How to make a daemon process

In Linux systems, creating a daemon process primarily involves the following steps:

1. Create a Child Process and Terminate the Parent Process

The daemon process must detach from terminal control, which is typically achieved by creating a child process and terminating the parent process. This ensures that the daemon is not the session leader and thus not associated with any terminal.

Example code:

c
pid_t pid = fork();
if (pid > 0) {
    exit(EXIT_SUCCESS); // Parent process exits
}

2. Change the Working Directory

To prevent the daemon from occupying a mountable filesystem, it is common to change its working directory to the root directory.

Example code:

c
if (chdir("/") < 0) {
    // Handle error
    exit(EXIT_FAILURE);
}

3. Reset the File Permission Mask

Call the umask() function to set the file mode creation mask for the daemon process, typically set to 0, so that created files have unrestricted permissions.

Example code:

c
umask(0);

4. Close All Inherited File Descriptors

The daemon should close all inherited file descriptors to avoid holding unnecessary resources.

Example code:

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

5. Redirect Standard Input, Output, and Error File Descriptors

Typically, redirect standard input, standard output, and standard error to /dev/null because the daemon should not interact with users.

Example code:

c
int fd = open("/dev/null", O_RDWR);
dup2(fd, STDIN_FILENO);
dup2(fd, STDOUT_FILENO);
dup2(fd, STDERR_FILENO);
if (fd > STDERR_FILENO) {
    close(fd);
}

6. Become the New Session Leader

Call setsid() to create a new session and make the calling process the session leader and process group leader.

Example code:

c
if (setsid() < 0) {
    // Handle error
    exit(EXIT_FAILURE);
}

7. Handle the SIGCHLD Signal

Handle the SIGCHLD signal to avoid zombie processes; it is optional to ignore this signal.

Example code:

c
signal(SIGCHLD, SIG_IGN);

8. Execute the Core Tasks of the Daemon

At this point, the daemon configuration is complete, and it can execute its core tasks.

Example code:

c
while(1) {
    // Execute the daemon's tasks
    sleep(1);
}

By following these steps, you can create a standard daemon process that runs in the background and performs specific tasks. Such processes are highly useful in various scenarios such as server management and file synchronization services.