Boost ASIO：Platform-specific I/O Objects

Abstract

Asio还提供了特定于平台的I/O对象，因为有些异步操作只在某些平台上可用，例如Windows或Linux。

How

目录监控

#include 
#include 
#include 
#include 
#include 

using namespace boost::asio;
using namespace boost::system;

int main()
{
  io_service ioservice;

  HANDLE file_handle = CreateFileA(".", FILE_LIST_DIRECTORY,
    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
    OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);

  char buffer[1024];
  DWORD transferred;
  OVERLAPPED overlapped;
  ZeroMemory(&overlapped, sizeof(overlapped));
  overlapped.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
  ReadDirectoryChangesW(file_handle, buffer, sizeof(buffer), FALSE,
    FILE_NOTIFY_CHANGE_FILE_NAME, &transferred, &overlapped, NULL);

  windows::object_handle obj_handle{ioservice, overlapped.hEvent};
  obj_handle.async_wait([&buffer, &overlapped](const error_code &ec) {
    if (!ec)
    {
      DWORD transferred;
      GetOverlappedResult(overlapped.hEvent, &overlapped, &transferred,
        FALSE);
      auto notification = reinterpret_cast(buffer);
      std::wcout << notification->Action << '\n';
      std::streamsize size = notification->FileNameLength / sizeof(wchar_t);
      std::wcout.write(notification->FileName, size);
    }
  });

  ioservice.run();
}

这个例子中使用了I/O对象boost::asio::windows::object_handle，该对象仅在windows上可用。boost::asio::windows::object_handle，它基于windows函数RegisterWaitForSingleObject()，让你启动对象句柄的异步操作。RegisterWaitForSingleObject()接受的所有句柄都可以通过boost::asio::windows::object_handle使用。使用async_wait()，可以异步等待对象句柄的更改。

初始化boost::asio::windows::object_handle类型的对象obj_handle，使用windows函数CreateEvent()创建的对象句柄。句柄是OVERLAPPED结构的一部分，其地址传递给Windows函数ReadDirectoryChangesW()。Windows使用OVERLAPPED结构来启动异步操作。

ReadDirectoryChangesW()可用于监视目录并等待更改。要异步调用该函数，必须将OVERLAPPED结构传递给ReadDirectoryChangesW()。通过Boost报告异步操作的完成情况。Asio，事件处理程序在传递给ReadDirectoryChangesW()之前存储在OVERLAPPED结构中。这个事件处理程序随后被传递给obj_handle。当在obj_handle上调用async_wait()时，当在观察到的目录中检测到更改时，将执行处理程序。

#include 
#include 
#include 
#include 
#include 

using namespace boost::asio;
using namespace boost::system;

int main()
{
  io_service ioservice;

  HANDLE file_handle = CreateFileA(".", FILE_LIST_DIRECTORY,
    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
    OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED, NULL);

  error_code ec;
  auto &io_service_impl = use_service(ioservice);
  io_service_impl.register_handle(file_handle, ec);

  char buffer[1024];
  auto handler = [&buffer](const error_code &ec, std::size_t) {
    if (!ec)
    {
      auto notification =
        reinterpret_cast(buffer);
      std::wcout << notification->Action << '\n';
      std::streamsize size = notification->FileNameLength / sizeof(wchar_t);
      std::wcout.write(notification->FileName, size);
    }
  };
  windows::overlapped_ptr overlapped{ioservice, handler};
  DWORD transferred;
  BOOL ok = ReadDirectoryChangesW(file_handle, buffer, sizeof(buffer),
    FALSE, FILE_NOTIFY_CHANGE_FILE_NAME, &transferred, overlapped.get(),
    NULL);
  int last_error = GetLastError();
  if (!ok && last_error != ERROR_IO_PENDING)
  {
    error_code ec{last_error, error::get_system_category()};
    overlapped.complete(ec, 0);
  }
  else
  {
    overlapped.release();
  }

  ioservice.run();
}

boost::asio::windows::overlapped_ptr是一个I/O对象，它没有启动异步操作的成员函数。异步操作是通过将内部OVERLAPPED变量的指针传递给一个Windows函数开始的。除了一个I/O服务对象之外，boost::asio::windows::overlapped_ptr的构造函数还期望在异步操作完成时调用一个处理程序。

异步写文件

#include 
#include 
#include 
#include 
#include 
#include 

using namespace boost::asio;

int main()
{
  io_service ioservice;

  posix::stream_descriptor stream{ioservice, STDOUT_FILENO};
  auto handler = [](const boost::system::error_code&, std::size_t) {
    std::cout << ", world!\n";
  };
  async_write(stream, buffer("Hello"), handler);

  ioservice.run();
}

boost::asio::posix::stream_descriptor can be initialized with a file descriptor to start an asynchronous operation on that file descriptor. In the example, stream is linked to the file descriptor STDOUT_FILENO to write a string asynchronously to the standard output stream.

Boost::asio::posix::stream_descriptor可以用一个文件描述符初始化，以启动对该文件描述符的异步操作。在示例中，流链接到文件描述符STDOUT_FILENO以异步方式将字符串写入标准输出流。