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Sending/receiving data with TCP

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• Sending messages on TCP sockets
  • Sending (= writing) with a TCP socket s:

    int write(int s, void *buf, int nbytes) Input parameters: s = the TCP socket buf = buffer address containing the data to be sent nbytes = number of bytes in buffer "buf" to send Return value: -1 = error >=0 = the actual number of bytes sent

    Effect:

    write(s, buf, nbytes) will try to send nbytes in buf through the socket s Usually: write(s, buf, nbytes) will send all of the nbytes However: If the send buffer space is (near) full: write(s, buf, nbytes) can send less than nbytes (Then the return value will be less than nbytes)

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• A commonly used TCPsend( ) function
  • The following function is guaranteed to send all nBytes:

    int TCPsend( int s, char *buf, int nBytes ) { int k, n; k = 0; // No bytes sent yet.... /* ------------------------------------------------------------ Call write( ) as long as we did not send nBytes.... ------------------------------------------------------------- */ while ( k < nBytes ) { n = write( s, &buf[k], nBytes-k ); // Send "remaining" nBytes-k bytes /* ======================================== Check if there was an error.... ======================================== */ if ( n == -1 ) return -1; // Return error.... k = k + n; // We sent n more bytes, add to k } return nBytes; // Success... }

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• Receiving messages on TCP sockets

  • Receiving (= reading) data with a TCP socket s:

    int read(int s, void *buf, int nbytes) Input: s = TCP socket buf = address of the buffer to received read data nbytes = # bytes that we wish to receive Return value: -1 = error >=0 = the actual number of bytes transfered

    Effect:

    read(s, buf, nbytes) will block until some data is available When data is available, read(s, buf, nbytes) will try to: Transfer nbytes from the receive buffer of socket s Store the data in the buf variable Usually: read(s, buf, nbytes) will transfer exactly nbytes However: When the receive buffer is near empty, the read operation may return with less than nbytes.

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• A commonly used TCPrecv( ) function
  • The following function is guaranteed to receive exactly nBytes:

    int TCPrecv( int s, char *buf, int nBytes ) { int k; k = 0; // No bytes received yet.... /* ------------------------------------------------------------ Call read( ) as long as we did not receive nBytes.... ------------------------------------------------------------- */ while ( k < nBytes ) { n = read( s, &buf[k], nBytes-k ); // Receive "remaining" nBytes-k bytes /* ======================================== Check if there was an error.... ======================================== */ if ( n == -1 ) return -1; // Return error.... k = k + n; // We sent n more bytes, add to k } return nBytes; }

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• Closing a TCP connection
  • Closing a TCP connection:

    int s; // TCP socket used to communicate close(s);

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• For CS450 students

  • Programming remark for CS450 students:

    A TCP socket behaves exactly like a pipe or an file descriptor of an open file ! The read( )/write( ) call can return with less than the specified amount of data when we reach the end of the file !!!

    The similarity is obvious in the behavior:

    a pipe or an opened file in UNIX is a stream-oriented device: the unit (smallest amount) of data that can be read/written is one byte