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    <p>Sorry, I can't reproduce your problem (Windows 7 x64, 32-bit Python 2.7.3). I'll describe what I tried in order to reproduce your problem, in the hope that it will help you.</p> <p>I created a new project and solution in Visual C++ Express 2008, both named "CDll". The project was set to compile as C code and use the stdcall calling convention. Apart from the stuff VC++ 2008 autogenerated, it had the following two files:</p> <p>CDll.h:</p> <pre><code>#ifdef CDLL_EXPORTS #define CDLL_API __declspec(dllexport) #else #define CDLL_API __declspec(dllimport) #endif typedef struct testResult_t { int testId; int TT; double fB; double mD; double mDL; int nS; int nL; } TestResult; TestResult CDLL_API __stdcall dummyTest(); </code></pre> <p>CDll.cpp (yes, I know the extension is '.cpp', but I don't think that matters):</p> <pre><code>#include "stdafx.h" #include "CDll.h" TestResult __stdcall dummyTest() { TestResult a = {0}; a.testId = 3; return a; }; </code></pre> <p>I then compiled and built the DLL. I then attempted to load it and call the function with the following Python script:</p> <pre><code>from ctypes import Structure, c_int, c_double, windll astdll = windll.CDll class TestResult(Structure): _fields_ = [ ("testId", c_int), ("TT", c_int), ("fB", c_double), ("mD", c_double), ("mDL", c_double), ("nS", c_int), ("nL", c_int) ] astdll.dummyTest.restype = TestResult result = astdll.dummyTest() print "Test ID: %d" % (result.testId) </code></pre> <p>When I ran this script I got the output <code>Test ID: 3</code>.</p> <hr> <p>The first thought I had about what your problem might be was that you were attempting to load the DLL using <code>CDLL</code> when you should be using <code>windll</code>, but when I tried using <code>CDLL</code>, I got a completely different error message. You haven't shown us how you are loading the DLL, but I suspect you are using <code>windll</code> as I have done above.</p>
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    1. USLuke Woodward
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    1. COThat was the problem, I was using oledll to load the library. Thank you very much!!
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      1. USJorge
    2. CO@Jorge Do beware that your function will only be callable from compilers that use the MS large struct return value ABI.
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      1. USDavid Heffernan
    3. COLuke, can you please take a look on my question [link](http://stackoverflow.com/questions/20773602/returning-struct-from-c-dll-to-python) ? It's very similar problem, difference is that I have strings in my struct, it gives me headache for days
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      1. USAleksandar
 

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