Sockets – Why is it assumed that send may return with less than requested data transmitted on a blocking socket

ccoding-stylesendsockets

The standard method to send data on a stream socket has always been to call send with a chunk of data to write, check the return value to see if all data was sent and then keep calling send again until the whole message has been accepted.

For example this is a simple example of a common scheme:

int send_all(int sock, unsigned char *buffer, int len) {
  int nsent;

  while(len > 0) {
    nsent = send(sock, buffer, len, 0);
    if(nsent == -1) // error
      return -1;

    buffer += nsent;
    len -= nsent;
  }
  return 0; // ok, all data sent
}

Even the BSD manpage mentions that

…If no messages space is available at the socket to hold the message to be transmitted, then send() normally blocks…

Which indicates that we should assume that send may return without sending all data. Now I find this rather broken but even W. Richard Stevens assumes this in his standard reference book about network programming, not in the beginning chapters, but the more advanced examples uses his own writen (write all data) function instead of calling write.

Now I consider this still to be more or less broken, since if send is not able to transmit all data or accept the data in the underlying buffer and the socket is blocking, then send should block and return when the whole send request has been accepted.

I mean, in the code example above, what will happen if send returns with less data sent is that it will be called right again with a new request. What has changed since last call? At max a few hundred CPU cycles have passed so the buffer is still full. If send now accepts the data why could'nt it accept it before?

Otherwise we will end upp with an inefficient loop where we are trying to send data on a socket that cannot accept data and keep trying, or else?

So it seems like the workaround, if needed, results in heavily inefficient code and in those circumstances blocking sockets should be avoided at all an non blocking sockets together with select should be used instead.

Best Answer

The thing that is missing in above description is, in Unix, system calls might get interrupted with signals. That's exactly the reason blocking send(2) might return a short count.

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