how to replace pthread_join() and pthread_create() by clone() [duplicate]

Question

This question already has an answer here:

How to wait for threads to finish their work, where threads created by clone in c? (1 answer)

Closed 9 years ago.

No doubt pthread_create() calls to clone, but is it possible to modify program having pthread_join()?

Actualy I am trying to modify this code to use clone()

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

void *childfun (void *para)
{
    sleep(2);
    printf("child terminating\n");
}

int main (void)
{
    void * stackptr;
    pthread_t readthread;
    pthread_create(&readthread,NULL,childfun,NULL);
    pthread_join(readthread,NULL);
    printf("exit\n");
}

first I confused which flag to use for clone,then i watched strace output of above code and replaced my main function by

int main (void)
{
    int ctid;
    void *stackptr;
    stackptr = malloc(getpagesize());
    ctid = clone(childfun , stackptr+getpagesize() , CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|   CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID,NULL);
    printf("exit\n");
}

but here main thread is terminating before new thread. How to achieve pthread_join functioning?

Answer 1

As alk said, if you use CLONE_THREAD you can not use wait() to wait for your thread to finish.

A new thread created with CLONE_THREAD has the same parent process as the caller of clone() (i.e., like CLONE_PARENT), so that calls to getppid(2) return the same value for all of the threads in a thread group. When a CLONE_THREAD thread terminates, the thread that created it using clone() is not sent a SIGCHLD (or other termination) signal; nor can the status of such a thread be obtained using wait(2). (The thread is said to be detached.)

The man page also tells us:

After all of the threads in a thread group terminate the parent process of the thread group is sent a SIGCHLD (or other termination) signal.

So if you have to use CLONE_THREAD you could use pause() or some other signal handling mechanism to wait for the whole thread group to finish.

...
    ctid = clone(childfun , stackptr+getpagesize() , CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|   CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID,NULL);
    pause();
    printf("exit\n");
}

If you dont need to create a new thread group (e.g. don't use CLONE_THREAD), you can use wait() as you are used to from 'normal' process handling:

...
    ctid = clone(childfun , stackptr+getpagesize() , CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND |CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID,NULL);

     ctid = waitpid( ctid, 0, 0 );
     if ( ctid == -1 ){             
        perror( "waitpid" );
        exit( 3 ); 
    }
}  

Hope this helps!