What is a synchronization primitive?

When the process calls the send primitive, after the message is sent, the sending process is in a blocking state, and the subsequent statements of the send primitive cannot continue to be executed until the message is sent. When a process calls a receiving primitive, it does not immediately return control, but waits until the message is actually received and put into the designated receiving area before returning control and continuing to execute the subsequent instructions of the primitive. This time has been blocked. The above transmission and reception are called synchronous communication primitives or blocking communication primitives.