c programming - socket 'connect' returns 0 even on invalid port

bill_nimmo · 04-27-2015, 11:25 AM

Greetings All,

This one confuses me. According to documentation, when I try to do a socket 'connect', the call will return 0 if everything went ok (I am assuming that means that it connected to a server or someone listening on that port). I have a program (example below) that attempts to create a connection to a valid IP BUT an invalid port number. Maybe I am missing the point (or doing something stupid in my code)??? In the code below, the call to connect returns 0 (I have tried many different valid and invalid port numbers...it always returns 0):

Code:

static struct sockaddr_in Server;
int connectToServer()
{
   int return_stat;
   int connection;
   int optval;
   int valid_ip;
   int SockFD;
   int SomeGlobalIP = nnn.nnn.nnn.nnn;
   int SomeInvalidPortNumber = 111222333444;

   memset(&Server, 0, sizeof(Server);
   signal(SIGPIPE,SIG_IGN);
   errno=0;
   SockFD = socket(AF_INET, SOCK_STREAM, 0);
   if (SockFD < 0)
      return -1;
   valid_ip = inet_aton(SomeGobalIP, &Server.sin_addr);
   if(valid_ip == 0)
     return -1;
   Server.sin_port = htons((short)SoemInvalidPortNumber);
   setsockopt(SockFD, SO_REUSEADDR, (char*)&optval, sizeof(int));
   connection - connect(SockFD, (struct sockaddr *)&Server, sizeof(Server));
   if (connection < 0)
      return -1;

   return 0;
}

SoftSprocket · 04-27-2015, 12:32 PM

If you use [code][/code] tags when posting your code will keep it's formatting and be easier to read.

On my system SHRT_MAX = 32767. At the very least it means that you code is invoking undefined behavior when it does:

Code:

Server.sin_port = htons((short)SoemInvalidPortNumber);

with SomeInvalidPortNumber = 111222333444.

Anything is possible strictly speaking, some things being more likely then others.

bill_nimmo · 04-27-2015, 01:47 PM

I apologize for the code being unformatted. I will use those tags next time.

Perhaps the port number that I put in my example was a bad one (ok, it WAS a bad example). To further explain my situation: I have a server application that is going to listen on a specific port number (8225). The server application is NOT running and when I use that port number (instead of SomeInvalidPortNumber) the 'connect' still returns 0. Whether I have the server app running or not, the 'connect' always seems to return 0. I would think that if the server wasn't running, the 'connect' would fail. Am I missing something, or some concept?

thanks,
Bill

SoftSprocket · 04-27-2015, 02:17 PM

If there's nothing to connect to at that ip/port pair then connect should return -1;

bill_nimmo · 04-27-2015, 03:00 PM

yes, that is what I thought. But, for some reason whatever number I use for the port number the call to connect returns 0. Its gotta be something that I am not doing right or something I am missing. ??? I didn't see anything obvious but I could just be blind.

rknichols · 04-27-2015, 03:35 PM

Quote:

Originally Posted by bill_nimmo

Code:

   connection - connect(SockFD, (struct sockaddr *)&Server, sizeof(Server));
              ^

Is that just a typo in your post, or is it in the actual code?

bill_nimmo · 04-27-2015, 05:05 PM

My apologies. I fat fingered it. It should be an equal sign

Code:

   connection = connect(SockFD, (struct sockaddr *)&Server, sizeof(Server));

ntubski · 04-27-2015, 06:51 PM

We can only find problems in what you posted, so you should really post some actual code that you have a problem with. It should be a Short, Self Contained, Correct (Compilable), Example.

Otherwise we can keep playing the is-this-mistake-just-from-posting game, but get no closer to solving anything.

Code:

    int SomeGlobalIP = nnn.nnn.nnn.nnn;
    // should be
    const char* SomeGlobalIP = "nnn.nnn.nnn.nnn";

Code:

   setsockopt(SockFD, SO_REUSEADDR, (char*)&optval, sizeof(int));

You never initialized optval.

NevemTeve · 04-27-2015, 11:43 PM

I think it is time to re-quote the actual program, without manual edits.

bill_nimmo · 04-28-2015, 07:55 AM

My apologies to all. The real code actually is written and running in a classified lab; thus, cannot be removed from the lab. I had to type it in from memory. I will re-post with code that compiles somehow. Again, I apologize.

Bill

bill_nimmo · 04-28-2015, 09:40 AM

Thank you all for your responses. I did some more digging and think I figured it out. At least, now I get it to fail with an invalid port, and succeed with a valid port. The piece of code that I was missing was:

Code:

   Server.sin_family = AF_INET;

From what I think I read, the call to connect will return 0 regardless of the port number if it thinks you want a raw socket.

Thank you, again.

Bill

SoftSprocket · 04-28-2015, 10:21 AM

I think more likely it's this (from the connect(2) man page):

Connectionless sockets may dissolve the association by connecting to an address with the sa_family member of sockaddr set to AF_UNSPEC (supported on Linux since kernel 2.2).

bill_nimmo · 04-28-2015, 10:42 AM

Ahhhh, ok. That's probably it. Thank you all again.

Bill