• Semaphores were the first and remain one of the most important synchronization tools
• They can be used in two functions:
• to protect critical sections, or/and
• to implement signaling and scheduling
• Semaphores can be implemented using busy-waiting techniques, or using kernel, i.e. interrupts.
• Implemention using busy-waiting techniques has the disadvantage that it wastes CPU time for spin-locking
• The concept and name of a semaphore is motivated by one of the ways in which railroad traffic is synchronized to avoid train collisions
• railroad semaphores can be viewed as mechanisms that signal conditions in order to ensure mutually exclusive occupancy of critical sections of track
• semaphores in concurrent programs are similar: they provide a basic signaling mechanism and are used to implement mutual exclusion and condition synchronization