Semaphores
Last updated
Last updated
A semaphore is a synchronization mechanism, defined by a data type plus two atomic operations, down and up.
Data type:
Operations:
down
block process if val is zero.
decrement val otherwise.
up
if queue is not empty, wake up one waiting process (accordingly to a given policy).
increment val otherwise.
Note that val can only be manipulated through these operations.
It is not possible to check the value of val.
Internally, the block_on_sem function must enable interruptions.
This implementation is typical of uniprocessor systems. Why?
Semaphores can be binary or not binary.
How to implement mutual exclusion using semaphores?
Using a binary semaphore.
How to implement using semaphores?
guaranteeing mutual exclusion and absence of busy waiting.
fifo.notEmpty()and fifo.notFull() are no longer necessary. Why?
One can easily make a mistake.
What is wrong with this solution? It can cause deadlock.
Concurrent solutions based on semaphores have advantages and disadvantages.
Advantages:
Support at the operating system level – operations on semaphores are implemented by the kernel and made available to programmers as system calls.
General – they are low level contructions and so they are versatile, being able to be used in any type of solution.
Disadvantages:
Specialized knowledge – the programmer must be aware of concurrent programming principles, as race conditions or deadlock can be easily introduced.
