检查套接字是否正在C中侦听

Question

在遍历套接字文件描述符时，如何检查其中一个是否来自被动套接字（侦听连接）？

Answer 1

This can be checked with getsockopt ( SO_ACCEPTCONN ). For example:

#include <sys/socket.h>

int val;
socklen_t len = sizeof(val);
if (getsockopt(fd, SOL_SOCKET, SO_ACCEPTCONN, &val, &len) == -1)
    printf("fd %d is not a socket\n", fd);
else if (val)
    printf("fd %d is a listening socket\n", fd);
else
    printf("fd %d is a non-listening socket\n", fd);

Answer 2

You can't really tell. You have to keep track of it yourself, and when you want to check if a socket is the listening socket you compare with the one you have saved.

Answer 3

Strictly speaking, you could try performing an operation on the socket which would incidentally determine what type of socket it is, like trying to accept a connection from it. If accept() fails with EINVAL, that's a pretty good sign that it isn't listening. :)

Keeping track of which sockets are which is a better solution overall, though. Unless you're building a really trivial application, chances are that you'll need to keep some sort of additional data on each socket anyway.

Answer 4

You can run in the command line (on a Mac/Linux enviroment):

lsof -i

and/or (Linux/Mac/Windows enviroment)

netstat -a