The typical way of doing this in Unix is to double fork. In bash, you can do this with
( sleep 30 & )
(..) creates a child process, and & creates a grandchild process. When the child process dies, the grandchild process is inherited by init.
If this doesn’t work, then your application is not waiting for child processes.
Other things it may be waiting for include the session and open lock files:
To create a new session, Linux has a setsid. On OS X, you might be able to do it through script, which incidentally also creates a new session:
# Linux:
setsid sleep 30
# OS X:
nohup script -q -c 'sleep 30' /dev/null &
To find a list of inherited file descriptors, you can use lsof -p yourpid, which will output something like:
sleep 22479 user 0u CHR 136,32 0t0 35 /dev/pts/32
sleep 22479 user 1u CHR 136,32 0t0 35 /dev/pts/32
sleep 22479 user 2u CHR 136,32 0t0 35 /dev/pts/32
sleep 22479 user 5w REG 252,0 0 1048806 /tmp/lockfile
In this case, in addition to the standard FDs 0, 1 and 2, you also have a fd 5 open with a lock file that the parent can be waiting for.
To close fd 5, you can use exec 5>&-. If you think the lock file might be stdin/stdout/stderr themselves, you can use nohup to redirect them to something else.