提高:: ASIO读取n个字节从插座到流缓冲 [英] boost::asio read n bytes from socket to streambuf

问题描述

我有一个序列化的结构，它正在通过套接字发送。我需要大块读它，因为它的领域之一包含剩余数据的大小，所以我需要阅读前几个字节，找出的长度和读取休息。这是我有：

I have got a serialized structure, which is being send via socket. I need to read it by chunks, since one of its fields contain size of data remaining, so I need to read first few bytes, find out the length and read the rest. This is what I have got:

    boost::asio::streambuf buffer;
    boost::system::error_code err_code;
    // here I need to read only first 16 bytes
    boost::asio::read(socket, buffer, err_code);
    std::istream is(&buffer);
    boost::archive::binary_iarchive ia(is);
    ia >> my_struct;

我已经采取了看

    boost::asio::async_read(s, boost::asio::buffer(data, size), handler);

但它只能读取数据，以提高:: ASIO ::缓​​冲区。我想知道如果我可以做的boost :: ASIO ::流缓冲一样吗？谢谢你在前进。

but it can only read data to boost::asio::buffer. I am wondering if I could do the same with boost::asio::streambuf? Thank you in advance.

推荐答案

有两个<一个重载href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/read.html\"><$c$c>boost::asio::read()和<一个href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/async_read.html\"><$c$c>boost::asio::async_read()接受的<一个实例href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/basic_streambuf.html\"><$c$c>boost::asio::basic_streambuf作为其缓冲区：

There are overloads for both boost::asio::read() and boost::asio::async_read() that accept instances of boost::asio::basic_streambuf as its buffer:

read(SyncReadStream&, basic_streambuf&);
read(SyncReadStream&, basic_streambuf&, CompletionCondition);

read(SyncReadStream&, basic_streambuf&, boost::system::error_code&);
read(SyncReadStream&, basic_streambuf&, CompletionCondition,
     boost::system::error_code&);

async_read(AsyncReadStream&, basic_streambuf&, ReadHandler);
async_read(AsyncReadStream&, basic_streambuf&, CompletionCondition,
           ReadHandler);

当调用一个不接受CompletionCondition过载，它是相当于调用其关联超载与<一个一个CompletionCondition href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/transfer_all.html\"><$c$c>boost::asio::transfer_all(),使操作来读取<一href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/basic_streambuf/max_size.html\"><$c$c>streambuf.max_size()字节。

When calling an overload that does not accept a CompletionCondition, it is the equivalent to calling its associated overload with a CompletionCondition of boost::asio::transfer_all(), causing the operation to read streambuf.max_size() bytes.

有关读取字节的已知量成流缓冲，可以使用：

For reading a known amount of bytes into a streambuf, either use:

• 的<一个href=\"http://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/transfer_exactly.html\"><$c$c>boost::asio::transfer_exactly(n) CompletionCondition限制从组成的操作传输的字节量：

• the boost::asio::transfer_exactly(n) CompletionCondition to limit the amount of bytes transferred from the composed operation:

std::size_t n = // ...
boost::asio::read(socket, streambuf, 
    boost::asio::transfer_exactly(n), error);

明确创建输出序列将用作缓冲，然后commiting读入流缓冲的输入序列的字节数：

Explicitly creating an output sequence that will serve as the buffer, then commiting the bytes read into the streambuf's input sequence:

std::size_t n = // ...
std::size_t bytes_transferred = boost::asio::read(socket,
    streambuf.prepare(n), // creates a boost::asio::buffer
    error);
streambuf.commit(bytes_transferred);

下面是一个完整的例子示范这两种方法：

Here is a complete example demonstrating both of these approaches:

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

// This example is not interested in the handlers, so provide a noop function
// that will be passed to bind to meet the handler concept requirements.
void noop() {}

std::string make_string(boost::asio::streambuf& streambuf)
{
  return {boost::asio::buffers_begin(streambuf.data()), 
          boost::asio::buffers_end(streambuf.data())};
}

int main()
{
  using boost::asio::ip::tcp;
  boost::asio::io_service io_service;

  // Create all I/O objects.
  tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 0));
  tcp::socket server_socket(io_service);
  tcp::socket client_socket(io_service);

  // Connect client and server sockets.
  acceptor.async_accept(server_socket, boost::bind(&noop));
  client_socket.async_connect(acceptor.local_endpoint(), boost::bind(&noop));
  io_service.run();

  // Write to server.
  boost::asio::streambuf write_buffer;
  std::ostream output(&write_buffer);
  output << "abc";
  std::cout << "Writing: " << make_string(write_buffer) << std::endl;
  auto bytes_transferred = boost::asio::write(server_socket, write_buffer);

  // Read from client.
  boost::asio::streambuf read_buffer;
  bytes_transferred = boost::asio::read(client_socket, read_buffer,
      boost::asio::transfer_exactly(bytes_transferred));
  std::cout << "Read: " << make_string(read_buffer) << std::endl;
  read_buffer.consume(bytes_transferred); // Remove data that was read.

  // Write to server.
  output << "def";
  std::cout << "Writing: " << make_string(write_buffer) << std::endl;
  bytes_transferred = boost::asio::write(server_socket, write_buffer);

  // Read from client.
  bytes_transferred = boost::asio::read(client_socket, 
      read_buffer.prepare(bytes_transferred));
  read_buffer.commit(bytes_transferred);      
  std::cout << "Read: " << make_string(read_buffer) << std::endl;
  read_buffer.consume(bytes_transferred); // Remove data that was read.
}

输出：

Writing: abc
Read: abc
Writing: def
Read: def

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