Single buffer; multiple sockets; single syscall under Linux

Question

Does Linux have any native kernel facility that enables send()ing a supplied buffer to a set of sockets? A sort-of vectored I/O, except for socket handles rather than for a buffer.

The objective being to reduce the number of u/k transitions involved in situations where-in for example you need to broadcast some state update to n clients which would require iterating through each socket and sending.

One restriction is that TCP sockets must be supported (not under my control)

Answer 1

The answer is no, neither linux nor posix systems have the call you want. I fear that you don't get any advantage of having it, as each of the data streams will follow different paths and that makes copying the buffers in kernel than in user space. Not making copies in user-to-kernel doesn't neccesarily mean doing in kernel mode is better.

Either way, in linux you can implement this kind of mwrite (or msend) system call, as you have the source code. But I'm afraid you won't get anything but a head pain. The only approach to this implementation is some kind of copy-on-divert method, but I don't think you'll get any advantage.

Finnaly, once you have finished the first write(2) call, the probability of having to swap in the user buffer again in the next is far too low, making the second and next copies of the buffer will have very low overhead, as the pages will be all in core memory. You need a very high loaded system to get the user buffer swapped out in the time between syscalls.