Read "/proc" to know if a process has opened a port

I need to know if a process with a given PID has opened a port without using external commands. I must then use the /proc filesystem. I can read the /proc/$PID/net/tcp file for example and get information about TCP ports opened by the process. However, on a multithreaded process, the /proc/$PID/task/$TID directory will also contains a net/tcp file. My question is : do I need to go over all the threads net/tcp files, or will the port opened by threads be written into the process net/tcp file.