How do I CloseHandle a handle that was converted to a FILE*? - c++

Question

I am trying to redirect the output of a child process and return a FILE* to the parent.
I am essentially following the code example at Creating a Child Process with Redirected Input and Output. The only modification is that i encapsulated the whole thing in a function and do
return _fdopen(_open_osfhandle((intptr_t)g_hChildStd_OUT_Rd, _O_RDONLY), "rb");
The problem is that I want to cleanup and fclose is throwing an error. Was I supposed to close handle after I did the _fdopen? How am I supposed to clean up here?

Answer 1

The correct way to close a file stream opened with _fdopen is to call fclose. If that's not working for you, then you have other problems.
Do not close the file descriptor after calling _fdopen. The documentation doesn't exactly say, but the FILE object owns that file descriptor and will close it when you call fclose. Likewise, the file descriptor owns the file handle, so you shouldn't call CloseHandle, either.

Answer 2

May be quite late to answer, but just adding for reference. I looked into call stack of fclose, and it is actually making a call to CloseHandle
auto NThdl = CreateFileW(L"D:\\s.txt", GENERIC_READ | GENERIC_WRITE, 0, nullptr, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, nullptr);
if (NThdl != INVALID_HANDLE_VALUE)
{
int iohdl = _open_osfhandle((intptr_t) NThdl, O_RDWR | O_BINARY);
if (iohdl != 0xFFFFFFFF)
{
FILE *hLstFile = _wfdopen(iohdl, L"w+bc");
fwrite("Bogus", 5, 1, hLstFile);
fclose(hLstFile);
// Exception
CloseHandle(NThdl);
}
}
And attempt to call CloseHandle after fclose would cause an exception saying that Handle is invalid.