What are the methods of process synchronization?

What are the ways of process synchronization ？
A critical region

The critical area is a piece of code , Within a critical region, processes will access critical resources . At most one process can enter the critical region at any time , in other words , The critical region is exclusive . therefore , For mutual exclusive access to critical resources , Before each process enters the critical area , It needs to be checked first .

The mutex

Is to use a mutually exclusive variable to directly restrict multiple processes , Each process has access to public resources only if it has this variable , Because there is only one mutex , So it can ensure the correct access to resources .

Semaphore

Semaphore （Semaphore） It's an integer variable , You can perform auto increment and auto decrement operations on it , Auto subtraction is also known as P operation , Auto increment operation is also called V operation . These two operations need to be designed as primitives , It's indivisible , The usual way to do this is to mask interrupts . Processes use these two operations to synchronize .

about P operation , If the semaphore is less than 0, Then the process performing the operation will block , Otherwise, continue to execute ;
about V operation , If the semaphore after operation is less than or equal to 0, Then it wakes a process from the blocking queue .
Tube side

Management uses object-oriented ideas , It will represent the data structure of the shared resource and related operations , Including synchronization mechanism , It's all concentrated and packaged together . All processes can only access critical resources indirectly through the pipe , The tube only allows one process to enter and perform operations , To achieve mutual exclusion of processes . Multiple condition variables are set in the tube , Indicates the condition under which multiple processes are blocked or suspended . Execute... On conditional variables wait() Operation will cause the calling process to block , Let the process out to another process to hold .signal() Operations are used to wake up blocked processes . The tube has an important feature , That is, only one process can use the tube at a time . The process can't always occupy the pipe when it can't continue to execute , Otherwise, other processes will never be able to use pipelining .

What are the ways of interprocess communication ？
The Conduit
The pipe is half duplex , Data can only flow in one direction ; If communication between the two parties is needed , Two pipes need to be built .
Pipes can only be used between parent-child processes or sibling processes, or processes with kinship ;
A pipe is a process at both ends of a pipe , It's a document , But it's not a normal document , It doesn't belong to some kind of file system , It only exists in memory .
The essence of the pipeline is a kernel buffer , The process accesses data from the buffer in a first in, first out fashion , The process at one end of the pipeline writes data to the buffer in sequence , The process at the other end reads the data in sequence . This buffer can be seen as a circular queue , The positions of reading and writing are automatically increasing , Can't change at will , A data can only be read once , After reading it out, the buffer no longer exists . When the buffer is empty or full , There are certain rules to control the corresponding read process or write process to enter the waiting queue , When the empty buffer has new data written or the full buffer has data read out , Just wake up and wait for the process in the queue to continue reading and writing .
The main limitation of pipeline lies in its characteristics , For example, only one-way data flow is supported , Can only be used between related processes , Isn't there a name , The buffer of the pipeline is limited and so on .
name pipes

This kind of pipe is also called FIFO. Naming pipes is different from pipes , It provides a pathname associated with it , Exists in the file system as a named pipe file , such , Even if the process is not related to the creation process of the named pipe , As long as you can access this path in the file system , You can communicate with each other through named pipes . Named pipes strictly follow the first in first out principle , It does not support data random location . The name of the named pipe exists in the file system , But the content is stored in memory .

Message queue

The message queue is a linked list of messages , Have a specific format , It's stored in memory , And each message queue has a unique identifier . Message queuing allows one or more processes to write and read messages to it , therefore , Using message queues , A process can send a block of data to another process , Each chunk has a type , The receiving process can independently receive data structures with different types , This process is asynchronous , We can avoid synchronization and blocking of named pipes by sending messages . But there is a limit to the size of the data block of the message queue .