Important steps in the Linux system startup process: user mode initialization

User mode initialization phase: a key link in the Linux system startup process

In the Linux system startup process, the user mode initialization phase is one of the most critical links. User-mode initialization refers to the initialization of user space and the startup process of user-level programs after the kernel completes startup. This article will introduce in detail the background, process and key code examples of user mode initialization.

1. Background introduction

The Linux system startup process is divided into two stages: kernel mode and user mode. Kernel mode refers to the privileged mode in which the operating system kernel runs, while user mode refers to the general mode in which applications run. After the kernel completes boot loading and system initialization, it will start a user-mode process to provide services to users.

2. User-mode initialization process

1. Execute the init process

In the Linux system, the first step of user-mode initialization is The first step is to execute the init process. The init process is the first process in user space, its process number is 1, and it is the parent process of the system. The init process is responsible for starting other user-level processes and services in the system. In newer Linux distributions, systemd may be used instead of traditional init as the system's initialization process.

2. Start system services

After the init process is started, it will start various services in the system according to the configuration file, including network services and logs Services, equipment management services, etc. These services will be started one by one during the user mode initialization phase.

3. Start the user-level program

The last step in user-mode initialization is to start the user-level program. These programs include user login management programs, graphical interface environments, applications, etc. The launch of user-level programs is a critical step before the user can interact with the operating system.

3. Key code examples

The following is a simple user-mode initialization code example, written in C language:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main() {
    pid_t pid;

    printf("Init process started
");

    pid = fork();

    if(pid < 0) {
        perror("Fork failed");
        exit(EXIT_FAILURE);
    } else if (pid == 0) {
        // Child process
        printf("Child process started
");
        // Start system services
        execl("/bin/systemd", "systemd", NULL);
    } else {
        // Parent process
        wait(NULL);
        // Start user-level programs
        execl("/bin/bash", "bash", NULL);
    }

    return 0;
}

The above code simply simulates the user-mode initialization phase. Process: First start the init process, and then create a child process through fork. The child process starts the system service (replaced by systemd here), and the parent process starts the user-level program (replaced by bash here).

Conclusion

The user mode initialization phase is a crucial part of the Linux system startup process, affecting the normal operation of the system and user experience. Through the introduction and code examples of this article, I hope readers can have a deeper understanding of the user mode initialization process and lay the foundation for in-depth study and understanding of the startup process of the Linux system.

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