Consider the following code using the POSIX Pthreads API:

#include #include #include #include

int myglobal = 0;

void* thread_function(void* arg)

{

int i, j;

for(i= 0; i < 20; i++)

{

j = myglobal;

j = j + 1;

printf(".");

fflush(stdout);

sleep(1);

myglobal = j;

}

return NULL;

}

int main(void)

{

pthread_t mythread;

int i;

if (pthread_create(&mythread, NULL, thread_function, NULL) != 0)

{

printf("error creating thread.");

abort(); }

for(i = 0; i < 20; i++)

{

3

myglobal= myglobal + 1;

printf("o");

fflush(stdout);

sleep(1);

}

if (pthread_join(mythread,NULL))

{

printf("error joining thread.");

abort(); }

printf(" myglobal equals %d ", myglobal);

exit(0); }

In main() we first declare a variable called mythread, which has a type of pthread_t. This is essentially an ID for a thread. Next, the if statement creates a thread associated with mythread. The call pthread_create() re- turns zero on success and a nonzero error code on failure. The third argument of pthread_create() is the name of a function that the new thread will ex- ecute when it starts. When this thread_function() returns, the thread terminates. Meanwhile the main program itself denes a thread, so there are two threads executing. The pthread_join() function causes the main thread to wait until the new thread completes.

Questions

1. What does this program accomplish? Please describe what happens with the global value myglobal. What does the thread_function() do and when does it run? What is the main() funciton doing? What is sleep() and what purpose does it serve?

2. Here is output from the above program being executed
3. $ ./thread2
4. ..o.o.o.o.oo.o.o.o.o.o.o.o.o.o..o.o.o.oo
5. myglobal equals 21

Is this the output you would expect? If not, what has gone wrong and how would you fix it (please show code)?