/*
 * ISA 563, Spring 2011
 * Copyleft by Muhammad Abdulla
 */

#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <signal.h>
#include <errno.h>
#include <stdio.h>
#include <syslog.h>
#include <fcntl.h>
#include <sys/resource.h>

/*
 * advanced_daemon.c -- demonstrates how to create daemons while taking more things into consideration
 */

#define LOCKFILE "/var/tmp/advanced_daemon.pid"
#define LOCKMODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH)

void daemonize(const char *progname)
{
    int                 i, fd, fd0, fd1, fd2;
    pid_t               pid;
    struct rlimit       rl;
    char    buf[16];

    // Clear file creation mask.
    umask(0);

    // Get maximum number of file descriptors.
    if (getrlimit(RLIMIT_NOFILE, &rl) < 0) {
        fprintf(stderr, "%s: can't get file limit", progname);
        exit(1);
    }

    if ((pid = fork()) < 0) {
        fprintf(stderr, "%s: can't fork", progname);
        exit(1);
    } else if (pid != 0) { // parent must quit
        exit(0);
    }

    // child continues from here


    // become a session leader
    setsid();

    /*
     * Change the current working directory to the root so
     * we won't prevent file systems from being unmounted.
     */
    if (chdir("/") < 0) {
        fprintf(stderr, "%s: can't change directory to /");
        exit(1); 
    }

    // Close all open file descriptors.
    if (rl.rlim_max == RLIM_INFINITY) {
        rl.rlim_max = 1024;
    }

    for (i = 0; i < rl.rlim_max; i++) {
        close(i);
    }

    /*
     * Attach file descriptors 0, 1, and 2 to /dev/null.
     * All terminal output to user should be printed before this.
     */
    fd0 = open("/dev/null", O_RDWR);
    fd1 = dup(0);
    fd2 = dup(0);

    /*
     * Initialize the log file.
     */
    openlog(progname, LOG_CONS, LOG_DAEMON);
    if (fd0 != 0 || fd1 != 1 || fd2 != 2) {
        syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);
        exit(1);
    }

    fd = open(LOCKFILE, O_RDWR|O_CREAT, LOCKMODE);
    if (fd < 0) {
        syslog(LOG_ERR, "%s: can't open %s: %s", progname, LOCKFILE, strerror(errno));
        exit(1);
    }

    if (lockf(fd,F_TLOCK,0) < 0) { // try to lock without getting blocked
        if (errno == EACCES || errno == EAGAIN) {
            syslog(LOG_ERR, "%s: can't lock %s: %s", progname, LOCKFILE, strerror(errno));
            close(fd);
            exit(1);
        }
        syslog(LOG_ERR, "%s: can't lock %s: %s", progname, LOCKFILE, strerror(errno));
        exit(1);
    }

    // write pid to lock file
    ftruncate(fd, 0);
    sprintf(buf, "%ld", (long)getpid());
    write(fd, buf, strlen(buf)+1);

    syslog(LOG_ERR, "Starting %s with pid %d", progname, getpid() );
}

int main(int argc, char *argv[])
{
   daemonize(argv[0]);

   // do some hard work in the background
   while(1) {
      sleep(1);
   }
}