如何从 QFile 或 QSaveFile 获取 Windows HANDLE 以用于 ReOpenFile？答案

【问题标题】：How to get a windows HANDLE from a QFile or QSaveFile for ReOpenFile?如何从 QFile 或 QSaveFile 获取 Windows HANDLE 以用于 ReOpenFile？
【发布时间】：2018-05-05 16:11:47
【问题描述】：

给定一个QFileDevice 实例（QFile 或QSaveFile） - 如何获得文件的本机 Windows HANDLE？这个句柄可以和ReOpenFile一起使用吗？

【问题讨论】：

标签： windows qt winapi handle qfile

【解决方案1】：

QFileDevice::handle() 返回从QFSFileEnginePrivate::nativeHandle 获得的 C 文件描述符（fd - 一个小整数）。那个文件描述符就是你从_open_osfhandle 得到的。您需要使用_get_osfhandle 回到HANDLE。

struct FdHandle {
  int fd = -1;
  HANDLE handle = INVALID_HANDLE_VALUE;
  operator HANDLE() const { return handle; }
  bool isFdValid() const { return fd != -1; }
  bool isHandleValid() const { return handle != INVALID_HANDLE_VALUE; }
};

FdHandle windowsHandleOf(QFileDevice* dev) {
  auto fd = dev->handle(); 
  return {fd, (HANDLE)_get_osfhandle(fd)};
}

ReOpenFile 可能会返回不同的文件句柄，因此您可能需要为它重新打开文件。一个重载的ReOpenFile 处理它：

struct ReOpenResult {
  enum { OK = 0, FileClosed = 10, NoHandle = 20, 
         OldHandle = 30, OpenFailed = 40 } rc;
  HANDLE handle = INVALID_HANDLE_VALUE;
  operator HANDLE() const { return handle; }
  explicit operator bool() const { return rc == OK; }
  bool isHandleValid() const { return handle != INVALID_HANDLE_VALUE; }
};

ReOpenResult ReOpenFile(QFile *file, DWORD access, DWORD shareMode, DWORD flags) {
  if (!file->isOpen())
    return {ReOpenResult::FileClosed};
  auto oldHandle = windowsHandleOf(file);
  if (oldHandle == INVALID_HANDLE_VALUE)
    return {ReOpenResult::NoHandle};
  auto newHandle = ReOpenFile(oldHandle, access, shareMode, flags);
  if (newHandle == INVALID_HANDLE_VALUE)
    return {ReOpenResult::OldHandle, oldHandle};
  if (!open(file, file->openMode(), newHandle))
    return {ReOpenResult::OpenFailed, oldHandle};
  return {ReOpenResult::OK, newHandle};
}

采用HANDLE 的“缺失”open 是：

struct OpenResult {
  enum { OK = 0, SameHandleOK = 1, InvalidHandle = 10, CCloseFailed = 20,
         InvalidFd = 30, OpenFailed = 40 } rc = OK;
  explicit operator bool() const { return rc < InvalidHandle; };
};

OpenResult open(QFile* file, QIODevice::OpenMode mode, HANDLE handle) {
  if (handle == INVALID_HANDLE_VALUE)
    return {OpenResult::InvalidHandle};
  if (file->isOpen() && windowsHandleOf(file) == handle)
    return {OpenResult::SameHandleOK};
  file->close();
  if (auto fd = file->handle() != -1)
    if (_close(fd)) // the C handle is still open, don't leak it
      return {OpenResult::CCloseFailed};
  int flags = 0;
  if (mode & QIODevice::Append)
    flags |= _O_APPEND;
  if (!(mode & QIODevice::WriteOnly))
    flags |= _O_RDONLY;
  auto fd = _open_osfhandle((intptr_t)handle, flags);
  if (fd == -1)
    return {OpenResult::InvalidFd};
  if (!file->open(fd, mode, QFileDevice::AutoCloseHandle))
    return {OpenResult::OpenFailed};
  return {};
}

【讨论】：