From book Advanced programming in the Unix Environment Charpter 13.4

daemonlize.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

#include <unistd.h>
#include <pthread.h>
#include <syslog.h>
#include <fcntl.h>
#include <sys/stat.h>

void daemonize(const char* cmd){
  	int i, fd0, fd1, fd2;
	pid_t pid;
	struct sigaction sa;
	//clear file creation mask
	umask(0);

	if ((pid = fork()) < 0) {
		printf("%s: can't fork\n", cmd);
		exit(1);
	}
	else if(pid != 0) {//parent
		exit(0);
	}
	setsid();
	
	sa.sa_handler = SIG_IGN;
	sigemptyset(&sa.sa_mask);
	sa.sa_flags = 0;
	if (sigaction(SIGHUP, &sa, NULL) < 0) {
		printf("%s: can't ignore SIGHUP\n", cmd);
		exit(1);
	}
	if ((pid = fork()) < 0) {
		printf("%s: can't fork\n", cmd);
		exit(1);
	}
	else if(pid != 0) {//parent
		exit(0);
	}

	//change current working dir to root
	if (chdir("/") < 0) {
		printf("%s: can't change dir to /\n", cmd);
		exit(1);
	}

	//close all open file descriptions
	if(rl.rlim_max == RLIM_INFINITY)
		rl.rlim_max = 1024;
	for(i = 0; i < rl.rlim_max; i++)
		close(i);

	//attach file descriptions 0, 1, 2 to /dev/null
	fd0 = open("/dev/null", O_RDWR);
	fd1 = dup(0);
	fd2 = dup(0);

	//initialize the log file 
	openlog(cmd, LOG_CONS, LOG_DAEMON);
	if (fd0 != 0 || fd1 != 0 || fd2 != 0){
		syslog(LOG_ERR, "unexpected file descriptions %d %d %d", fd0, fd1, fd2);
		exit(1);
	}
}