Unix Socket Programming Interview Question:

How can I make my server a daemon?

Submitted by: Administrator

There are two approaches you can take here. The first is to use inetd to do all the hard work for you. The second is to do all the hard work yourself. If you use inetd, you simply use stdin, stdout, or stderr for your socket. (These three are all created with dup() from the real socket) You can use these as you would a socket in your code. The inetd process will even close the socket for you when you are done.



#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/wait.h>

/* Global variables */
volatile sig_atomic_t keep_going = 1;
/* controls program termination */

/* Function prototypes: */
void termination_handler (int signum);
/* clean up before termination */

int
main (void)
{
...

if (chdir (HOME_DIR))
/* change to directory containing data
files */
{
fprintf (stderr, "`%s': ", HOME_DIR);
perror (NULL);
exit (1);
}

/* Become a daemon: */
switch (fork ())
{
case -1:
/* can't fork */
perror ("fork()");
exit (3);
case 0:
/* child, process becomes a daemon: */
close (STDIN_FILENO);
close (STDOUT_FILENO);
close (STDERR_FILENO);
if (setsid () == -1)
/* request a new session (job control) */
{
exit (4);
}
break;
default:
/* parent returns to calling process: */
return 0;
}


/* Establish signal handler to
clean up before termination: */
if (signal (SIGTERM, termination_handler)
== SIG_IGN)

signal (SIGTERM, SIG_IGN);
signal (SIGINT, SIG_IGN);
signal (SIGHUP, SIG_IGN);

/* Main program loop */
while (keep_going)
{
...
}
return 0;
}

void
termination_handler (int signum)
{
keep_going = 0;
signal (signum, termination_handler);
}
Submitted by: Administrator