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This article mainly talks about "what are the ways of communication between Linux processes". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what are the ways of communication between Linux processes?"

The concept of process

Process is the concept of the operating system, whenever we execute a program, for the operating system to create a process, in this process, accompanied by the allocation and release of resources. It can be considered that a process is an execution of a program.

The concept of process communication

The process user space is independent of each other and generally speaking cannot access each other. However, in many cases, processes need to communicate with each other to complete some function of the system. Processes coordinate their behavior by communicating with the kernel and other processes.

Application scenario of process Communication

Data transfer: one process needs to send its data to another process, the amount of data sent is between one byte and several megabytes.

Shared data: multiple processes want to manipulate shared data, and changes made by one process to shared data should be seen by other processes immediately.

Notification event: one process needs to send a message to another or group of processes informing it (they) that something has happened (such as notifying the parent process when the process terminates).

Resource sharing: sharing the same resources among multiple processes. To do this, the kernel is required to provide locking and synchronization mechanisms.

Process control: some processes want to have complete control over the execution of another process (such as the Debug process). At this time, the control process wants to be able to intercept all traps and exceptions of another process, and be able to know its state change in time.

The way the process communicates

1. Pipeline

Pipes are divided into famous pipes and unnamed pipes.

Unnamed pipe is a half-duplex communication mode, in which data can only flow in one direction and can only be used between related processes. The kinship of process generally refers to the relationship between father and son. Unknown pipes are generally used for communication between two different processes. When a process creates a pipe and calls fork to create its own child process, the parent process closes the read pipe end and the child process closes the write pipe end, which provides a way to flow data between the two processes.

A named pipeline is also a half-duplex mode of communication, but it allows communication between unrelated processes.

two。 Semaphore

A semaphore is a counter that can be used to control the access of multiple threads to shared resources. it is not used to exchange large amounts of data, but for synchronization between multiple threads. It is often used as a locking mechanism to prevent a process from accessing a resource when other processes access it. Therefore, it is mainly used as a means of synchronization between processes and between different threads in the same process.

Linux provides a set of well-designed semaphore interfaces to manipulate signals, not just for binary semaphores. These functions are described below, but note that these functions are used to manipulate groups of semaphore values. They are declared in the header file sys/sem.h.

Semget function

Its function is to create a new semaphore or get an existing semaphore

Semop function

Its function is to change the value of the semaphore.

Semctl function

This function is used to directly control semaphore information.

3. Signal

Signal is a complex way of communication, which is used to notify the receiving process that an event has occurred.

4. Message queue

Message queues are linked lists of messages, stored in the kernel and identified by message queue identifiers. Message queue overcomes the characteristics of less signal transmission information, pipeline can only carry unformatted byte flow and limited buffer size. Message queuing is a mechanism for sharing resources among different processes in UNIX. UNIX allows different processes to send formatted data streams to any process in the form of message queues. Processes with permission to operate on the message queue can use msget to control the operation of the message queue. By using message types, processes can read information in any order or prioritize messages.

5. Shared memory

Shared memory is to map a piece of memory that can be accessed by other processes, which is created by one process, but can be accessed by multiple processes. Shared memory is the fastest IPC (inter-process communication) mode, which is specially designed for the low efficiency of other inter-process communication modes. It is often used with other communication mechanisms, such as semaphores, to achieve synchronization and communication between processes.

6. Socket

Socket, or socket, is a communication mechanism. With this mechanism, the development of the client / server (that is, the process to communicate) can be carried out either on the local stand-alone machine or across the network. In other words, it allows processes that are not on the same computer but are connected to the computer through the network to communicate. Because of this, sockets clearly distinguish between the client and the server.

The characteristics of sockets are determined by three properties: domain, type, and protocol.

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