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POPython ctypes and DLL that uses a COM object
 

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    <p>Under Windows, I'm trying to use a 3rd party DLL (<code>SomeLib.dll</code>) programmed in C++ from Python 2.7 using <code>ctypes</code>. For some of its features, this library uses another COM DLL (<code>SomeCOMlib.dll</code>), which itself uses other DLL (<code>LibA.dll</code>).</p> <p><em>Please note that this isn't about using a COM DLL directly from Python, but it's about using a DLL that uses it from Python.</em></p> <p>To make the integration with Python easier, I've grouped the calls I want to use into my own functions in a new DLL (<code>MyLib.dll</code>), also programmed in C++, simply to make the calls from <code>ctypes</code> more convenient (using <code>extern "C"</code> and tailoring the functions for specific scenarios). Essentially, my library exposes 2 functions: <code>doSomethingSimple()</code>, <code>doSomethingWithCOMobj()</code> (all returning <code>void</code>, no parameters).</p> <p>The "effective" dependency hierarchy is as follows:</p> <pre class="lang-none prettyprint-override"><code>MyLib.dll SomeLib.dll SomeCOMlib.dll LibA.dll </code></pre> <p>I'm able to write a simple C++ console application (Visual C++) that uses <code>MyLib.dll</code> and makes those 2 consecutive calls without problem.</p> <p>Using Python/ctypes, the first call works fine, but the call that makes use of COM throws a <code>WindowsError: [Error -529697949] Windows Error 0xE06D7363</code>. From the rest of the library behaviour, I can see that the problem comes exactly where that COM call is made.</p> <p>(The simple test C++ application also fails more or less at the same place if <code>LibA.dll</code> is missing, but I'm not sure if it's related.)</p> <p>I've looked at the dependency hierarchy using <a href="http://www.dependencywalker.com/" rel="noreferrer">Dependency Walker</a>. <code>SomeCOMlib.dll</code> isn't listed as a dependency of <code>SomeLib.dll</code>, although it's clearly required, and <code>LibA.dll</code> isn't listed as a dependency of <code>SomeCOMlib.dll</code>, although it's also clearly required at run time.</p> <p>I'm running everything from the command line, from within the directory where these DLLs are (and the C++ sample executable works fine). I've tried to force the PATH to include that directory, and I've also tried to copy the DLLs to various places where I guessed they might be picked up (<code>C:\Windows\System32</code> and <code>C:\Python27\DLLs</code>), without success. <code>SomeCOMlib.dll</code> was also registered with <code>regasm.exe</code>.</p> <p><strong>What could cause this difference between using this DLL from a plain C++ application and from Python's <code>ctypes</code> when it comes to its own usage of the COM mechanism (and possibly the subsequent loading of other DLLs there)?</strong></p> <p><strong>Which steps would give at least a bit more information than <code>Windows Error 0xE06D7363</code> from Python, to be able to investigate the problem further?</strong></p> <p>The Python code looks like this:</p> <pre class="lang-python prettyprint-override"><code>import ctypes myDll = ctypes.WinDLL("MyLib.dll") myDll.doSomethingSimple() myDll.doSomethingWithCOMobj() # This statement throws the Windows Error </code></pre> <p>(The test C++ standalone application that works, linked to <code>MyLib.dll</code> makes exactly the same calls within <code>main</code>.)</p>
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