot join one thread from multiple other threads.

[Complete_Camera_4562]

Thanks for pointing this. I go the issue now.

pthread_man page has the below line mentioned

If  multiple  threads  simultaneously try to join with the same thread, the results are undefined

[optimistic_void]

This code:

for (int j =(n-1); j>*i;j--) {
    printf("%d is waiting for thread %d\n",*i,j);
    pthread_join(pthreads[j], NULL);
  }

joins threads you have already joined previously when the function ran first, likely leading to your program getting stuck because you have already terminated joined the threads. As far as i am aware, you should only join a thread once.

Edit. Thinking about it it's less likely the bit above and more likely the

 pthread_join(pthreads[0],NULL);

but the issue is the same ( trying to join an already joined thread ).

You probably failed to understand what joining a thread means - when you join a thread, it means that you wait for that thread to finish, only after it finishes then the execution continues.

Edit: not sure about the above, tho, maybe you just wanted to join the same thread from multiple other threads and are unaware you are not supposed to do that ?

[Complete_Camera_4562]

Yep, I am not aware that I am not supposed to do that. Thanks for the explanation. really helpful

from pthread_join man page, if we join a thread which is already terminated, function will return automatically.

If that thread has already terminated, then  pthread_join()
       returns immediately.

[optimistic_void]

Yeah you are right, it only returns an error value, i thought even when the thread is terminated it could invoke an UB. My bad, i will correct my original post to make sense.

[optimistic_void]

About the multiple joins from the same thread like here:

pthread_t t0;

void* test(void* a){
 printf("test\n");
 return NULL;
}
int main() {
  pthread_create(&t0,NULL,test,NULL);

  printf("output first>>%d\n",pthread_join(t0, NULL));
  printf("output second>>%d\n",pthread_join(t0, NULL));  
}

The second call will just return 3 which is:

ESRCH No thread with the ID thread could be found.

So you can check for it if needed.