How to fetch an IP address of a socket inside a fd_set?

Question

Hey Im storing all connected sockets in a fd_set. I want to iterate through the sockets and display the IP address.

ZeroMemory(host, NI_MAXHOST);
                ZeroMemory(service, NI_MAXSERV);

                inet_ntop(AF_INET, &socketAddr.sin_addr, host, NI_MAXHOST);
                std::cout << host << " connected on port " << ntohs(socketAddr.sin_port) << std::endl;

                if (getnameinfo((sockaddr*)&socketAddr, sizeof(socketAddr), host, NI_MAXHOST, service, NI_MAXSERV, 0) == 0) {
                    std::cout << host << " connected on port " << service << std::endl;
                }
                else {
                    inet_ntop(AF_INET, &socketAddr.sin_addr, host, NI_MAXHOST);
                    std::cout << host << " connected on port " << ntohs(socketAddr.sin_port) << std::endl;
                }

I use this way to display the info when a new client joins but this does not seem to work when iterating through sockets already in the fd_set. Im new to socket programming and any help will be greatly appreciated.

Edit: the remote address the socket is connected to

Answer 1

Assuming you are interested in IPv4 and the IP address of the connecting client (as opposed to the server-side IP address), this should work:

#include <stdio.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>

// theFDSet is a pointer to your FD_SET object
// maxFD is the largest file-descriptor value in the set
void PrintClientAddressesInFDSet(FD_SET * theFDSet, int maxFD)
{
   for (int i=0; i<=maxFD; i++)
   {
      if (FD_ISSET(i, theFDSet))
      {
         struct sockaddr_in addr;
         socklen_t length = sizeof(sockaddr_in);
         if (getpeername(i, (struct sockaddr *)&addr, &length) == 0)
         {
            printf("Socket FD %i is connected to a peer at IP address %s\n", i, inet_ntoa(addr.sin_addr));
         }
         else perror("getpeername");
      }
   }
}

For IPv6, you'd need to use inet_ntop() rather than inet_ntoa(), and a sockaddr_in6 rather than a sockaddr.