https, is a daemon, as is sshd, which listens for ssh requests on remote servers you can ssh into).So, without further ado, here's the code sample:
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <syslog.h>
#include <sys/types.h>
#include <sys/stat.h>
int main(int argc, char *argv[]) {
// Set our uid to root so we get root permissions.
setuid(0);
// Fork off.
pid_t pid;
int current_arg = 0;
pid = fork();
if (pid < 0) {
exit(EXIT_FAILURE);
}
if (pid > 0) {
exit(EXIT_SUCCESS);
}
umask(0);
// Open a syslog so we can post messages, as daemons shouldn't access
// stdout.
openlog(argv[current_arg], LOG_NOWAIT|LOG_PID, LOG_USER);
syslog(LOG_NOTICE, "Successfully started daemon\n");
// Create a new process group.
pid_t sid;
sid = setsid();
if (sid < 0) {
syslog(LOG_ERR, "Could not create process group\n");
exit(EXIT_FAILURE);
}
// Set our working directory to / so we know where we're writing files
// from. Also good practice.
if ((chdir("/")) < 0) {
syslog(LOG_ERR, "Could not change working directory to /\n");
exit(EXIT_FAILURE);
}
// Close the standard streams like stdout so we're not attached to the
// console (and thus killable with Control-C).
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);
// Now mimic some daemon activity. A real daemon would actually do
// something here, such as monitor CPU temperature or something...
do {
syslog(LOG_NOTICE, "Sending a message\n");
sleep(10);
} while (1);
// Close our access to the syslog.
closelog();
} Put this into a text file, and then compile it with your C compiler. You then go into the terminal, run your program, and… it exits immediately - what gives?
The truth is, something did happen - you just don't notice. Thanks to our
fork() call, we split off from the main process and run in the background.In the second part of this series, we'll write a more complicated daemon program. See you later!
