Asked  7 Months ago    Answers:  5   Viewed   26 times

I am working on a rather large codebase in which C++ functionality is P/Invoked from C#.

There are many calls in our codebase such as...


extern "C" int __stdcall InvokedFunction(int);

With a corresponding C#:

[DllImport("CPlusPlus.dll", ExactSpelling = true, SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
    private static extern int InvokedFunction(IntPtr intArg);

I have scoured the net (insofar as I am capable) for the reasoning as to why this apparent mismatch exists. For example, why is there a Cdecl within the C#, and __stdcall within the C++? Apparently, this results in the stack being cleared twice, but, in both cases, variables are pushed onto the stack in the same reverse order, such that I do not see any errors, albeit the possibility that return information is cleared in the event of attempting a trace during debugging?

From MSDN:

// explicit DLLImport needed here to use P/Invoke marshalling
[DllImport("msvcrt.dll", EntryPoint = "printf", CallingConvention = CallingConvention::Cdecl,  CharSet = CharSet::Ansi)]

// Implicit DLLImport specifying calling convention
extern "C" int __stdcall MessageBeep(int);

Once again, there is both extern "C" in the C++ code, and CallingConvention.Cdecl in the C#. Why is it not CallingConvention.Stdcall? Or, moreover, why is there __stdcall in the C++?

Thanks in advance!



This comes up repeatedly in SO questions, I'll try to turn this into a (long) reference answer. 32-bit code is saddled with a long history of incompatible calling conventions. Choices on how to make a function call that made sense a long time ago but are mostly a giant pain in the rear end today. 64-bit code has only one calling convention, whomever is going to add another one is going to get sent to small island in the South Atlantic.

I'll try to annotate that history and relevance of them beyond what's in the Wikipedia article. Starting point is that the choices to be made in how to make a function call are the order in which to pass the arguments, where to store the arguments and how to cleanup after the call.

  • __stdcall found its way into Windows programming through the olden 16-bit pascal calling convention, used in 16-bit Windows and OS/2. It is the convention used by all Windows api functions as well as COM. Since most pinvoke was intended to make OS calls, Stdcall is the default if you don't specify it explicitly in the [DllImport] attribute. Its one and only reason for existence is that it specifies that the callee cleans up. Which produces more compact code, very important back in the days when they had to squeeze a GUI operating system in 640 kilobytes of RAM. Its biggest disadvantage is that it is dangerous. A mismatch between what the caller assumes are the arguments for a function and what the callee implemented causes the stack to get imbalanced. Which in turn can cause extremely hard to diagnose crashes.

  • __cdecl is the standard calling convention for code written in the C language. Its prime reason for existence is that it supports making function calls with a variable number of arguments. Common in C code with functions like printf() and scanf(). With the side effect that since it is the caller that knows how many arguments were actually passed, it is the caller that cleans up. Forgetting CallingConvention = CallingConvention.Cdecl in the [DllImport] declaration is a very common bug.

  • __fastcall is a fairly poorly defined calling convention with mutually incompatible choices. It was common in Borland compilers, a company once very influential in compiler technology until they disintegrated. Also the former employer of many Microsoft employees, including Anders Hejlsberg of C# fame. It was invented to make argument passing cheaper by passing some of them through CPU registers instead of the stack. It is not supported in managed code due to the poor standardization.

  • __thiscall is a calling convention invented for C++ code. Very similar to __cdecl but it also specifies how the hidden this pointer for a class object is passed to instance methods of a class. An extra detail in C++ beyond C. While it looks simple to implement, the .NET pinvoke marshaller does not support it. A major reason that you cannot pinvoke C++ code. The complication is not the calling convention, it is the proper value of the this pointer. Which can get very convoluted due to C++'s support for multiple inheritance. Only a C++ compiler can ever figure out what exactly needs to be passed. And only the exact same C++ compiler that generated the code for the C++ class, different compilers have made different choices on how to implement MI and how to optimize it.

  • __clrcall is the calling convention for managed code. It is a blend of the other ones, this pointer passing like __thiscall, optimized argument passing like __fastcall, argument order like __cdecl and caller cleanup like __stdcall. The great advantage of managed code is the verifier built into the jitter. Which makes sure that there can never be an incompatibility between caller and callee. Thus allowing the designers to take the advantages of all of these conventions but without the baggage of trouble. An example of how managed code could stay competitive with native code in spite of the overhead of making code safe.

You mention extern "C", understanding the significance of that is important as well to survive interop. Language compilers often decorate the names of exported function with extra characters. Also called "name mangling". It is a pretty crappy trick that never stops causing trouble. And you need to understand it to determine the proper values of the CharSet, EntryPoint and ExactSpelling properties of a [DllImport] attribute. There are many conventions:

  • Windows api decoration. Windows was originally a non-Unicode operating system, using 8-bit encoding for strings. Windows NT was the first one that became Unicode at its core. That caused a rather major compatibility problem, old code would not have been able to run on new operating systems since it would pass 8-bit encoded strings to winapi functions that expect a utf-16 encoded Unicode string. They solved this by writing two versions of every winapi function. One that takes 8-bit strings, another that takes Unicode strings. And distinguished between the two by gluing the letter A at the end of the name of the legacy version (A = Ansi) and a W at the end of the new version (W = wide). Nothing is added if the function doesn't take a string. The pinvoke marshaller handles this automatically without your help, it will simply try to find all 3 possible versions. You should however always specify CharSet.Auto (or Unicode), the overhead of the legacy function translating the string from Ansi to Unicode is unnecessary and lossy.

