Problems with UDP sockets

This is a discussion on Problems with UDP sockets within the Networking/Device Communication forums, part of the General Programming Boards category; hi, It's the first time I use UDP socket so my questions might seem very basic... I'm building a client ...

  1. #1
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    Problems with UDP sockets

    hi,

    It's the first time I use UDP socket so my questions might seem very basic...

    I'm building a client server application using UDP protocols...


    can anyone explain to me the recvfrom() function works? for example the first parameter is the client socket but how do I initialize the client socket since there is no accept() in UDP??

  2. #2
    and the hat of wrongness Salem's Avatar
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    I think it's socket() / bind() / recvfrom()
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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    another question...

    something really wierd is happeneing to my UDP server...I just can't open a socket...the socket descriptor keeps getting a negative value...

    the funny thing is that the socket I opened in the client application works perfectly...

    does this has something to do with the OS?? I'm using WIndows...

  4. #4
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    You might want to post some of your code, especially the initialization.

    Are you writing with MFC or straight win32? If MFC are you using CAsyncSockets? Furthermore are you wanting blocking or non-blocking sockets? All these sort of things would be helpful (well to me at least).

    Have you bound the socket to a port yet?

  5. #5
    and the hat of wrongness Salem's Avatar
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    > the socket descriptor keeps getting a negative value
    Which means there is an error.
    What error checking code have you put in, and what is it telling you?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
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    ok here's one part of my code...
    Code:
    int main(int argc, char **argv)
    {
    	int sock;
    	int port, queryCode, length;
    	int clientSock;
    	char message[260];
    	struct sockaddr_in servSock_in;
    	struct sockaddr_in client_in;
    	char *serverIP;			//the IP of the FTP server
    	char *hostname;
    	
    	
    	debug = 0;  //debug is OFF by default
    	port = 53000; 
    	
    	if(argc == 2)
    	{
    		if( strcmp(argv[1], "1") == 0 )
    		{
    			debug = 1;	//debug mode is ON
    			printf("debug mode is ON\n\n");
    		}
    		
    	}
    
    	sock = socket(PF_INET, SOCK_DGRAM,0);   //the dns server socket
    
    	if ( sock< 0)
    	{
    		printf("I am too tired... I failed to open gate...\n");
    		return -1;
    	}
    	
    	memset((char *)&servSock_in,0,sizeof(servSock_in));
    	servSock_in.sin_family = PF_INET;
    	servSock_in.sin_addr.s_addr = htonl(INADDR_ANY);
    	servSock_in.sin_port = htons((u_short)port);
    
    
    	//bind server
    	if (bind(sock, (struct sockaddr *)&servSock_in, sizeof(servSock_in)) < 0)
    	{
    		printf("I couldn't attach gate to port...\n");
    		return -1;
    	}
    	else
    		printf("Waiting for messages...\n");
    you asked me about blocking or non blocking sockets..actually I don't know the difference between them (i'm new in network programming..)

  7. #7
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    i'm sure that the code is OK but I keep on getting a negative descriptor here:
    -> sock = socket(PF_INET, SOCK_DGRAM,0);

    I'm using Windows visual C++... I made a link to wsock32.lb....

  8. #8
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    You have to call WSAStartup() before any other socket calls.

  9. #9
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    that's right!
    I can't believe I forgot to do this!!
    thanks a lot

  10. #10
    and the hat of wrongness Salem's Avatar
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    Which, if you'd actually BOTHERED to look at some error information rather than the programming equivalent of *shrug*, you would have found out.

    I won't be telling you to check for errors again.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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