how to check if a port is free in linux using c

Question

i have never written anything like it, how do i check things like if a port is empty using c program in Linux environment thanks a lot.

ps looking for a way, by not using bind or connect and checking if it failed.

edit i cant use bind or connect, looking for faster way to find 3k ports that are free in a row

Answer 1

Better way is to use next free port,You can also use 0 port bind will use the next available port.

You can get port selected by bind() by following code

struct sockaddr_in sin;
socklen_t len = sizeof(sin);
if (getsockname(sock, (struct sockaddr *)&sin, &len) != -1)
  printf("port number %d\n", ntohs(sin.sin_port)); 

Also refer How to bind to any available port? for more inforamtion

Answer 2

Run following command using system() or popen()

netstat -antu

It will give list of all used port of your machine. You need to parse output of that command and then you will have list of all busy port.