Create Linux Daemon: Step-by-Step Guide

To create a daemon in a Linux system, you can follow these steps.

Creating a child process and then exiting the parent process ensures that the child process is not a process group leader, allowing it to create a new session using the setsid() function.

pid_t pid = fork();
if (pid < 0) {
    exit(EXIT_FAILURE);
}
if (pid > 0) {
    exit(EXIT_SUCCESS);
}

Setting the file permissions mask (umask) can ensure that the files created by the daemon process have the appropriate permissions.

umask(0);

Switch the working directory to the root directory. This ensures that the daemon does not occupy any mounted file systems.

chdir("/");

Close the file descriptors to release any unnecessary file descriptors inherited from the parent process by the daemon.

int fd;
for (fd = sysconf(_SC_OPEN_MAX); fd > 0; fd--) {
    close(fd);
}

Redirect standard input, standard output, and standard error output to /dev/null. This ensures that any attempts to read from or write to standard input or output will not affect the daemon.

open("/dev/null", O_RDONLY); // 标准输入
open("/dev/null", O_RDWR); // 标准输出
open("/dev/null", O_RDWR); // 标准错误输出

6. Implementing the core logic of the daemon process.

// 守护进程核心逻辑

This creates a simple daemon process. You can put the above code into a function and then call that function at the program’s entry point.