C++ – Simple cross platform TCP IP API

On macOS Big Sur and later, use this command:

sudo lsof -i -P | grep LISTEN

or to just see just IPv4:

sudo lsof -nP -i4TCP:$PORT | grep LISTEN

On older versions, use one of the following forms:

sudo lsof -nP -iTCP:$PORT | grep LISTEN
sudo lsof -nP -i:$PORT | grep LISTEN

Substitute $PORT with the port number or a comma-separated list of port numbers.

Prepend sudo (followed by a space) if you need information on ports below #1024.

The -n flag is for displaying IP addresses instead of host names. This makes the command execute much faster, because DNS lookups to get the host names can be slow (several seconds or a minute for many hosts).

The -P flag is for displaying raw port numbers instead of resolved names like http, ftp or more esoteric service names like dpserve, socalia.

See the comments for more options.

For completeness, because frequently used together:

To kill the PID:

sudo kill -9 <PID>
# kill -9 60401

You will need to call bind() if you want your program to receive UDP packets. connect() only sets the address that the socket will send packets to if you call send(); it does not associate the socket with a local UDP port to receive on; for that you must call bind().