some rules from <Advanced Programming in the UNIX Environment> and <Internetworing With TCP/IP: Vol3>

1. close inherited file descriptors.
for(i = getdtablesize() -1; i>=0; i--)
close(i);

2. separate with controy terminal.
fd = open("/dev/tty", O_RDWR);
ioctl(fd, TIOCNOTTY, 0);

3. change to a security directory
chdir("/");

4. set umask
umask(027);

5. set a new session , and become session leader
setsid();

6. set process group
setpgrp(0, getpid());

7. open 3 standard file descriptors to /dev/null
fd = open("/dev/null", O_RDWR); /* stdin */
dup(fd); /*stdout*/
dup(fd); /*stderr*/

8. keep only one copy of daemon
use flock(), and the file lock should be keep in daemon's life.

9. record daemon's pid
write pid to lock file is a good idea.

10. ignore all unneeded signals