## What it will do

A common use for multiple threads is to do something that takes a long time on a second thread, so that the main thread can continue working.

This program supports 3 commands:

• exit - exits the main thread.
• status - shows how many alarm threads are pending.
• alarm n - sets an alarm for n seconds from now.
• when the alarm goes off, it prints a message.

This seems like a silly program, but the way it works is exactly how more complex programs work. Once you learn this pattern, you can apply it to many other kinds of problems, like:

• doing complicated calculations in the background
• periodically reminding the user of something
• pretty much any long-running task in a GUI application

Here is an example interaction with the program:

$./lab8 > status 0 alarm(s) pending. > alarm 5 > alarm 8 > status 2 alarm(s) pending. > (RING RING!) status 1 alarm(s) pending. > sta(RING RING!)tus 0 alarm(s) pending. > alarm 3 > exit still 1 alarm(s) pending... (RING RING!)$


Above, you can see it act kind of funny… the (RING RING!) are the alarms going off. Sometimes, they go off in the middle of the user typing something. Yep! That’s the point! :D

## What to do

Compile it like so: gcc -lpthread -Wall -Werror --std=c99 -o lab8 lab8.c

Don’t forget the -lpthread gcc flag!

If you compile and run it right now, it won’t do much. The functions you have to implement are at the bottom. See below for details.

• Refer to the threading examples I gave in class on the examples page, particularly 21_thread_test.c.
• Some of the functions really are “that easy” and only take a couple lines of code.
• If you are only reading a shared global variable, technically you don’t have to lock its mutex…
• You can print out the \a escape character to make your console go “ding” :)

Don’t do everything at once. Implement and test each function below in the order given.

• exit_main_thread() - for the exit command
• If there are any threads running, it should say how many there are left.
• Then you can safely exit the main thread with pthread_exit(). Look it up!
• When you use this, the other threads will keep running.
• If there are no other threads running, then this behaves like normal exit().
• show_status() - for the status command
• This should show how many alarm threads are running.
• I gave you the num_threads variable to count them.
• change_thread_counter(int delta)
• This should properly lock/unlock the mutex around its code.
• See 22_thread_cooperating.c in the examples.
• set_alarm(long duration) - for the alarm command
• See 21_thread_test.c to see how we “sneak” an integer into the thread’s main function.
1. sleep()
• use fflush(stdout) after printing, if you don’t use \n!
3. decrement the number of threads using change_thread_counter