[Boost-users] [boost::asio] Wait for multiple async_read()'s

Subject: [Boost-users] [boost::asio] Wait for multiple async_read()'s
From: Adam Zegelin (adam_at_[hidden])
Date: 2017-01-21 07:09:16

Hi,

I'm writing a boost::asio application that communicates with multiple
streams. I'd like to write data to these streams then wait for
everyone to reply before performing computation on the resulting data.

What is the best way to implement this?

The route I've ventured down involves me calling async_write() on each
stream, with the callback hander executing async_read(). But this
where I'm stuck. What is the best way to execute one final hander once
all read operations have completed?

One solution I've found, though it feels "un-asio" is to pass the
use_future option as the hander for the reads. The result is futures
for each read, which I can then place into a list<future>, and have an
additional io_service.post()'ed handler call get() on these futures.
The only issue with this is that I must call io_service.run() from
more than one thread, since the handler calling get() blocks —
otherwise I can potentially deadlock the whole application.

Another alternative I can think of is that the function that schedules
all the async_writes knows ahead-of-time the number of reads that need
to be completed. Each async_read() handler would put its results into
a shared list and then, if the list's size is equal to the expected
number of results, fires the "completion" handler. While easy enough
to implement, this seems fraught with issues. If there is more than
one thread calling io_service.run(), then access to the shared list
has to be synchronised. Also, it doesn't seem very generic — I have a
number of places where this write n, read n, execute (or is it map,
reduce?) pattern needs to happen, and I'd prefer a solution that was
write one, worked everywhere.

Any help would be appreciated.

- Adam