Convert your C Program into Linux Daemon

v What is Daemon?

A daemon (or service) is a background process that is designed to run autonomously, with little or no user intervention. The Apache web server http daemon (httpd) is one such example of a daemon. It waits in the background listening on specific ports, and serves up pages or processes scripts, based on the type of request.

v What Daemon is going to do?

A Daemon can be made to do one specific thing such as writing log to some event or managing many mailboxes on multiple domains.

Daemons should never have direct communication with a user through a terminal. In fact, a daemon shouldn't communicate directly with a user at all. All communication should pass through some sort of interface (which you may or may not have to write), which can be GUI, or as simple as a signal set.

v Steps to create a Daemon.

1.      Fork off the parent process (using fork system call)

2.      Change file mode mask (using umask system call)

3.      Open any logs for writing (optional but recommended)

4.      Create a unique Session ID (using setsid system call)

5.      Change the current working directory to a safe place (using chdir system call)

6.      Close standard file descriptors

7.      Enter actual daemon code

v Example to implement above steps :

int main(void) {

       

        /* process ID and Session ID */

        pid_t pid, sid;

       

        /* Fork off the parent process */

        pid = fork();

        if (pid ==  -1) {

                exit(EXIT_FAILURE);

        }

        /* If forking success, then

           We can exit the parent process. */

        if (pid > 0) {

                exit(EXIT_SUCCESS);

        }

        /* Change the file mode mask */

        umask(0);

               

        /* Open any logs here */       

               

        /* Create a new session (SID) for the child process */

        sid = setsid();

        if (sid < 0) {

                /* Log the failure */

                exit(EXIT_FAILURE);

        }

       

        /* Change the current working directory */

        if ((chdir("/")) < 0) {

                /* Log the failure */

                exit(EXIT_FAILURE);

        }

       

        /* Close out the standard file descriptors */

        close(STDIN_FILENO);

        close(STDOUT_FILENO);

        close(STDERR_FILENO);

       

        /* Daemon-specific initialization goes here */

       

        /* The Big Loop */

        while (1) {

           /* Do Actual task of Daemon here ... */

           

           sleep(_SOME_SECOND_); /* wait for specific seconds */

        }

   exit(EXIT_SUCCESS);

}

That’s it.  Now, your C program is running as a Daemon.