POSIX Sockets: How to detect Ctrl-C sent over Telnet?

Question

Short Question
What's the right way to handle a Ctrl-C event sent over Telnet on the server side?

Long Question
After calling recv() on a socket, I'd like to handle some situations appropriately. One of them is to return a certain error code when Ctrl-C was received. What's the correct way to detect this? The following works, but it just doesn't seem right:

size_t recv_count;
static char ctrl_c[5] = {0xff, 0xf4, 0xff, 0xfd, 0x06};

recv_count = recv(socket, buffer, buffer_size, 0);

if (recv_count == sizeof(ctrl_c) &&
    memcmp(buffer, ctrl_c, sizeof(ctrl_c) == 0)
{
    return CTRL_C_RECEIVED;
}

I found a comment on Ctrl-C in a side-note in this UNIX Socket FAQ:

[...] (by the way, out-of-band is often used for that ctrl-C, too).

As I understand, receiving out-of-band data is done using recv() with a certain flag as the last parameter. But when I'm waiting for data using recv() as I do in the code above, I can't read out-of-band data at the same time. Apart from that, I'm getting something using recv() without that oob-flag.

Answer 1

Set the socket to non-blocking with fcntl(), use select() (pselect() on some systems) to check for arriving data. That is how to sample a socket's current condition, i.e. whether it has data to recv() and if can accept a send(), or there is an exception. Don't simply sit there blocking.

A recv() returns as much available information as the size of the buffer supplied can hold. If the socket has been configured to receive of out-of-band data (socket option SO_OOBINLINE) and there is unread OOB data, only out-of-band data is returned. Call ioctl() SIOCATMARK to determine whether any more out-of-band data remains un-read.

When you receive OOB data you cannot recv() past the end of the OOB packet in a single recv() call, so it is goof-proof in that regard.

I don't know what is considered best practice, but the idea of grabbing ctrl-c ahead of other already buffered socket data is a good one.