IPC mechanism

Preface

IPC - Interprocess communication , It is often used in multiprocess programming to share data , System learning , Conceptual things , It is often used in past operations socket Conduct IPC, But other things need to be understood , This article briefly summarizes

communication mode

1. The Conduit

1.1 Anonymous pipeline

scene

cat xx.txt|grep a

Knock over linux Those who command can understand this command , This is the use of anonymous pipes , Anonymous pipes can only pass through fork The way to create a process under the communication , So the information flow is unidirectional

principle

Kernel space unformatted byte stream data ,
Dependent file system , The process ends and is automatically destroyed
One-way communication , Upstream and downstream processes default to Sync
fork The process of will inherit the opened by the parent process fd therefore , Applicable to parent-child process or kinship process

summary

Anonymous pipeline , Its communication range can only exist between father and son / The process of kinship

1.2 name pipes FIFO

scene

Two way information flow

principle

Kernel space unformatted byte stream data
Device file
Create a pipe file in advance , Use mkfifo fifo Create named pipes

summary

Can be used for any interprocess communication

Whether anonymous or named , It is rarely used in actual scenarios , But some specific scenarios may use it more efficiently than others , For example, the transmission of large files on this computer

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2. Message queue

principle

Kernel space message linked list
The message body is user-defined , Each message body is stored in a fixed size

problem

Kernel mode user mode switching and data copying
Release... Must be displayed , Otherwise, the memory will not be released as the process ends , Unless the operating system restarts

summary

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3. Shared memory

principle

Virtual memory maps to the same physical address , Avoid kernel mode user mode switching and data copying

problem

Concurrency issues , Non concurrent secure , Overwriting may occur when multiple processes write data to the same shared memory , How to solve ？ Lock it , namely Semaphore mechanism

4. Semaphore

scene

Used for mutual exclusion and synchronization between processes

principle

The kernel directly manages , Is an integer counter , Realize mutual exclusion and synchronization between processes , Not used to cache communication data between processes , There are two types of ： Binary semaphore , Count the semaphore , Binary semaphores are 1/0 Used to realize mutual exclusion / Sync , The counting semaphore is used for program counting

• Two atomic operations

  • P operation
    Semaphore -1, After subtraction, if the semaphore <0, Indicates that the resource has been occupied , The process needs to block and wait ; After subtraction, if the semaphore >=0, It means that there are still resources available , The process can continue normally .
  • V operation
    Semaphore +1, After addition, if the semaphore <=0, Indicates that there is a blocking process , The process will then wake up and run ; After addition, if the semaphore >0, Indicates that there is no blocking process .

• example

  • Mutex semaphore ： The initial value of semaphore is 1, This mechanism ensures that only one process can access the memory at any time .

    You can see that mutual exclusion requires two factors , ① Semaphore =1 ② The same process executes PV operation

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  • Synchronous semaphore ： The initial value of semaphore is 0, This mechanism guarantees mutual exclusion , Also guaranteed Sync The order

  The figure above shows two processes , The process 1 In the process 2 From the previous execution, we can see that synchronization requires two factors , ① Semaphore =0 ②PV The operation is not in the same process

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  The above two figures mentioned A critical region There is a corresponding Critical resources , How to understand ？
  Critical resources ： At the same time , A resource that only one process or thread is allowed to access
  A critical region ： Code snippet for accessing critical resources , The process in this code / Threads will access shared resources , When another process / When the thread is already running in this code . This process / Threads cannot execute in this code , To control access to critical resources is to control the program to enter the critical area

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5. The signal

asynchronous communication , A number , Signal is the only asynchronous communication mechanism among inter process communication mechanisms

• scene
  kill -9 xx：SIGKILL The signal , End the process immediately
  ctrl+c： SIGINT The signal , To force the execution of a program to be interrupted , Kill the process of the program
  ctrl+z： SIGTSTP The signal , Interrupt tasks , Suspend process , fg Re execution ,bg Directly kill , vim A file can be tested

6. Socket

Cross network and different hosts need to use socket We're communicating , The above 5 A mechanism that is all on the same host
Generally, this kind of is through TCP/UDP agreement