Missing the setsid step causes child processes to set a SIGHUP when the session is exited.

Daemonization is weird and complicated under unix. To correctly daemonize the following steps should be taken in order

1. explicitly set umask since the inherited umask is unknown
2. call fork() and exit in the parent. This forces the child to have a process group ID which is not the same as the process id (thus ensuring we're not the process group leader).
3. calls setsid() (this is now safe since we're not the process group leader). This forces us to become the PGL of a new session, and ensures we have no controlling terminal.
4. At this point in highly portable programs it is typical to fork() yet again to prevent the child from being a session leader. This is known as the 'double fork daemonize' method.
5. change the cwd to / to ensure that if the working directory is removed from underneath it the daemons continue to work. This also allows for the filesystem to unmounted.
6. close open file descriptors (and possibly reopen 0,1,2).

In C without error checking this looks like

umask(0);
int pid = fork();
if (pid != 0)
	exit(0);
setsid();
int pid = fork();
if (pid != 0)
	exit(0);
chdir("/");
closefrom(0);
open("/dev/null", O_RDWR);
dup(0);
dup(0);

ran the daemons

note well: I wrote this all after 1:00am so something is probably wrong.

also note: I suspect the 'TODO' to learn how to unix works is hard