Creating a daemon process 创建守护进程

Creating a daemon process involves several steps to ensure that the process is detached from the terminal, runs in the background, and is independent of the user session. Below is a detailed explanation of each step:

1. Fork a Child Process

pid_t pid = fork();
if (pid < 0) {
    // Error handling for fork failure
    perror("fork");
    exit(1);
} else if (pid > 0) {
    // Parent process exits
    exit(0);
}

2. Create a New Session and Set the Process Group ID

if (setsid() < 0) {
    // Error handling for setsid failure
    perror("setsid");
    exit(1);
}

3. Fork Again and Terminate the Parent Process

pid = fork();
if (pid < 0) {
    // Error handling for fork failure
    perror("fork");
    exit(1);
} else if (pid > 0) {
    // Parent process exits
    exit(0);
}

4. Change Working Directory to Root

chdir("/");

5. Set File Creation Mask (umask)

umask(0);

6. Close File Descriptors

close(0);
close(1);
close(2);

7. Optional: Handle Signals

#include <signal.h>

// Function to handle signals
void signal_handler(int signum) {
    // Cleanup and exit the daemon
    // (e.g., close files, free resources)
    exit(0);
}

// Register signal handlers
signal(SIGTERM, signal_handler);
signal(SIGINT, signal_handler);

8. Daemon Logic

while (1) {
    // Your main daemon loop
    // Perform background tasks, write to log files, etc.
    sleep(1);
}

9. Cleanup on Exit (if needed)

// Cleanup code goes here

10. Compile and Run

Compile your program and run it in the background:

gcc your_daemon_program.c -o your_daemon_program
./your_daemon_program &

This sequence of steps ensures that the process becomes a daemon, detached from the terminal and capable of running independently. Note that the details of the daemon logic will depend on the specific requirements of your application. Always remember to handle errors gracefully and ensure proper cleanup mechanisms.