Comment by kazinator

It's very easy to end up with small writes. E.g.

  1. Write four bytes (length of frame)
  2. Write the frame (write the frame itself)

The easiest fix in C code, with the least chance of introduce a buffer overflow or bad performance is to keep these two pieces of information in separate buffers, and use writev. (How portable is that compared to send?)

If you have to combine the two into one flat frame, you're looking at allocating and copying memory.

Linux has something called corking: you can "cork" a socket (so that it doesn't transmit), write some stuff to it multiple times and "uncork". It's extra syscalls though, yuck.

You could use a buffered stream where you control flushes: basically another copying layer.