Dynamically sized boost::asio::buffer

Question

I'm reading from a boost::asio::ip::udp::socket like this:

using boost::asio::ip::udp;

// ...

char recv_buf[128];
udp::endpoint sender_endpoint;
size_t len = socket.receive_from(boost::asio::buffer(recv_buf), sender_endpoint);

Now, this works perfectly fine, but the maximum amount of characters that I am able to recieve is now 127. However I am facing a problem because I need to accept some data input of which the length can greatly vary (and is not of well-defined length with prefixed headers, for example). A solution to this would be a dynamically expanding buffer, like a vector. Is it possible to create a dynamically expanding boost::asio::buffer to accept (theoretical) infite amounts of input and store it in a container?

Tripping

Answer 1

UDP datagram size does not vary all that greatly: it will never be greater than 65535, leaving room for 65,527 bytes of data after the 8-byte header.

Cocainism

+1 You are totally right! Still I would be interested in a dynamic boost::asio::buffer, not because 64KiB is so huge, but a dynamic buffer often seems a lot more appropriate, and for reuse in TCP. – Tripping

@nightcracker with TCP, asio calls the read handler when the buffer is full, so you can empty it and return to reading. – Cocainism

Answer 2

Boost 1.66.0 added dynamic_buffer which can adapt a reference to std::string or std::vector<CharType>:

• http://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/reference/dynamic_buffer.html

Keele

Answer 3

If you use smaller buffers, you can easily chain them together via the *BufferSequences concepts. For example, you can pass in a MutableBufferSequence to accept data from a read(2) call or pass a ConstBufferSequence for a list of buffers that you are going to write(2) out. That said, I tend to recommend using a single buffer in each direction because it tends to simplify code (though that's not always possible).

Clinkstone

Answer 4

There does not appear to be any provision for dynamic sizing. And it makes sense that there would not be. Think about what would have to happen:

• A single UDP datagram can only be received once, and at once, therefore:
• the buffer given to the low-level system call needs to be large enough for the largest valid message, so,
• for it to be efficient, the buffer must be allocated up-front by the caller.

So it does not make sense for there to be a dynamically-sized buffer available. As Cubbi points out, UDP datagrams have a smallish maximum size anyway, so just make your buffer as large as the largest valid message in your system and be done with it.

Pontias

boost::asio::buffer is a general ASyncronous Input/Output buffer, and is not only used for UDP. – Tripping

Sure. And in general it does not make sense to have a dynamic one. Neither for your application nor for TCP nor any other protocol which eventually calls the low-level C routines to do their work, because it will only lead to inefficient memory copying, which anathema in C++ and in Boost. – Pontias