c++ - 调用 ReadDirectoryChangesW 时，只有第一次调用返回任何更改(同步和异步)

Question

以下是使用 ReadDirectoryChangesW 的最小程序。我遇到的问题是只有第一次调用 GetQueuedCompletionStatus 返回。第二次通过循环，无论对目录进行多少更改，它都会永远阻塞。

我也尝试过使用同步版本，但遇到了完全相同的问题。

#include <array>
#include <cassert>
#include <iostream>
#include <Windows.h>

int main() {
  // Open the directory to monitor.
  HANDLE dir = ::CreateFileA(
      "G:\\Program Files (x86)\\Steam\\steamapps\\common\\eve online"
    , FILE_LIST_DIRECTORY
    , FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE
    , NULL
    , OPEN_EXISTING
    , FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED
    , NULL
    );

  if (dir == INVALID_HANDLE_VALUE) {
    std::cout << "Failed to open directory for change notifications!\n";
    return 1;
  }

  // Setup IOCP.
  HANDLE iocp = ::CreateIoCompletionPort(
      dir
    , NULL
    , NULL
    , 1
    );

  // Monitor.
  while (true) {
    std::array<char, 1024 * 8> buf;
    DWORD bytes_read;
    OVERLAPPED overlapped;
    std::memset(&overlapped, 0, sizeof(overlapped));
    BOOL result = ::ReadDirectoryChangesW(
        dir
      , &buf.front()
      , buf.size()
      , false
      , FILE_NOTIFY_CHANGE_FILE_NAME // Includes file creation.
      , &bytes_read
      , &overlapped
      , NULL
      );

    if (result == FALSE) {
      DWORD error = ::GetLastError();
      std::cout << "Call to ReadDirectoryChangesW failed! " << error << "\n";
      return 1;
    }

    // Wait for completion.
    ULONG_PTR key;
    LPOVERLAPPED overlapped_result;

    result = ::GetQueuedCompletionStatus(
        iocp
      , &bytes_read
      , &key
      , &overlapped_result
      , INFINITE
      );

    if (result == FALSE) {
      std::cout << "Call to GetQueuedCompletionStatus failed!\n";
      return 1;
    }

    // Print results!
    for (FILE_NOTIFY_INFORMATION *fni =
           reinterpret_cast<FILE_NOTIFY_INFORMATION *>(&buf.front());
         ;
         fni = reinterpret_cast<FILE_NOTIFY_INFORMATION *>(
           reinterpret_cast<char *>(fni) + fni->NextEntryOffset)) {
      std::wstring filename(fni->FileName, fni->FileName + fni->FileNameLength);
      std::wcout << "Got change: " << filename.c_str() << "\n";

      if (fni->NextEntryOffset == 0) break;
    }
  }

}

Answer 1

一些问题。

首先，您尝试将多字节字符串文字输出到 wcout。您应该通过在 L 之前将它们变成宽字符串。

其次，FileNameLength 变量表示名称的字节长度，而不是字符长度。您应该将它除以 2 以获得字符数。