C: Sending long strings with send()

Question

I'm using Linux and trying to send a long message through send(). The message is 1270 bytes, but my client application is only receiving 1024 bytes.

Since 1024 bytes is such a convenient number, I'm guessing that send() can only send 1024 bytes at a time. I looked at the man page for send, but all it says about long messages is:

When the message does not fit into the send buffer of the socket, send() normally blocks, unless the socket has been placed in nonblocking I/O mode. In nonblocking mode it would fail with the error EAGAIN or EWOULD- BLOCK in this case. The select(2) call may be used to determine when it is possible to send more data.

I'm using blocking mode, and the man page doesn't say what to do. My exact call to send looks like this: send(socket, message, strlen(message), 0);

Would I need to split up the string into 1024 byte chunks and send them separately? And how would my client handle this? If my client needs to do anything, I'll just mention that it's in Java and it uses InputStreamReader to receive data.

Answer 1

There is a misunderstanding on your part as to how send actually works. It does not send whatever you throw at it, it sends at most the number of bytes you pass in as a parameter. That means you have to check the return value of send, remove the number of bytes that actually got send from your send queue and try to send the remaining stuff with another call to send. Same holds true for recv, by the way.

Answer 2

In addition to what Jim Brissom said, it is worth pointing out that SOCK_STREAM sockets (ie. TCP) do not have a concept of message boundaries - the connection is just one long unstructured stream of bytes.

Just because you sent 100 bytes (say) with one send() call does not mean it will arrive as 100 bytes one recv() call. In the extreme, it may arrive in 100 separate recv()s of 1 byte each - or at the other, it may arrive as part of a larger recv(), along with prior or following data.

Your receiver must be able to handle this - if you need to define individual messages within the stream, you must impose some sort of structure upon the stream yourself, at the application layer.