Linux Daemon Process

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

int main(void) {
        
        /* Our process ID and Session ID */
        pid_t pid, sid;
        
        /* Fork off the parent process */
        pid = fork();
        if (pid < 0) {
                exit(EXIT_FAILURE);
        }
        /* If we got a good PID, 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 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 some task here ... */
           
           sleep(30); /* wait 30 seconds */
        }
   exit(EXIT_SUCCESS);
}

---------------

1) The fork() function returns either the process id (PID) of the child process (not equal to zero), or -1 on failure. If the process cannot fork a child, then the daemon should terminate right here.

2) In order to write to any files (including logs) created by the daemon, the file mode mask (umask) must be changed to ensure that they can be written to or read from properly. By setting the umask to 0, we will have full access to the files generated by the daemon. Even if you aren't planning on using any files, it is a good idea to set the umask here anyway, just in case you will be accessing files on the filesystem.

3)From here, the child process must get a unique SID from the kernel in order to operate. Otherwise, the child process becomes an orphan in the system. Again, the setsid() function has the same return type as fork().

4)One of the last steps in setting up a daemon is closing out the standard file descriptors (STDIN, STDOUT, STDERR). Since a daemon cannot use the terminal, these file descriptors are redundant and a potential security hazard.