boost::asio::streambuf -如何重用缓冲区？

Question

我正在实现TCP服务器，它使用asio socket.async_read()和boost::asio::async_read_until()方法从套接字异步读取数据。两者都使用相同的处理程序从boost::asio::streambuf读取数据。

通过async_read()完全调用的处理程序：

void handle_read(const boost::system::error_code& ec, std::size_t ytes_transferred) )
{
    m_request_buffer.commit(bytes_transferred);
    boost::asio::streambuf::const_buffers_type rq_buf_data = m_request_buffer.data();
    std::vector<uint8_t> dataBytes(boost::asio::buffers_begin(rq_buf_data), boost::asio::buffers_begin(rq_buf_data) + bytes_transferred);

    //process data here

    m_request_buffer.consume(bytes_transferred);
    bytes_transferred = 0;
}

我的服务器依赖于对数据的处理，可能会关闭连接或通过同一个套接字继续读取。

但是，如果handle_read()是从第二个boost::asi::async_read_until()调用中调用的，我将在dataBytes中得到一个零的值，然后有效的数据就会出现。

我尝试了一个简单的测试用例，发现在将数据写入streambuf之后，并提交()+消费()，streambuf中的数据仍然保留先前的缓冲区。

那么，有什么方法可以清除boost::asio::streambuf中的数据并在boost::asio::async_read_until()中重用它吗？

活Coliru

如果使用USE_STREAM=1进行编译，则该活生生的示例可以正常工作。但是，与缓冲区消耗()相比，istream有什么不同呢？

Answer 1

当使用在Boost.Asio上操作的streambuf操作或使用streambuf的流对象(如std::ostream和std::istream )时，将正确地管理底层输入和输出序列。如果向操作提供缓冲区，例如将prepare()传递给读操作，或者将data()传递给写操作，则必须显式地处理commit()和consume()。

该示例中的问题是，它违反了API协议，导致未初始化的内存被提交到输入序列。commit()文档声明：

需要前面的prepare(x)调用x >= n，并且不需要修改输入或输出序列的中间操作。

std::ostream在prepare()和commit()之间的使用违反了该契约，因为它将修改输入序列：

// Prepare 1024 bytes for the output sequence.  The input sequence is
// empty.
boost::asio::streambuf streambuf;
streambuf.prepare(1024);

// prepare() and write to the output sequence, then commit the written
// data to the input sequence.  The API contract has been violated.
std::ostream ostream(&streambuf);
ostream << "1234567890";

// Commit 10 unspecified bytes to the input sequence.  Undefined
// behavior is invoked.
streambuf.commit(10);

下面是一个完整的示例示范，它使用带注释的streambuf：

#include <iostream>
#include <vector>
#include <boost/asio.hpp>

int main()
{
  std::cout << "with streams:" << std::endl;
  {
    boost::asio::streambuf streambuf;

    // prepare() and write to the output sequence, then commit the written
    // data to the input sequence.  The output sequence is empty and
    // input sequence contains "1234567890".
    std::ostream ostream(&streambuf);
    ostream << "1234567890";

    // Read from the input sequence and consume the read data.  The string
    // 'str' contains "1234567890".  The input sequence is empty, the output
    // sequence remains unchanged.
    std::istream istream(&streambuf);
    std::string str;
    istream >> str;
    std::cout << "str = " << str << std::endl;

    // Clear EOF bit.
    istream.clear();

    // prepare() and write to the output sequence, then commit the written
    // data to the input sequence.  The output sequence is empty and
    // input sequence contains "0987654321".
    ostream << "0987654321";

    // Read from the input sequence and consume the read data.  The string
    // 'str' contains "0987654321".  The input sequence is empty, the output
    // sequence remains unchanged.
    istream >> str;
    std::cout << "str = " << str << std::endl;
  }

  std::cout << "with streams and manual operations:" << std::endl;
  {
    boost::asio::streambuf streambuf;

    // prepare() and write to the output sequence, then commit the written
    // data to the input sequence.  The output sequence is empty and
    // input sequence contains "1234567890".
    std::ostream ostream(&streambuf);
    ostream << "1234567890";

    // Copy 10 bytes from the input sequence.  The string `str` contains
    // "1234567890".  The output sequence is empty and the input
    // sequence contains "1234567890".
    auto data = streambuf.data();
    std::string str(boost::asio::buffers_begin(data),
                    boost::asio::buffers_begin(data) + 10);
    std::cout << "str = " << str << std::endl;

    // Consume 10 bytes from the input sequence.  The input sequence is
    // now empty.
    streambuf.consume(10);

    // prepare() and write to the output sequence, then commit the written
    // data to the input sequence.  The output sequence is empty and
    // input sequence contains "0987654321".
    ostream << "0987654321";

    // Copy 10 bytes from the input sequence.  The string `str` contains
    // "0987654321.  The output sequence is empty and the input
    // sequence contains "0987654321".    
    data = streambuf.data();
    str.assign(boost::asio::buffers_begin(data),
               boost::asio::buffers_begin(data) + 10);
    std::cout << "str = " << str << std::endl;

    // Consume 10 bytes from the input sequence.  The input sequence is
    // now empty.
    streambuf.consume(10);
  }
}

输出：

with streams:
str = 1234567890
str = 0987654321
with streams and manual operations:
str = 1234567890
str = 0987654321

有关streambuf使用的更多信息，请考虑阅读这答案。