boost::asio::streambuf - 如何重用缓冲区？

Question

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

通过 async_read() 调用的完美处理程序：

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;
}

根据数据处理，我的服务器可能会关闭连接或通过同一套接字继续读取。

但是，如果从第二次 boost::asi::async_read_until() 调用中调用了 handle_read()，我会在 dataBytes 中得到多个零，然后有效数据会消失。

我尝试了一个简单的测试用例，发现在将数据写入streambuf后，commit()+consume()后streambuf中的数据仍然保留之前的缓冲区。

那么，有没有办法清除 boost::asio::streambuf 中的数据并在 boost::asio::async_read_until() 中重用它？

Live Coliru

如果使用 USE_STREAM=1 编译，现场示例可以正常工作。但是 std::istream 与 buffer consume() 相比有何不同？

Answer 1

当使用对streambuf 进行操作的Boost.Asio 操作或使用streambuf 的流对象（例如std::ostream 和std::istream）时，将正确管理底层输入和输出序列。如果为操作提供缓冲区，例如将prepare() 传递给读操作或将data() 传递给写操作，则必须显式处理commit() 和consume()。

示例中的问题是它违反了 API 约定，导致未初始化的内存被提交给输入序列。 commit() 文档指出：

需要前面调用 prepare(x) where x >= n，并且没有修改输入或输出序列的干预操作。

在prepare() 和commit() 之间使用std::ostream 违反了这个约定，因为它会修改输入序列：

// 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);

---

这是一个完整的示例 demonstrating 使用带有注释 cmets 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this 答案。