socat duplicate stdin to each connected client

Question

ncat (from the nmap folk) has a neat default action of duplicating any input to all connected clients. E.g.:

Start a server on terminal 1:

% mkfifo messages
% exec 8<>messages  # hold the fifo open
% ncat -l 5555 -k --send-only < messages

Start clients listening on terminals 2 & 3:

% nc localhost 5555

Output something to the fifo on terminal 4 and watch the same message appear on all connected clients (terminals 2 & 3):

% printf 'Hello, clients.\n' > messages

Is this same pattern possible with socat?

Update: screenshot of Philippe's solution:

Answer 1

What you want to do is, I'm afraid, impossible. The closest you can get is:

On terminal 1:

mkfifo messages
socat PIPE:messages TCP4-LISTEN:5555,fork

On terminals 2 & 3:

nc localhost 5555

Then, text sent to messages will appear sometimes on terminal 2 and sometimes on terminal 3. This is because socat will 'consume' the text in messages before making them available through the socket open on localhost:5555. Then, the first netcat process to wake up and read from the socket will get the messages and the other will get nothing.

Answer 2

This command will do:

socat tcp-listen:5555,fork,reuseaddr \
'system:
PIPE=$(mktemp -u /tmp/pns_XXX)
mkfifo $PIPE
while read PIPE_MESSAGE<$PIPE; do
  echo $PIPE_MESSAGE
done &
PID=$!
while read CLIENT_MESSAGE; do
  for OTHER_PIPE in $(ls /tmp/pns_*); do
    [ $PIPE != $OTHER_PIPE ] && echo $CLIENT_MESSAGE > $OTHER_PIPE
  done
done
kill $PID
rm $PIPE'

The idea is to create a named pipe for every new connection and then echoing to std output (i.e. send to client) whatever is read from the pipe. Std input (i.e. send by client to server) is read from and whatever is received is echoed to all the named pipes, except the own pipe.