Before you can run atariserver or atarixfer, you will need the atarisio
kernel module loaded. If you're dedicating a serial port for use with
atariserver, you can add the following line to /etc/rc.d/rc.modules or
rc.local (as you prefer):

/sbin/modprobe atarisio port=/dev/ttyS0

(Replace ttyS0 if you're using a different serial port, of course)

If you need to use your serial port for other purposes, it's a little
less cut-and-dried. You will have to either manually modprobe and rmmod
the module as needed, or write yourself a script to do the job (possibly
also starting up agetty when the module is unloaded, or SLIP mode if
you're using FujiChat on your Atari, etc etc).

#### READ THIS! ####

# By default, atariserver and atarixfer are installed setuid root,
# group owner "users". This is done for two reasons: (a) so the programs
# can access the /dev/atarisio* devices, and (b) so they can set POSIX
# realtime scheduling mode, which prevents timing issues that can cause
# SIO frames to be retransmitted or (on a loaded system) dropped.

# If run setuid root, atariserver and atarixfer will drop their root
# privileges after setting realtime mode and opening the device.
# There are no currently known exploits against atarisio, but the code
# hasn't exactly been audited by the NSA either.

# You have several options here:

# 1. The default. Simply run this script. On a single-user system, this
# is reasonable, though it's the least secure. atariserver and atarixfer
# will be setuid root, and runnable by anyone in the users group.

# 2. Run atariserver and atarixfer setuid root, but restrict access to
# some group other than "users". To do this, run the script as:
# SETUID=yes GROUP=wheel ./atarisio.SlackBuild
# (replace "wheel" with any other group, as you prefer).
# This option is more secure than option 1 and outperforms option 3.

# 3. Run atariserver and atarixfer as a normal user. This can cause
# performance problems, but on most setups it works OK. To do this,
# run the script as:
# SETUID=no ./atarisio.SlackBuild
# This should be more secure than 1 or 2.

# 4. Same as 3, but extra paranoid: use a group other than "users" (I like
# "wheel"), and be very picky about who you add to the group.
# SETUID=no GROUP=wheel ./atarisio.SlackBuild