[c] When to use pthread_exit() and when to use pthread_join() in Linux?

I am new to pthreads, and I am trying to understand it. I saw some examples like the following.

I could see that the main() is blocked by the API pthread_exit(), and I have seen examples where the main function is blocked by the API pthread_join(). I am not able to understand when to use what?

I am referring to the following site - https://computing.llnl.gov/tutorials/pthreads/. I am not able to get the concept of when to use pthread_join() and when to use pthread_exit().

Can somebody please explain? Also, a good tutorial link for pthreads will be appreciated.

#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: creating thread %ld\n", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         exit(-1);
      }
   }

   /* Last thing that main() should do */
   pthread_exit(NULL);

Realized one more thing i.e.

pthread_cancel(thread);
pthread_join(thread, NULL);

Sometimes, you want to cancel the thread while it is executing. You could do this using pthread_cancel(thread);. However, remember that you need to enable pthread cancel support. Also, a clean up code upon cancellation.

thread_cleanup_push(my_thread_cleanup_handler, resources);
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, 0);

static void my_thread_cleanup_handler(void *arg)
{
  // free
  // close, fclose
}

Hmm.

POSIX pthread_exit description from http://pubs.opengroup.org/onlinepubs/009604599/functions/pthread_exit.html:

After a thread has terminated, the result of access to local (auto) variables of the thread is 
undefined. Thus, references to local variables of the exiting thread should not be used for 
the pthread_exit() value_ptr parameter value.

Which seems contrary to the idea that local main() thread variables will remain accessible.

When pthread_exit() is called, the calling threads stack is no longer addressable as "active" memory for any other thread. The .data, .text and .bss parts of "static" memory allocations are still available to all other threads. Thus, if you need to pass some memory value into pthread_exit() for some other pthread_join() caller to see, it needs to be "available" for the thread calling pthread_join() to use. It should be allocated with malloc()/new, allocated on the pthread_join threads stack, 1) a stack value which the pthread_join caller passed to pthread_create or otherwise made available to the thread calling pthread_exit(), or 2) a static .bss allocated value.

It's vital to understand how memory is managed between a threads stack, and values store in .data/.bss memory sections which are used to store process wide values.

The pthread_exit() API

as has been already remarked, is used for the calling thread termination. After a call to that function a complicating clean up mechanism is started. When it completes the thread is terminated. The pthread_exit() API is also called implicitly when a call to the return() routine occurs in a thread created by pthread_create(). Actually, a call to return() and a call to pthread_exit() have the same impact, being called from a thread created by pthread_create().

It is very important to distinguish the initial thread, implicitly created when the main() function starts, and threads created by pthread_create(). A call to the return() routine from the main() function implicitly invokes the exit() system call and the entire process terminates. No thread clean up mechanism is started. A call to the pthread_exit() from the main() function causes the clean up mechanism to start and when it finishes its work the initial thread terminates.

What happens to the entire process (and to other threads) when pthread_exit() is called from the main() function depends on the PTHREAD implementation. For example, on IBM OS/400 implementation the entire process is terminated, including other threads, when pthread_exit() is called from the main() function. Other systems may behave differently. On most modern Linux machines a call to pthread_exit() from the initial thread does not terminate the entire process until all threads termination. Be careful using pthread_exit() from main(), if you want to write a portable application.

The pthread_join() API

is a convenient way to wait for a thread termination. You may write your own function that waits for a thread termination, perhaps more suitable to your application, instead of using pthread_join(). For example, it can be a function based on waiting on conditional variables.

I would recommend for reading a book of David R. Butenhof “Programming with POSIX Threads”. It explains the discussed topics (and more complicated things) very well (although some implementation details, such as pthread_exit usage in the main function, not always reflected in the book).

You don't need any calls to pthread_exit(3) in your particular code.

In general, the main thread should not call pthread_exit, but should often call pthread_join(3) to wait for some other thread to finish.

In your PrintHello function, you don't need to call pthread_exit because it is implicit after returning from it.

So your code should rather be:

void *PrintHello(void *threadid)  {
  long tid = (long)threadid;
  printf("Hello World! It's me, thread #%ld!\n", tid);
  return threadid;
}

int main (int argc, char *argv[]) {
   pthread_t threads[NUM_THREADS];
   int rc;
   intptr_t t;
   // create all the threads
   for(t=0; t<NUM_THREADS; t++){
     printf("In main: creating thread %ld\n", (long) t);
     rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
     if (rc) { fprintf(stderr, "failed to create thread #%ld - %s\n",
                                (long)t, strerror(rc));
               exit(EXIT_FAILURE);
             };
   }
   pthread_yield(); // useful to give other threads more chance to run
   // join all the threads
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: joining thread #%ld\n", (long) t);
      rc = pthread_join(&threads[t], NULL);
      if (rc) { fprintf(stderr, "failed to join thread #%ld - %s\n",
                                (long)t, strerror(rc));
               exit(EXIT_FAILURE);
      }
   }
}

