pthread_exit behavior from main

[Bharath Ramesh <bramesh at vt dot edu>]

I am attaching a small code snippet and I am little bit curious
on the behavior of pthread_exit from main based on the output. I
am passing the address of local stack variable to the threads
main is creating. In the thread function I am printing its value.
The issue I am noticing is that the value printed isnt what I am
expecting it to be "42" for each thread every run. If I run many
instances of the application simultaneously some threads report
"0" instead of "42".

I suspect this behavior is because probably main's stack isnt
valid. I agree that it is bad programming practice to pass a
local stack variable to the threads spawned. I am confused by the
behavior based on what is stated in the pthread_exit manpage [1]
in the notes section. The manpage states "To allow other threads
to continue execution, the main thread should terminate by
calling pthread_exit() rather than exit(3)". Does this imply that
main's stack will be valid till all the threads exit?

I would really appreciate it if somebody could clarify this
behavior I am seeing.

[1] http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_exit.3.html

-- 
Bharath

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

void *printhello(void *arg)
{
	int *pa;

	pa = (int *) arg;
	printf("Hello world: %d\n", *pa);
	pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
	int a;
	pthread_t thread1, thread2;

	a = 42;
	pthread_create(&thread1, NULL, printhello, (void *) &a);
	pthread_create(&thread2, NULL, printhello, (void *) &a);
	pthread_create(&thread2, NULL, printhello, (void *) &a);
	pthread_create(&thread2, NULL, printhello, (void *) &a);

	pthread_exit(NULL);

	return EXIT_SUCCESS;
}

Attachment: smime.p7s
Description: S/MIME cryptographic signature