udp client in c receiving a text file

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I am making a udp client to receive a simple text file,the code builds fine but gives me a blank console when run,after some observation I found out that the problem is occuring at the time of recetion,so I am pasting that part of my code,could someone please guide me...

C++

size_t data=0;
if(data=recvfrom(sd, file_buffer, sizeof(file_buffer), 0, (struct sockaddr *) &server, &server_length)<0)
{
    printf("Error receiving file.");
    exit(1);
}

if(data==sizeof(file_buffer))
{
    printf("Received Data:[%s]",file_buffer);
}

the point where I am receiving data:

data=recvfrom(sd, receivedData, sizeof(receivedData)-1, 0, (struct sockaddr *) &server,  &server_length);
receivedData[data]='\0';

the point where I am writing data to the file:

if(fwrite(receivedData, 1, data, fp)!=data);
       {
           printf("Error writing file! \n");
           exit(1);
       }

Posted 5-Apr-13 19:23pm

Aayman Khalid

Updated 5-Apr-13 22:59pm

v4

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Richard MacCutchan 6-Apr-13 3:45am

You should not be using UDP to receive data such as this. UDP datagrams do not have guaranteed delivery or order; switch to TCP.

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SoMad · Answer 1 · 2013-04-05T20:03:00

Solution 1

At first I thought the whole problem was in the first if condition, because it is somewhat busy with calling recvfrom(), assigning the return value to data and then doing a less than comparison.
But after looking at it for a while, I realized you have size_t data = 0;, but size_t is an unsigned integer type, so it will never have a value less than zero. My guess is that you actually do get a negative return value from recvfrom(), but it never gets caught because of this.

It looks like you should change it to int data = 0; and then see if you land on exit(1);.

I wish I could do more, but that is tough without seeing more of the code, so try that change out and do some troubleshooting in the code. Outputting the return value of recvfrom() when it fails would be helpful.

Soren Madsen

Posted 5-Apr-13 20:03pm

SoMad

Updated 5-Apr-13 20:10pm

v2

Comments

Aayman Khalid 6-Apr-13 2:52am

as you said I've changed the data type of data to "int" and well it appears that the result is less than zero...

SoMad 6-Apr-13 2:56am

Well, this is progress. Now you have to find out why it fails. What is the return value?

Soren Madsen

Aayman Khalid 6-Apr-13 3:16am

the return value is "-1" so basically the client is not receiving the file,when I send some string from the server side the client receives it,so I thought that I will send the contents of the text file to client and will first display it on the console but here rises the issue!

SoMad 6-Apr-13 4:12am

Yes, that is the value of SOCKET_ERROR. You need to call WSAGetLastError() to get the actual error. See http://msdn.microsoft.com/en-us/library/windows/desktop/ms740120(v=vs.85).aspx[^].

Soren Madsen

Aayman Khalid 6-Apr-13 4:27am

finally received the data:) but there is still one problem:although the file gets written, still I get on the console:"Error writing file" and I also get a little bit of useless data although I set the receiving buffer to zero before receiving the text file...

SoMad 6-Apr-13 4:44am

Unfortunately it is quite difficult for us to speculate what might be your problem without seeing the code.

Soren Madsen

Aayman Khalid 6-Apr-13 5:00am

I've uploaded the relevant code.

SoMad 6-Apr-13 5:25am

Alright. Well, as Richard pointed out, UDP does not have guaranteed delivery, so you could be missing packets. However, I believe I see the problem.

You have a semicolon at the end of the if statement that writes to the file. Remove that semicolon. You should also make sure data has a valid value.

if (data > 0)
{
if(fwrite(receivedData, 1, data, fp)!=data)
{
printf("Error writing file! \n");
exit(1);
}
}

Soren Madsen

Aayman Khalid 10-Apr-13 1:19am

I removed the semicolon but the result is still the same...

SoMad 13-Apr-13 5:31am

Sorry about the delay. I was pretty sure that semicolon was the problem. Tell me something, are you sure the file is open when you write to it?

Soren Madsen

Aayman Khalid 15-Apr-13 1:02am

the code is fine now,Thank you so much:)

SoMad 15-Apr-13 1:09am

Great. Glad I could help.

Soren Madsen

H.Brydon 23-Apr-13 23:19pm

+5 for the answer and the extra effort.

SoMad 23-Apr-13 23:50pm

Thanks (x3) Harvey.

Soren Madsen