C++ Winsock recv() buffer junk

Question

I'm writing a console appication in cpp that sends control commands from a file via TCP to a host machine and receives a response. All those informations are shown on screen and logged to a file and this is the actual problem. The output string seems to store junk for any reason, even if I try to set a fixed length.

EDIT: cleaned up the code and took care of the return value of recv(). The only thing I don't get yet is that the 2nd recv line in my logfile is filled with junk. Maybe one of you guys is able to spot the problem.

        string cmd="";
        char *sendstr=(char*)cmd.c_str();
        fflush(stdin);
        int n = 1, total = 0;
        char temp[1024];
        string inStr;
        if(cmdin.is_open())
        {           
            while(!cmdin.eof())
            {
                total=0;
                cmd=fread();
                send(serverPC, sendstr, (int)strlen(sendstr),0);
                n=recv(serverPC,&temp[total],sizeof(temp)-total-1,0); // FIX THIS
                total+=n;
                temp[total]='\0';
                inStr=temp;
                fwrite(inStr,cmd);
            }
            cout << "Data successfully sent!\n";
        }
        else{
            cerr<<"can't find 'cmd.cfg' file"<<endl;
        }

The output I expect :

<11:40:00> INIT
received: VELO=0.00km/h  DOT=FORW  
----

This is what i get :

<10:05:56> INIT
received: VELO=0.00km/h  DOT=FORW  
----
<10:05:56> VELO=50.00
received: VELO=0.00km/h  DOT=FORW  VELO=0.00km/h  DOT=FORW  VELO=0.00km/h
DOT=FORW  VELO=0.00km/h  DOT=FORW  VELO=0.00km/h  DOT=FORW  VELO=0.00km/h  DOT=FORW  
VELO=0.00km/h  DOT=FORW  
----
<10:05:56> VELO=100.00
received: VELO=50.00km/h  DOT=FORW  
----
<10:05:56> DOT=BACK
received: VELO=50.00km/h  DOT=FORW

Separate networking support from data handling. Split the code into functions. Then rewrite main code using those functions. As it is it is very cumbersome to read. — Dariusz
I agree with Dariusz about functions. Your code is very difficult to read. Also you should be using sizeof instead of constant values for the length of tbuf, or do a define. — DAB
Ignore the return value of recv, get junk in the buffer. A fair and just deal. — n. 1.8e9-where's-my-share m.
gee ... thanks man. haven't even thought about the return value of recv I'll care about that right after I've done some clean-up in my code^^ — Madmob

icabod icabod · Accepted Answer · 2013-04-15T13:52:56

A key point to remember when writing a TCP receiver is that TCP is considered as a data stream. This means that when you perform a receive from something sending packets of N bytes of data, you can receive:

N bytes of data
less than N bytes of data
more than N bytes of data

Assuming there is no disconnection, buffer overflow, or other error, you will always receive what the server sends, in the order it sends it, but messages may be truncated or concatenated at the receiving end. It's one of the reasons that a lot [citation needed?] of custom TCP protocols include either a length field (so you know how much data you need to receive), or some way of denoting the end of the data. That way your receiver can loop until it has received just the right amount, leaving the rest in the TCP buffer for the next receive.

When you say you are seeing "junk", it doesn't look like junk to me, it looks like several packets of data concatenated together into one.

C++ Winsock recv() buffer junk

1 Answers