Winsock Select() Function

**PetrolMan** · 03-25-2009

I'm trying to make a real basic program that will connect to something read data until there is no more to read, then allow for input, then loop. I can create the socket, connect and read the first round of data and respond just fine, but there seems to be a problem in the second iteration of the loop.

I'm using the winsock select() function and I cannot seem to figure out the problem. I was hoping somebody might have a snippet of sample code that I might look at in comparison.

Thanks,

PetrolMan

**iMalc** · 03-26-2009

Certain non-blocking selects often don't work in Windows when some third party firewall programs are installed because they turn a non-blocking select into a blocking one! It's fustrating as hell having to write a workaround just for certain PCs.

You're betting off posting a snippet of your code rather than asking for someone else's.

**Tonto** · 03-26-2009

I revived an old windows specific code using WSAAsyncSelect -- it's bare bones but it's windows code and it's pretty verbose about the connection

http://zxcvbn.googlecode.com/files/Asyncdaemon.zip
http://zxcvbn.googlecode.com/files/asyncdaemon_src.zip

Code:

[+] Attempting go
[+] WSASocket returned valid socket
[+] Bind'ed socket to service - port 5678
[+] Connection from 127.0.0.1
[+] Echo: "This is a test"
[+] Read from 127.0.0.1: g
[+] Read from 127.0.0.1: o
[+] Read from 127.0.0.1: o
[+] Read from 127.0.0.1: d
[+] Read from 127.0.0.1:  
[+] Read from 127.0.0.1: q
[+] Read from 127.0.0.1: u
[+] Read from 127.0.0.1: i
[+] Connection 'quit' 127.0.0.1 closed
[+] Connection 127.0.0.1 closed
[-] No systems go
[-] Definitely closed a connection

**PetrolMan** · 03-26-2009

I appreciate the code. As for the firewall issue, how do you recognize this issue and create a workaround? For some protocols such as POP3 blocking sockets isn't much of an issue since there is a call and response setup but IRC, for example, is a free for all and I'm honestly not sure how someone could even attempt a program without nonblocking sockets.

Again, thanks for the help.

PetrolMan

**PetrolMan** · 03-26-2009

Code:

		while (select(0, &sockets, 0, 0, &timeout))
		{
			test = recv(mysock, bufs, sizeof(bufs), 0);
			if (test == 0)
			{
				cout << "Connection Closed\n";
				return 1;
			}
			cout << bufs[0];
		}

I'm pretty sure my error is in this code. It works on the first iteration (the whole thing is in an infinite for loop) but blocks on the second which suggests to me that the select is returning a value even though the socket isn't ready to read. Perhaps I'm simply using this wrong though. The other possibility is that select is returning some kind of error that I'm simply not checking for but how would I distinguish? BTW, I'm receiving everything in a single char at a time but I figured that would be safest.

I'm sure I'm just making a stupid and obvious mistake but I can't figure it out.

Thanks for all of the help.

PetrolMan

**caroundw5h** · 03-26-2009

The other possibility is that select is returning some kind of error that I'm simply not checking for but how would I distinguish?

you should check if there is an error using WSAGetLastError() it returns a int that specifies exactly what the error is. haven't used WinSock in a while so i hope that's helpful

**Tonto** · 03-26-2009

Definitely check recv for -1 socket error, and check WSAGetLastError for WSAEWOULDBLOCK. if you get this message you can just wait for another select notification.

**PetrolMan** · 03-26-2009

Progress: I've run the program with error checking and the first iteration goes just fine. On the second go around the select function seems to be the function that is failing. Interestingly enough, the error code is 10022, which for the select function means the timeval structure is wrong. The problem is the code runs fine the first iteration. Any ideas?

PetrolMan

**Tonto** · 03-26-2009

I found on this man page on select and fd messages this code

Code:

   retval = select(1, &rfds, NULL, NULL, &tv);
    /* Don't rely on the value of tv now! */

fd_zero(3): synchronous I/O multiplexing - Linux man page

lol, you might have to reset the timeout value or something

**PetrolMan** · 03-26-2009

I made the dumb mistake of not reading the MSDN reference very carefully.

Upon return, the structures are updated to reflect the subset of these sockets that meet the specified condition. The select function returns the number of sockets meeting the conditions. A set of macros is provided for manipulating an fd_set structure.

So once select() returns a negative on a socket's readability it is removed from the list. And that is the source of all of my problems. I was trying to select() against an empty list which causes an error.

I need a Nerf Bat to whack against my head for trying to skim the reference text. Sorry about all of the wasted time for those who have helped. I promise to be more careful in the future.

PetrolMan

**caroundw5h** · 03-26-2009

I brain fart real loud when coding at night, of course if it wasn't late where your at then by all means get that nerf bat out.

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