DuplicateHandle, why duplicate instead of just acquire?

Question

Why would a process want to call DuplicateHandle from the Win32API, and get it from another process instead of just acquiring the handle on some object itself?

Is there some advantage to calling DuplicateHandle or something?

Answer 1

You may find the answer in Chapter 6.8 of 'Programming Applications for Microsoft Windows'.

Gaining a Sense of One's Own Identity
Sometimes you might need to acquire a real handle to a thread instead of a pseudo-handle. By "real," I mean a handle that unambiguously identifies a unique thread. Examine the following code:

DWORD WINAPI ParentThread(PVOID pvParam) {
   HANDLE hThreadParent = GetCurrentThread();
   CreateThread(NULL, 0, ChildThread, (PVOID) hThreadParent, 0, NULL);
   // Function continues...
}

DWORD WINAPI ChildThread(PVOID pvParam) {
   HANDLE hThreadParent = (HANDLE) pvParam;
   FILETIME ftCreationTime, ftExitTime, ftKernelTime, ftUserTime;
   GetThreadTimes(hThreadParent,
      &ftCreationTime, &ftExitTime, &ftKernelTime, &ftUserTime);
   // Function continues...
}

Can you see the problem with this code fragment? The idea is to have the parent thread pass to the child thread a thread handle that identifies the parent thread. However, the parent thread passes a pseudo-handle, not a real handle. When the child thread begins executing, it passes the pseudo-handle to the GetThreadTimes function, which causes the child thread to get its own CPU times, not the parent thread's CPU times. This happens because a thread pseudo-handle is a handle to the current thread— that is, a handle to whichever thread is making the function call.

To fix this code, we must turn the pseudo-handle into a real handle. The DuplicateHandle function (discussed in Chapter 3) can do this transformation

Answer 2

See here on the MSDN what it has to say about the usage of 'DuplicateHandle'. The best way I can think of it is this way, an analogy if you like - suppose you open a file using the CreateHandle routine for writing only, then you call DuplicateHandle to pass the handle onto another thread in which the thread will read from the file, only the handle is duplicated hence the thread does not have to call CreateHandle again...

Hope this helps, Best regards, Tom.

Answer 3

One possible use of DuplicateHandle is to duplicate a handle between a 32-bit process and a 64-bit process.

Note: cannot be used on I/O Completion ports or Sockets.

Answer 4

Another use of DuplicateHandle is to open a file in multiple processes when the file uses FileOptions.DeleteOnClose. (such a file cannot be opened by multiple processes if the file path is used to open the file)

See my answer at https://stackoverflow.com/a/36606283/2221472