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Semaphores are primitive yet powerful and flexible tool for enforcing mutual exclusion and for coordinating processes. However, it may be difficult to produce a correct program using semaphores. i.e., sem_wait() and sem_signal() may be scattered throughout a program, and it is not easy to see the overall effect of these operations on the semaphores they affect.
The monitor is a programming language construct that provides equivalent functionality to semaphores, but is easier to control and verify.
A monitor is a software module consisting one or more procedures, an initialization sequence and local data.
The local data variables are accessible only by the monitor's procedures and not any external procedure (as private attributes in a class object).
A process enters the monitor by invoking one of its procedures.
In basic monitors, only one process may be executing in the monitor at a time, the others block to wait (i.e., mutual exclusion).
Still possible to cause deadlock if programmed incrrectly.
Monitors use condition variables to manage synchronization.
Condition variables are internal to the monitor (i.e., accessible only within the monitor), and therefore, cannot be scattered elsewhere in the code.
Condition variables are a special data type in monitors, which are operated on by two functions:
cwait(c)
Suspend execution of the calling process on condition c. The monitor is now available for use by another process.
csignal(c)
Resume execution of some process blocked after a cwait() on the same condition. If there are several such processes, choose one of them; if there are none, do nothing.
[!] Note: Note that wait() and signal() for condition variables are different from those of Semaphores'. In POSIX Threads library they are pthread_cond_wait() and pthread_cond_signal(), respectively. See Thread Synchronization - Condition Variables.
Examples:
Producer/consumer would use a monitor for the buffer data structure and the condition variables would be isFull and isEmpty. Still only one can be in the monitor at a time.
All Java objects can behave as a monitor. They have a single built in condition variable.
Use the synchronized keyword to indicate that Java threads using an object should wait for each other.
Java also has other mechanisms for synchronizing threads, this is just one of them!
Stallings, W. (2018). Operating Systems: Internals and Design Principles (9th ed.). Pearson Education, Inc.