client-server programme

Question

We have made simple client.c and server.c programme in an UNIX environment. We are using it transfer a simple text file by first opening it, then reading it and sending using open, read, and send system calls; on client side I am receiving it, and writing it by creating a file on server machine. The transfer is taking place quite smoothly but the file recieved at the client side is not exactly the same on the server side. In between the readable characters there are some unreadable characters. Can you please tell me what could be the possible reason for this? Though the most part of the file is same, only a small part in between has some discrepancies.

Code can be seen here.

Thanks!

server side loop:

do
{
    n=read(t,buf,100);
    write(1,buf,strlen(buf));
    send(connected, buf,strlen(buf), 0);    
} while(n!=0);

on client side

do
{
    bytes_recieved=recv(sock,recv_data,100,0);
    count=write(t,recv_data,strlen(recv_data));  
} while(bytes_recieved!=0);

Answer 1

MS Windows systems use carriage return-linefeed as a line ending, linux using just linefeed.

The "unreadable character" is Ctrl-M, the carriage return, character number 13.

Answer 2

You should check your send()s to see if they succeed. You may find that at some point the write buffer (that is, the socket's output buffer) is full and the send fails. This is normal behavior. Send returns the number of bytes sent. If you are not sending multibyte characters, this would probably manifest as missing data rather than corruted data. If you lose one byte of a two byte character, it could result in corruption.

Answer 3

In the first instance, don't use strlen(). There's absolutely no guarantee that there'll be a NUL byte anywhere in the file stream, and in fact with a text file there never should be.

Each time around the loop you should output the actual number of bytes that were received, not how many strlen() thinks might be in the buffer:

do
{
    bytes_received = recv(sock, recv_data, 100, 0);
    if (bytes_received < 0) {
        perror("read");
        break;
    } else if (bytes_received == 0) {
        break;
    } else {
        count = write(t, recv_data, bytes_received);
        // in theory - add error check here too!
    }
} while (count > 0);