How can I make the parent thread wait

Applications Programming
1

Hi All,

I will be glad if you could help me resolve this problem.

I have created two detachable threads and wanted to them execute independent of the parent thread ( the main task which creates the detachable threads). But I see no output coming from the execution of two detachable threads. When I debugged the application, I found that the parent thread terminates before the newly created threads complete the execution and this causes the child threads to terminate without completing their job. I have no idea what is causing the detachable threads to stop their execution. As far as I know we should be able to execute the detachable threads irrespective of the parent thread status(of course there is nothing like parent child hierarchy in posix lib by default) , please correct me if I am wrong.

Following is the sample code to reproduce the problem.

class Sample
{
public:
Sample()
{ cout <<"Sample:: constructor \n"; }
~Sample()
{ cout <<"Sample :: destructor\n"; }

};

void* ThreadFun(void * data)
{
Sample s1;
cout<<" ThreadFun "<<*((int*)(data))<<"\n";
}

int main()
{
pthread_t tid1,tid2;
pthread_attr_t attr,attr1;
int i=11;
int a =10, b=20;
cout<<" from main \n";
pthread_attr_init(&attr);
pthread_attr_init(&attr1);
pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_DETACHED);
pthread_attr_setdetachstate(&attr1,PTHREAD_CREATE_DETACHED);

int j=pthread_create(&tid1,&attr,&ThreadFun,(void*)&a);

j=pthread_create(&tid2,&attr1,&ThreadFun,(void*)&b);

pthread_attr_destroy(&attr);
pthread_attr_destroy(&attr1);

//sleep(2);
return 0;
}

OUTPUT:
from main

And If I remove the comments at "sleep(2)" , then it works fine. Following is the output with sleep statement.

from main
Sample:: constructor
ThreadFun 10
Sample :: destructor
Sample:: constructor
ThreadFun 20
Sample :: destructor

2

You have two problems (1) the pthreads model and API is defined for the C language but not for C++ and (2) you are creating your threads as DETACHED. Therefore they are not JOINABLE and thus main() does not and cannot wait for your threads to terminate.

3

Thank you for your response Murphy!

I have rewritten the code in C, but it doesn't seem to help.

My primary intention is not to have the parent wait for other threads, but to execute the detachable threads. As I did not find any other alternative mechanism to execute the detachable threads, I introduced sleep statement; it worked . However, I would like to replace the sleep() with some function of Pthread lib or find a way to execute the detachable threads even if the parent terminates.

4

main() isn't exactly a parent thread. exit() is called when it returns, that kills the process, and you can't have threads without a process. If you want things to be truly independent you need to create new processes.

If you just want main() to wait, don't make them detachable and use pthread_join() like everyone says, that's what it's for.

5

Thanks Corona ! I got it. I forgot the basic concept. All these threads execute in the address space of the process and these threads will die when the process dies.