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6-a-failed-flush-keeps-references-the-echo-pattern-then-overwrites
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October 2, 2026 11:10
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sendmmsg stops at the first datagram it cannot send. flush returned that
error and left the whole batch staged, so a destination the kernel
always refuses (EMSGSIZE, EPERM, EHOSTUNREACH) failed every later flush
and stopped replies to everyone. A short send dropped its tail even
when it stopped on EAGAIN. In the echo pattern a failed flush also kept
references to receive slots that the next receive overwrote, sending
new peers' bytes to old peers.
flush now returns a flushed report. An error that belongs to one
datagram drops that entry and the rest still go. An error that belongs
to the socket (EAGAIN, ENOBUFS, EINTR, EBADF, ...) stops the flush and
keeps everything unsent staged, in order, behind a head offset. The
error classification lives in detail::send_all, tested against scripted
sendmmsg results. The echo example and README discard after a stall.
Breaking: flush returns flushed instead of resultstd::size_t.