pthreads-2: Creating a detachable thread

In the post "pthread creation" we saw how to create a simple thread and we had used the default attributes for the thread.
One of the default attribute is that the thread is creates always as a joinable i.e. the parent thread has to call pthread_join for the thread to successfully terminate.
On the other hand if we do not want to call pthread_join from the parent, but let the child thread finish and in then inform the parent, then we will have to change the attribute to "detachstate".
If we create a thread in the "detachable" state, then the parent thread need not call pthrea_join, it will just have to wait till the thread exits and informs about it .
The two states supported by POSIX are

PTHREAD_CREATE_JOINABLE: Which is the default state. PTHREAD_CREATE_DETACHED: Which will have to be set specifically if needed.

To set the attributes to default value we can use

pthread_attr_t attr; pthread_attr_init(&attr);

Now to modify the detachstate from joinable we will use the function:

int pthread_attr_setdetachstate(pthread_attr_t *attr, int detachstate);

Arguments:

attr: The attribute initialized previously detachstate: Either PTHREAD_CREATE_JOINABLE or PTHREAD_CREATE_DETACHED.

Thus to modify the default initialization we will use

pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_DETACHED).

Now we can create a thread using the above attribute variable and we need not call pthread_join on this thread.

pthread_create(thread_id,&attr,&hello,NULL);

Note: hello is the same function we defined in the post "pthread creation"
The full code will look as follows.
detachable_thread.c

#include <stdio.h> #include <pthread.h> #include <stdlib.h> void * hello(void *temp) { char *ret; ret="Finished"; printf("\n Hello from thread \n"); exit(0); } main() { pthread_t thread_id; pthread_attr_t attr; int ret; void *res; ret = pthread_attr_init(&attr); if(ret != 0) { printf("Error initalizing attributes"); exit(0); } pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_DETACHED); ret=pthread_create(&thread_id,&attr,&hello,NULL); if(ret != 0) { printf("Error creating thread"); exit(0); } printf("\n Created thread"); pause(); }

Please note that the function "hello" has the statement

exit(0)

This is because the parent is waiting for the child to exit and does not call join to check the child's status.
Now compile it using the options -pthread

cc-pthread detachable_thread.c -o detachable_thread

Execute it :

./detachable_thread

Output:

Created thread Hello from thread

To see the effect of creating a detachable thread, remove the exit from the function "heelo" and the execute the program. We will see that the process goes into an infinite wait as the parent thread keeps waiting for the child thread to call the exit.