sending and receiving 2D array using UDP

Posted on 2003-11-04
Last Modified: 2010-04-15
I am trying to send and recieve a 5x5 int array.

In my code I have for sending(UDP) int distvector[5][5]:

        remoteServAddr.sin_port = htons(5000+i*1000);
        rc = sendto(sd, &distvector, sizeof((int)distvector[5][5]), 0,
        (struct sockaddr *) &remoteServAddr,

And for recieving I have (UDP) int recv_DV[5][5]:

        recvfrom(sd, &recv_DV, sizeof((int)recv_DV[5][5]), 0,
         (struct sockaddr *) &cliAddr, &cliLen);

i dont recieve the right array, recv_DV[0][0] has an odd number in it (I suppose its the array's address) and the rest of the array is '0'.  Also in the sendto() function I used distvector instead of &distvector but still the same result.

ne help would be much appreciated.

Question by:imranqurai
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LVL 22

Accepted Solution

grg99 earned 500 total points
ID: 9684022
your size expression "sizeof((int)distvector[5][5]), " is probably computing the size of an integer.

Try making a new variable:  Length = sizeof( distvector ), print out it's value to be sure, and use it in the send and recv calls.
That should work a heck of a lot better.

And you do realize that UDP isnt guaranteed to deliver anything at all, and no guarantee what order packets arrive?

LVL 10

Expert Comment

ID: 9684198
-----> rc = sendto(sd, &distvector, sizeof((int)distvector[5][5]), 0,
        (struct sockaddr *) &remoteServAddr,

U are using sizeof ( (int) distvector[5][5] ) ---.> This would give u the size of the last element in your array i.e. the size of int on your machine

For getting the size of whole array, u should use         sizeof ( distvector )
LVL 10

Expert Comment

ID: 9684203
Same comment above applies to recvfrom also
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LVL 10

Expert Comment

ID: 9684246
Also I don't think you would need to pass &distvector in sendto OR recvfrom

both these functions expect a void pointer *. So U can just pass szDataFromUdp as the second parameter to sendto and recvfrom.

Basically the second parameter expects the starting address of the data to be passed and the amount of data passed is governed by the next parameter.

Thus only using szDataFromUdp  as second parameter would give u the base address of the data and sizeof( szDataFromUdp ) would give u the amount of data to be passed

Expert Comment

ID: 9684401
Your code looks clean and should work .. just a suggestion calculate the length before hand and use that in send and recv functions like

//for sending
int distvector[5][5],len;
len = sizeof(distvector);
sendto(sd,&distvector,len,0,(struct sockaddr *) &remoteServAddr,sizeof(remoteServAddr);

// for receiving
int recv_DV[5][5],len;
len = sizeof(recv_DV);
recvfrom(sd, &recv_DV, len, 0,(struct sockaddr *) &cliAddr, &cliLen);
LVL 10

Expert Comment

ID: 9684435
Hi Ajar,

I think, as I already posted, the second parameter shoudl be distvector and NOT &distvector

Your code would pass the len amount of data starting from ADDRESS of distvector

LVL 10

Expert Comment

ID: 9684478
Sorry Guys,

I am really sorry

Did not come to mind that distvector and &distvector would be same

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