Counting semaphore.

This module implements counting semaphores on top of protothreads. Semaphores are a synchronization primitive that provide two operations: acquire() and release(). Sometimes these operations are also called wait and signal. The acquire() operation checks the semaphore counter and "blocks" the thread if the counter is zero. The release() operation increases the semaphore counter but does not block. If another thread has blocked waiting for the semaphore that is signalled, the blocked thread will become runnable again.

This class can for example be used for resource limiting or credit tracking:

Resource limit:

• initial = number of resources
• acquire() to gain access to a resource
• release() to release the resource

Credit tracking:

• initial = 0
• acquire() to increment (credit)
• release() to consume (debit)

As protothreads implement a form cooperative multithreading no synchronisation is needed for accessing global variables or for critical sections. Just don't use PT_YIELD() or PT_WAIT_UNTIL() inside a critical sections and you're fine.

Therefore there's no Mutex implementation, it isn't needed.