Output Streams
==============

'lib/ostream.h' describes Dovecot's output streams. Output streams can be
stacked on top of each others as many times as wanted.

Currently there are only two output streams:

 * file: Write to given fd using 'pwrite()' for files and 'write()' for
   non-files.
 * buffer: Write to <buffer> [Design.Buffers.txt].

Writes are non-buffered by default. To add buffering, use 'o_stream_cork()' to
start buffering and 'o_stream_uncork()' to stop/flush. When output buffer gets
full, it's automatically flushed even while the stream is corked. The term
"cork" is used because with TCP connections the call actually sets/removes TCP
cork option. It's quite easy to forget to enable the corking with files, making
the performance worse.

When doing a lot of writes, you can simplify the error handling by delaying the
error checking. Simply do a lot of writes and finally check if
'stream->last_failed_errno' is non-zero.

If output buffer's size isn't unlimited, the writes can also fail because
there's no more space in the buffer and 'write()' syscall is returning
'EAGAIN'. This of course doesn't happen with blocking fds (e.g. files), but you
need to handle this in some way with non-blocking network sockets. A common way
in Dovecot to handle this is to just use unlimited buffer sizes and after each
write check if the buffer size becomes too large, and when it does it stops
writing until more space is available.

(This file was created from the wiki on 2017-10-10 04:42)