What three parts does the linux process include?

The three parts of the Linux process: 1. The process control block makes a program (including data) that cannot run independently in a multi-program environment become a basic unit that can run independently, and can be combined with other Processes that are executed concurrently; 2. Program segment is the program code segment in the process that can be executed by the process scheduler on the CPU; 3. Data segment is the data segment of a process, which can be the original processed by the program corresponding to the process. Data can also be intermediate or final data generated after program execution.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

1. Definition of process

A process (Process) is a program in the computer that performs a running activity on a certain data collection. It is a system The basic unit for resource allocation is the basis of the operating system structure.

In short, it is the execution process of program.
A process is an instance of a running program, that is, a task that is being executed.
The process has a life cycle. It is created as the program runs and terminates as the program ends.
The process is the smallest unit for allocating resources. As long as a process is created, [0-3G] of user space is allocated.
As long as the user executes a program, the kernel will create a task_struct (PCB) structure, which represents the current process.
Maintains its own set of file descriptors and buffers within the process. As soon as the process execution ends, all its resources will be reclaimed by the operating system.

Time slice polling to achieve concurrency

2. Characteristics of the process

• Dynamicity: The essence of a process is the execution of a program in a multi-programming system. The process is dynamically generated and dies dynamically.

• Concurrency: Any process can execute concurrently with other processes

• Independence: A process is a basic unit that can run independently. It is also an independent unit for system resource allocation and scheduling;

• Asynchronicity: Due to the mutual constraints between processes, the process has intermittent execution, that is, the processes are independent and unpredictable. Advance at a faster speed

Multiple different processes can contain the same program: a program constitutes different processes in different data sets and can obtain different results; but execution During the process, the program cannot be changed.

3. The composition and function of the process

The composition of the process consists of three parts: process control block PCB (task_struct), data segment , program segment.

Process control block: Make a program (including data) that cannot run independently in a multiprogramming environment become a basic unit that can run independently, and a process that can execute concurrently with other processes.
Program segment: It is the program code segment in the process that can be executed on the CPU by the process scheduler.
Data segment: The data segment of a process can be the original data processed by the program corresponding to the process, or the intermediate or final data generated after the program is executed.

4. Process control block

##4.1 Process control block definition

In order to describe the operation of the control process, the data structure in the system that stores the management and control information of the process is called the process control block (PCB Process Control Block). It is part of the process entity and is a part of the operating system. The most important record-keeping data structure. It is the most important data structure for process management and control. Each process has a PCB. When a process is created, the PCB is established and accompanies the entire process of running the process until the process is cancelled.

The essence of PCB is a structure. The name of PCB is different in different operating systems. In Linux, the PCB is called task_struct, and the PCB is the only means of controlling the process. Each process has a process descriptor. This "process descriptor" is task_struct. A lot of information about process control is stored in task_struct.

4.2 Contents of task_struct

Each process puts its information in the task_struct data structure. The task_struct contains the following content:

• Identifier (pid): The unique identifier that describes this process and is used to distinguish other processes.

• Status: task status, exit code, exit signal, etc.

• Priority: relative to the priority of other processes (the smaller the number, the higher the priority).

• Program counter: The address of the next instruction to be executed in the program.

• Memory pointers: including pointers to program code and process-related data, as well as pointers to memory blocks shared with other processes.

• Context data: Saving the context is to save the value in the CPU register to the memory; restoring the context is to restore the register value in the memory to the CPU;

• I/O status information: includes displayed I/O requests, I/O devices assigned to the process and a list of files being used by the process.

• Accounting information: may include the total processor time, the total number of clocks used, time limits, accounting accounts, etc.

• Other information

##5. The difference between process and program

• A process is an execution process of a program. It is dynamic, has a life cycle, and is stored in memory.

• The program is static and has no life cycle. Stored on disk, a program is an executable file.

• Processes can more truly describe concurrency, but programs cannot.

• Processes have the ability to create other processes, but programs do not.

• The same program can correspond to multiple processes.

6. The difference between processes and threads

Usually a process can contain several threads, and they can Using the resources owned by the process, in the operating system that introduces threads, the process is usually used as the basic unit for allocating resources, and the thread is used as the basic unit for independent running and independent scheduling. Since threads are smaller than processes, they are basically not With system resources, the overhead for its scheduling will be much smaller, and it can more efficiently increase the degree of concurrent execution among multiple programs in the system.

7. Types of processes

There are three types of processes: interactive process, batch process, and daemon process

• Interactive process: The interactive process is maintained by the shell and interacts with the user through the shell.

  For example, a text editor is an interactive process.
  

• Batch process: The batch process will be placed in a queue in the kernel and run as the queue runs. Its priority is relatively low.

  For example, the process of gcc compiling a program.
  

• Daemon process: The daemon process is a process running in the background. It starts when the system starts and terminates when the system terminates.

  For example, various services on windows.
  

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