  #include<stdio.h>
  #include<pthread.h>
  #include<semaphore.h>
 
   sem_t st;
   void *fun_t(void *arg);
   void *fun_t(void *arg)
   {
       printf("Linux\n");
       sem_post(&st);
       //pthread_exit("Bye"); 
       while(1);
       pthread_exit("Bye");
   }
   int main()
   {
       pthread_t pt;
       void *res_t;
       if(pthread_create(&pt,NULL,fun_t,NULL) == -1)
           perror("pthread_create");
       if(sem_init(&st,0,0) != 0)
           perror("sem_init");
       if(sem_wait(&st) != 0)
           perror("sem_wait");
       printf("Sanoundry\n");
       //Try commenting out join here.
       if(pthread_join(pt,&res_t) == -1)
           perror("pthread_join");
       if(sem_destroy(&st) != 0)
           perror("sem_destroy");
       return 0;
   }

Copy and paste this code on a gdb. Onlinegdb would work and see for yourself.

Make sure you understand once you have created a thread, the process run along with main together at the same time.

1. Without the join, main thread continue to run and return 0
2. With the join, main thread would be stuck in the while loop because it waits for the thread to be done executing.
3. With the join and delete the commented out pthread_exit, the thread will terminate before running the while loop and main would continue
4. Practical usage of pthread_exit can be used as an if conditions or case statements to ensure 1 version of some code runs before exiting.

void *fun_t(void *arg)
   {
       printf("Linux\n");
       sem_post(&st); 
       if(2-1 == 1)  
           pthread_exit("Bye");
       else
       { 
           printf("We have a problem. Computer is bugged");
           pthread_exit("Bye"); //This is redundant since the thread will exit at the end
                                //of scope. But there are instances where you have a bunch
                                //of else if here.
       }
   }

I would want to demonstrate how sometimes you would need to have a segment of code running first using semaphore in this example.

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

sem_t st;

void* fun_t (void* arg)
{
    printf("I'm thread\n");
    sem_post(&st);
}

int main()
{
    pthread_t pt;
    pthread_create(&pt,NULL,fun_t,NULL);
    sem_init(&st,0,0);
    sem_wait(&st);
    printf("before_thread\n");
    pthread_join(pt,NULL);
    printf("After_thread\n");
    
}

Noticed how fun_t is being ran after "before thread" The expected output if it is linear from top to bottom would be before thread, I'm thread, after thread. But under this circumstance, we block the main from running any further until the semaphore is released by func_t. The result can be verified with https://www.onlinegdb.com/

Both methods ensure that your process doesn't end before all of your threads have ended.

The join method has your thread of the main function explicitly wait for all threads that are to be "joined".

The pthread_exit method terminates your main function and thread in a controlled way. main has the particularity that ending main otherwise would be terminating your whole process including all other threads.

For this to work, you have to be sure that none of your threads is using local variables that are declared inside them main function. The advantage of that method is that your main doesn't have to know all threads that have been started in your process, e.g because other threads have themselves created new threads that main doesn't know anything about.

pthread_exit() will terminate the calling thread and exit from that(but resources used by calling thread is not released to operating system if it is not detached from main thread.)

pthrade_join() will wait or block the calling thread until target thread is not terminated. In simple word it will wait for to exit the target thread.

In your code, if you put sleep(or delay) in PrintHello function before pthread_exit(), then main thread may be exit and terminate full process, Although your PrintHello function is not completed it will terminate. If you use pthrade_join() function in main before calling pthread_exit() from main it will block main thread and wait to complete your calling thread (PrintHello).

Using pthread_exit in the main thread(in place of pthread_join), will leave the main thread in defunct(zombie) state. Since not using pthread_join, other joinable threads which are terminated will also remain in the zombie state and cause resource leakage.

Failure to join with a thread that is joinable (i.e., one that is not detached), produces a "zombie thread". Avoid doing this, since each zombie thread consumes some system resources, and when enough zombie threads have accumulated, it will no longer be possible to create new threads (or processes).

Another point is keeping the main thread in the defunct state, while other threads are running may cause implementation dependent issues in various conditions like if resources are allocated in main thread or variables which are local to the main thread are used in other threads.

Also, all the shared resources are released only when the process exits, it's not saving any resources. So, I think using pthread_exit in place of pthread_join should be avoided.

pthread_exit terminates the calling thread while pthread_join suspends execution of calling thread until target threads completes execution.

They are pretty much well explained in detail in the open group documentation:

• pthread_exit
• pthread_join