Boost C++ Libraries: Ticket #3627: boost::asio::async_read() cannot be used with null_buffers()

severity changed

jkp@… — Tue, 12 Jan 2010 14:16:27 GMT

severity Problem → Regression

I can also confirm that this is the case on OS X. The question I have is whether the change is intended? The new documentation seems to imply it is:

If the total size of all buffers in the sequence mb is 0, the asynchronous read operation shall complete immediately and pass 0 as the argument to the handler that specifies the number of bytes read.

Also, the changelist for 1.40.0 says the following:

Changed the buffered*_stream<> templates to treat 0-byte reads and writes as no-ops, to comply with the documented type requirements for SyncReadStream, AsyncReadStream, SyncWriteStream and AsyncWriteStream.

It's unclear what the intendended behaviour is from these two snippets. Certainly pre 1.40.0 you could call async_read with null_buffers and get a callback when some new data had arrived. This is extremely useful functionality, esp for code that has layered other streams on-top of the asio ones. Right now I have no way to check for data (async) without reading it off the socket.

severity changed

chris_kohlhoff — Tue, 12 Jan 2010 21:28:45 GMT

severity Regression → Problem

Not a regression. The null_buffers() type never worked with async_read().

You may confirm that it doesn't work by making the following changes to the nonblocking example:

@@ -117,7 +117,8 @@

if (session_impl_.want_read() && !read_in_progress_) {

read_in_progress_ = true;

socket_.async_read_some(

+ boost::asio::async_read(socket_, + socket_.async_read_some(

boost::asio::null_buffers(), boost::bind(&connection::handle_read,

shared_from_this(),

@@ -138,6 +139,8 @@

void handle_read(boost::system::error_code ec) {

+ printf("handle_read\n"); +

read_in_progress_ = false;

Notify third party library that it can perform a read.

I just checked against 1.39.

type, severity, milestone changed

chris_kohlhoff — Tue, 12 Jan 2010 21:46:23 GMT

type Bugs → Feature Requests
severity Problem → Not Applicable
milestone Boost 1.41.0 → To Be Determined

The null_buffers type is only intended for use with async_read_some() and async_write_some() on the "lowest layer", i.e. the socket or descriptor. It is not part of any of the stream type requirements, and so not supported by composed operations such as async_read.

If it worked with the buffered*_stream<> templates then that was just a side effect of the 0-byte read/write bug.

It may or may not be useful to support null_buffers as part of the stream type requirements. I'm reclassifying this as a feature request as something to think about in the future.

In any case:

Right now I have no way to check for data (async) without reading it off the socket.

is not correct. It works just fine if you use async_read_some() on the socket.