Using Non Blocking socket in a port scanner - c

Question

I have written an simple port scanner code to see the whether the host is available or not by establishing the socket connection .The logic is to check for connection if the connection established is successful then the host is available or else if we get error code as WSACONNREFUSED then the host is present.The socket connection is a blocking one and if there's no host on that ip address the program will be blocked until an timeout occurs . I read a couple of examples on the internet on how to use a non blocking socket but they use read and write to check whether we can write to server side or read from the server side to establish the connection .and for that there should be a server code to accept.Is there a way to modify the existing code for the port scanner using a non blocking socket,as i want the present application to run a bit faster any valuable tips would be useful .
The present code is below .
#ifndef UNICODE
#define UNICODE
#endif
#define WIN32_LEAN_AND_MEAN
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>
// Need to link with Ws2_32.lib
#pragma comment(lib, "ws2_32.lib")
int port[]={80,139};
int wmain()
{
// Initialize Winsock
WSADATA wsaData;
int i=0,flag=0;
char ip[20];
int iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
if (iResult != NO_ERROR) {
wprintf(L"WSAStartup function failed with error: %d\n", iResult);
return 1;
}
SOCKET ConnectSocket;
sockaddr_in clientService;
// Create a SOCKET for connecting to server
ConnectSocket = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
if (ConnectSocket == INVALID_SOCKET) {
wprintf(L"socket function failed with error: %ld\n", WSAGetLastError());
WSACleanup();
return 1;
}
// The sockaddr_in structure specifies the address family,
// IP address, and port of the server to be connected to.
printf("\n Enter the Ip Address : ");
scanf("%s",ip);
clientService.sin_family = AF_INET;
clientService.sin_addr.S_un.S_addr = inet_addr(ip);
for(i=0;i<2;i++)
{
clientService.sin_port = htons((unsigned short)port[i]);
iResult = connect(ConnectSocket, (SOCKADDR *) & clientService, sizeof (clientService));
if((iResult==0)||((iResult=WSAGetLastError())==WSAECONNREFUSED))
{
printf(" %d ",iResult);
printf("\n Port Number : %d",port[i]);
printf("\n Machine Found ");
flag=1;
break;
}
}
if(flag==0)
{
printf("\n Machine not found ");
}
iResult = closesocket(ConnectSocket);
if (iResult == SOCKET_ERROR) {
wprintf(L"closesocket function failed with machine %d error: %ld\n",i, WSAGetLastError());
WSACleanup();
return 1;
}
WSACleanup();
return 0;
}

Answer 1

After doing a non-blocking connect(), the socket will become writable when the connect completes. At that point you should check SO_LASTERROR on the socket to see if the connection has completed successfully.