On linux, how to check if port is in listen state without trying to connect

Question

How do I check with C if a port on my local machine (if required by passing an IP or interface, too), is in listen state? I don't want to connect to this port for checking because I don't want to irritate the service behind this port.

I want to use this to add the missing net.tcp.listen item to Zabbix.

EDIT - THIS IS THE REAL ANSWER:

The correct way is to read the socket tables:

/proc/net/tcp /proc/net/tcp6

They contain lines like:

sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
0: 00000000:1F40 00000000:0000 0A 00000000:00000000 00:00000000 00000000   101        0 4083927 1 f5d15240 750 0 0 2 -1
1: 00000000:2742 00000000:0000 0A 00000000:00000000 00:00000000 00000000  1002        0 6100 1 decd76c0 750 0 0 2 -1

and can easily parsed for listening sockets (dst:00000000:0000). An strace on netstat shows that netstat works the same way.

Answer 1

Just try to open the port for listening. You will get an error.

There is no way you can steal a port from another process, so this will be safe, and of course easy to implement as it requires no additional code other than proper error handling.

Answer 2

The correct way is to read the socket tables:

/proc/net/tcp /proc/net/tcp6

They contain lines like:

sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
0: 00000000:1F40 00000000:0000 0A 00000000:00000000 00:00000000 00000000   101        0 4083927 1 f5d15240 750 0 0 2 -1
1: 00000000:2742 00000000:0000 0A 00000000:00000000 00:00000000 00000000  1002        0 6100 1 decd76c0 750 0 0 2 -1

and can easily parsed for listening sockets (dst:00000000:0000). An strace on netstat shows that netstat works the same way.