Re: [Boost-users] Buffering and ASIO

Subject: Re: [Boost-users] Buffering and ASIO
From: Vadim Shmelev (vadim.shmelev_at_[hidden])
Date: 2008-12-15 06:54:38

> Good day.
> I'm trying to read video data transferred by HTTP. I use the following function in order 
> to read HTTP header:
> boost::asio::async_read_until(m_socket, m_headerBuffer, "\r\n\r\n",    boost::bind(&CAXISParser::read_header, this,  boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));

> , where m_headerBuffer is asio::streambuf object. During header parsing i can get video sample length. Now I want to read all video data in one function call, for example:

> boost::asio::async_read(m_socket,  boost::asio::buffer(pSample->GetBody(), m_currentSampleLength),  boost::bind(&CAXISParser::read_body, this, pSample,         boost::asio::placeholders::error,  boost::asio::placeholders::bytes_transferred));

>, where pSample is object with buffer for video data. There is problem that async_read_until captures > some data from video data, and that call's behavior is incorrect. How can i fight that problem? For example, how can i figure out number of bytes left in streambuf and copy them to my buffer, and then read rest with async_read? I'm will very appreciate for any advices.

> It's not clear to me exactly what you're asking here.  From what I can
> tell, you have created a buffer of size m_currentSampleLength, and
> asked ASIO to call CAXISParser::read_body when it has received this
> many bytes.  Is that what you intended to do?  Is that what boost is
> doing?  Or is the problem something else?
>
> -----Scott.

Hello, Scott.
Sorry for bad english, i'm will try explain.

Yep, there is problem. When i'm read HTTP header with acync_read_until, it
consume header and some bytes from video data (i think). After i'm
calculate video data length from header and trying read data with
async_read. But it works incorrect as previous call async_read_until
captures some bytes from video data and video data readed with async_read
have wrong offset.
How can i understand, the problem that async_read_until and async_read
used different buffers for reading (asio::streambuf and custom buffer),
and some video data consumed by streambuf.
Question: how to get video data captured with async_read_until and read
rest from tcp buffer?There is is code sample:void read_header(PSample pSample,     const boost::system::error_code& error, size_t bytes_transferred){    // Calculate data length with regex    boost::asio::const_buffer data = m_headerBuffer.data();    const char* beg = boost::asio::buffer_cast<const char*>(data);    const char* end = beg + bytes_transferred;    CalculateDataLength(beg, end);     // Clear buffer      std::vector<char> vec(bytes_transferred);    m_headerBuffer.sgetn(&vec[0], bytes_transferred);     ReadBody();} Before ReadBody call, i reject streambuf content with sgetn. But i think there ismore data in streambuf that is video data. How can i get it? Below non working sample, but it describe my wishes: // Add before read bodysize_t restBytes = m_headerBuffer.size() - bytes_transferred;if (0 != restBytes){    m_headerBuffer.sgetn(pSample->GetBody(), restBytes);} Then in ReadBody i can to read rest video data with single async_readcall. There is implementation sample: void ReadBody(NMMSS::PSample& pSample){1)//boost::asio::async_read(m_socket, m_headerBuffer.prepare(m_currentSampleLength),2)//boost::asio::async_read_until(m_socket, m_headerBuffer, "\r\n",3)//boost::asio::async_read(m_socket, boost::asio::buffer(pSample->GetBody(), m_currentSampleLength),    boost::bind(&CAXISParser::read_body, this,        pSample, boost::asio::placeholders::error,        boost::asio::placeholders::bytes_transferred));} I trying use 3 variants there:1) Trying read to buffer all video data (dont understand why it doesn't work)2) Very slow variant, as video data is big.3) Preferrable variant

P.S. One question more: how can i correctly reject data consumed by
streambuf? Now i used the following strategy:

streambuf buf;
.....
std::vector<char> vec(size);
buf.sgetn(&vec[0], size);

Is there another strategy for that? For example, commit and consume strategy? I dont need streambuf content in some
cases.

Best regards,
Vadim