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Socket Programming (Client recieving invalid characters)

Posted on 2004-09-21
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Last Modified: 2013-11-20
Hi, I am currently developing a application that involves socket programming. Currently developing the server application on visual studio 6.0, and client application on embedded visual 3.0.

I did a socket connection between the server & client. As the server sends back information back to the client, it displays invalid charcters after the actual data (Eg. Apple*^@*^, Orange #&$($%@)

I suppose its the size, and I have to set a null at the end of the string. Is that so, could anyone guide me along on ways that I could solve this problem. Thank you.

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Question by:mwcmp
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Accepted Solution

by:
jkr earned 250 total points
ID: 12115297
If you receive data like

    int bytesRecv = SOCKET_ERROR;
    char recvbuf[32] = "";

    while( bytesRecv == SOCKET_ERROR ) {
        bytesRecv = recv( m_socket, recvbuf, 32, 0 );
        if ( bytesRecv == 0 || bytesRecv == WSAECONNRESET ) {
            // error
        }

    // zero-terminate string according to the amount of bytes received
    recvbuf[bytesRecv + 1] = 0;

the last line will correctly zero-terminate the string.
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Author Comment

by:mwcmp
ID: 12115952
Ok, i shall try it out and let you know again. Thanks!
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Expert Comment

by:AaronReams
ID: 12119157
Hi mwcmp,

jkr has the right idea, but the code has a couple errors.  you need to make sure you null terminate the buffer before the end of the buffer... so for a buffer of length 32 you only want to read 31 bytes, and then put the 0 terminator in the 31st position (since arrays are 0 based).  


    int bytesRecv = SOCKET_ERROR;
    char recvbuf[32] = "";

    while( bytesRecv == SOCKET_ERROR ) {
        bytesRecv = recv( m_socket, recvbuf, 31, 0 );
        if ( bytesRecv == 0 || bytesRecv == WSAECONNRESET ) {
            // error
        }

    // zero-terminate string according to the amount of bytes received
    recvbuf[bytesRecv] = 0;


good luck,
aaron
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LVL 8

Expert Comment

by:AaronReams
ID: 12119185
<disclaimer>

hey jkr, you da man around these parts so i by no means meant to step on anyones toes.  keep up the great info.

peace. -a
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Expert Comment

by:cool_alok
ID: 12120033
HI Friend,
            i dont know what are using , I wanna ask are u making the client/server socket architecture in MFC based classes like CSocket or CAsyncSocket or are using raw Socket Api's
if you are using the raw Api like this

int recv(
  SOCKET s,      
  char FAR *buf,  
  int len,        
  int flags      
);

the total byte of data recv is returned by the above function
and  you create the string like that

    char buffer[1000];
     int iRec=recv(hSocket,buffer,999,0);
     buffer[iRec]=NULL;
   is the required string by you

Now in MFC
int Receive( void* lpBuf, int nBufLen, int nFlags = 0 );

it s same functionality

i think it will help

   
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LVL 86

Expert Comment

by:jkr
ID: 12123300
>>jkr has the right idea, but the code has a couple errors

That's a sample just taken to illustrate the zero-termination :o)
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Author Comment

by:mwcmp
ID: 12140176
  int bytesRecv = SOCKET_ERROR;
    char recvbuf[32] = "";

    SOCKET AcceptSocket;

      // Receive data to the server
    while (1)
      {
        AcceptSocket = SOCKET_ERROR;
        while ( AcceptSocket == SOCKET_ERROR )
            {
            AcceptSocket = accept( m_socket, NULL, NULL );
        }
            tmp_socket = m_socket;
        m_socket = AcceptSocket;
        break;
    }


That's my current codes, how should i change to remove those invalid characters?
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LVL 8

Assisted Solution

by:AaronReams
AaronReams earned 250 total points
ID: 12140212
Well you aren't actually receiving any characters with that code.  After you accept the socket you need to do a recv to get characters.

After your while loop... add the following code to continually receive characters.

char buffer[1024];
while(true)
{
          bytesRecv = recv(AcceptSocket, buffer, 1023, 0);
          if(bytesRecv==0)
          {
            //Connection closed
              break;
          }
      else
      if(bytesRecv<0)
      {
            //Connection failed
            break;
      }
      buffer[bytesRecv]=0;
      printf("Your characters received are = %s",buffer);

}
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