Variable threads running on basic pthreaded code...not sure why

[sudobangbang]

I've got a simple pthread example. When I run the executable I get a different number of threads running each time when I assumed I should be always generating and running 5 threads.

My compile statement:

$ gcc -Wall ptdetached.c -o runmain -pthread

Here is a sample of the last few runs:

$ ./runmain 
Thread 296150784 started
Thread 296150784 has detached and will end on it's own
Thread 279365376 started
Thread 279365376 has detached and will end on it's own
Thread 287758080 started
Thread 287758080 has detached and will end on it's own
Thread 262579968 started
Thread 262579968 has detached and will end on it's own
Thread 270972672 started
Thread 270972672 has detached and will end on it's own
$ ./runmain 
Thread -100919552 started
Thread -100919552 has detached and will end on it's own
$ ./runmain 
Thread 1415571200 started
Thread 1415571200 has detached and will end on it's own
$ ./runmain 
Thread 469026560 started
Thread 469026560 has detached and will end on it's own
$ ./runmain 
Thread -1016367360 started
Thread -1016367360 has detached and will end on it's own

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

//GLOBALS
#define MAX_THREADS 5

void *threadWorker( void *arg )
{
    //LOCALS
    int ret;

    printf( "Thread %d started\n" , (int)pthread_self() );
    
    ret = pthread_detach( pthread_self() );
    if ( ret != 0 ){
        printf( "error detaching pthread %d\n", (int)pthread_self() );
    }   
    else{  printf( "Thread %d has detached and will end on it's own\n" , (int)pthread_self() ); }
   
    pthread_exit( arg );
}

int main()
{
    //LOCALS
    int ret, i, status;
    pthread_t threadIds[MAX_THREADS];

    for (i=0; i < MAX_THREADS ; i++)
    {   
        ret = pthread_create( &threadIds[i], NULL, threadWorker, (void *)i );  
        if (ret != 0)
        {   
            printf( "error creating thread %d\n" , (int)threadIds[i] );
        }   
     
    }   
    
    return 0;
}

[anduril462]

With pthreads, the thread that spawned the others does not automatically wait for "child" threads to finish before it exits. Couple that with the fact that the pthreads library does not guarantee any particular thread execution order, and you return to your "parent" thread after an arbitrary number of child threads have executed. That parent thread makes no attempt to wait for all it's children to finish, and just exits, killing off all your threads, sometimes before all of them have had a chance to finish.

It's a little bit analogous to what would happen if you forked several processes to do a large load of work, and then immediately exited from the parent. You would have a bunch of orphaned children that init would have to come along and reap. Except with threads, there is no init to reap them and keep them alive until they finish. They simply die off.

[Bayint Naung]

If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes.

https://computing.llnl.gov/tutorials/pthreads/
Exit main function by calling pthread_exit()

[anduril462]

Here's a good tutorial and reference for the pthreads library: https://computing.llnl.gov/tutorials/pthreads/.

Also, if you want to guarantee that all your threads finish before exiting, put a pthread_exit(NULL); at the end of your main function.

EDIT: Beaten by Bayint!