Purpose: You can use a semaphore to limit the number of threads accessing a shared resource
Meaning: Semaphores refer to objects that control access to common resources. Before accessing the resource, the thread must obtain a permission from the semaphore. After accessing the resource, the thread must return the permission to the semaphore. Once permission is granted, the total number of available permissions in the semaphore is decremented by 1. Once the permission is released, the number of permissions in the semaphore is increased by 1.
# Only one permitted semaphore can be used to simulate a mutually exclusive lock.
Example:Similarities and differences between locks and semaphores Where?
(The lock here refers to the mutex lock)
Scope
Mutex lock: inter-thread
Execution when locking
Mutex lock: Only one object is locked (locked), then no other thread can access the locked object.
One is synchronization and the other is mutual exclusion
Mutex lock: used for mutual exclusion in multi-threads and multi-tasks. After a thread occupies a certain resource, other threads cannot access it. Only when this thread is unlocked can other threads use it.
In other words, the semaphore is not necessarily about locking a certain resource, but a process concept. For example: there are two threads A and B, and thread B has to wait for thread A to complete. After a certain task, you can perform the following steps. This task does not necessarily involve locking a certain resource, but can also perform some calculations or data processing. The thread mutex is the concept of "locking a certain resource". During the lock period, other threads cannot operate on the protected data. In some cases the two are interchangeable.
Related articles:
Examples of mutex semaphores and multi-thread waiting mechanisms in Java
Detailed explanation of basic usage examples of PHP semaphores
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