Opening a socket opens a socket, which is what you see listed as file descriptor 3 in your output (
socket:). The other three file descriptors are the standard input, output and error descriptors, which are opened by default for any process. In your case, these point to the terminal where the program is running, which is why “surni” shows up there when you write to the file descriptor.
To write to the socket, you need to use appropriate mechanisms, such as
netcat in the other direction:
echo Hello | nc localhost 9999
or, if you’re using Bash:
echo Hello > /dev/tcp/localhost/9999
However it appears you already have a connection established to port 9999 using another
netcat, so that won’t actually work in your case. You need to use the established connection...
If you’re expecting a long-running
nc to provide a way for other processes to feed into the socket, you need to set that up e.g. using a FIFO:
nc localhost 9999 < socket-input
echo Hello > socket-input