This chapter introduces several synchronization methods: semaphore, mutex, and event. It describes inter-thread synchronization. Multiple threads operate and access the same area. This block of code is called the critical section, and the shared memory block in the earlier example is the critical section. A semaphore is a light-duty kernel object that can solve the problems of synchronization between threads. The semaphore value corresponds to the actual number of instances of the semaphore object and the number of resources. The semaphore control block contains important parameters related to the semaphore and acts as a link between various states of the semaphore. A semaphore lock protects an array of critical region resources, ensuring the exclusivity of number taking for the consumers each time, and achieving inter-thread synchronization. Semaphore can also be easily applied to synchronize between the thread and interrupt, such as an interrupt trigger. When interrupting a service routine, the thread needs to be notified to perform corresponding data processing.