A deadlock is a state in which each member of a group is waiting for some other member to take action, such as sending a message or, more commonly, releasing a lock, or a resource.

A simple example will help you get the picture. Imagine two little kids playing together. Find a toy that is made of two parts, and give each of them one part. Naturally, neither of them will want to give the other one their part, and they will want the other one to release the part they have. So neither of them will be able to play with the toy, as they each hold half of it, and will indefinitely wait for the other kid to release the other half.

Another example could be having two threads execute the same procedure again. The procedure requires acquiring two resources, A and B, both guarded by a separate lock. Thread 1 acquires A, and Thread 2 acquires B, and then they will wait indefinitely until the other one releases the resource it has. But that won't happen, as they both are instructed to wait and acquire the second resource in order to complete the procedure. Threads can be much more stubborn than kids.

You can solve this problem in several ways. The easiest one might be simply to apply an order to the resources acquisition, which means that the thread that gets A, will also get all the rest: B, C, and so on.

Another way is to put a lock around the whole resources acquisition procedure, so that even if it might happen out of order, it will still be within the context of a lock, which means only one thread at a time can actually gather all the resources.

Let's now pause our talk on threads for a moment, and explore processes.