  • The standard decoration for __stdcall functions is _foo@4. Leading underscore and a @n postfix that indicates the combined size of the arguments. This postfix was designed to help solve the nasty stack imbalance problem if the caller and callee don't agree about the number of arguments. Works well, although the error message isn't great, the pinvoke marshaller will tell you that it cannot find the entrypoint. Notable is that Windows, while using __stdcall, does not use this decoration. That was intentional, giving programmers a shot at getting the GetProcAddress() argument right. The pinvoke marshaller also takes care of this automatically, first trying to find the entrypoint with the @n postfix, next trying the one without.

  • The standard decoration for __cdecl function is _foo. A single leading underscore. The pinvoke marshaller sorts this out automatically. Sadly, the optional @n postfix for __stdcall does not allow it to tell you that your CallingConvention property is wrong, great loss.

  • C++ compilers use name mangling, producing truly bizarre looking names like "??2@YAPAXI@Z", the exported name for "operator new". This was a necessary evil due to its support for function overloading. And it originally having been designed as a preprocessor that used legacy C language tooling to get the program built. Which made it necessary to distinguish between, say, a void foo(char) and a void foo(int) overload by giving them different names. This is where the extern "C" syntax comes into play, it tells the C++ compiler to not apply the name mangling to the function name. Most programmer that write interop code intentionally use it to make the declaration in the other language easier to write. Which is actually a mistake, the decoration is very useful to catch mismatches. You'd use the linker's .map file or the Dumpbin.exe /exports utility to see the decorated names. The undname.exe SDK utility is very handy to convert a mangled name back to its original C++ declaration.

So this should clear up the properties. You use EntryPoint to give the exact name of the exported function, one that might not be a good match for what you want to call it in your own code, especially for C++ mangled names. And you use ExactSpelling to tell the pinvoke marshaller to not try to find the alternative names because you already gave the correct name.

I'll nurse my writing cramp for a while now. The answer to your question title should be clear, Stdcall is the default but is a mismatch for code written in C or C++. And your [DllImport] declaration is not compatible. This should produce a warning in the debugger from the PInvokeStackImbalance Managed Debugger Assistant, a debugger extension that was designed to detect bad declarations. And can rather randomly crash your code, particularly in the Release build. Make sure you didn't turn the MDA off.

Tuesday, June 1, 2021
answered 7 Months ago

Python is a higher level language than C, which means it abstracts the details of the computer from you - memory management, pointers, etc, and allows you to write programs in a way which is closer to how humans think.

It is true that C code usually runs 10 to 100 times faster than Python code if you measure only the execution time. However if you also include the development time Python often beats C. For many projects the development time is far more critical than the run time performance. Longer development time converts directly into extra costs, fewer features and slower time to market.

Internally the reason that Python code executes more slowly is because code is interpreted at runtime instead of being compiled to native code at compile time.

Other interpreted languages such as Java bytecode and .NET bytecode run faster than Python because the standard distributions include a JIT compiler that compiles bytecode to native code at runtime. The reason why CPython doesn't have a JIT compiler already is because the dynamic nature of Python makes it difficult to write one. There is work in progress to write a faster Python runtime so you should expect the performance gap to be reduced in the future, but it will probably be a while before the standard Python distribution includes a powerful JIT compiler.

Wednesday, June 2, 2021
answered 7 Months ago

It stands for member. I personally find this convention unhelpful, but it's subjective.

Saturday, June 12, 2021
answered 6 Months ago

The calling convention has to be one where the caller clears the arguments from the stack (because the callee doesn't know what will be passed).

That doesn't necessarily correspond to what Microsoft calls "__cdecl" though. Just for example, on a SPARC, it'll normally pass the arguments in registers, because that's how the SPARC is designed to work -- its registers basically act as a call stack that gets spilled to main memory if the calls get deep enough that they won't fit into register anymore.

Though I'm less certain about it, I'd expect roughly the same on IA64 (Itanium) -- it also has a huge register set (a couple hundred if memory serves). If I'm not mistaken, it's a bit more permissive about how you use the registers, but I'd expect it to be used similarly at least a lot of the time.

Why does this matter to you? The point of using stdarg.h and its macros is to hide differences in calling convention from your code, so it can work with variable arguments portably.

Edit, based on comments: Okay, now I understand what you're doing (at least enough to improve the answer). Given that you already (apparently) have code to handle the variations in the default ABI, things are simpler. That only leaves the question of whether variadic functions always use the "default ABI", whatever that happens to be for the platform at hand. With "stdcall" and "default" as the only options, I think the answer to that is yes. Just for example, on Windows, wsprintf and wprintf break the rule of thumb, and uses cdecl calling convention instead of stdcall.

Thursday, November 4, 2021
Boas Enkler
answered 1 Month ago

What's happening here is that when you switch from __cdecl to __stdcall in the C++ code, the compiler decorates the name by which the function is exported. Instead of myFunc1 it is exported as myFunc1@0 or perhaps _myFunc1@0. All the same, the name is decorated. You can check that this is so with dumpbin or Dependency Viewer.

When you call GetProcAddress, it cannot find a function named myFunc1 and so returns NULL. You don't check for return values, and so carry on regardless. When you try to call the function, a run time error is thrown.

I've had to guess most of this because you did not show complete code. The other big lesson is to check for errors when calling Win32 functions.

Thursday, December 2, 2021
Ash Burlaczenko
answered 1 Week ago
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