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To avoid these naming conflicts, I recommend defining the P/Invoke function in a namespace or managed class, as shown here: namespace NativeFuncs { [System::Security::SuppressUnmanagedCodeSecurity] [System::Runtime::InteropServices::DllImport( "kernel32.dll", SetLastError=false)] void fNativeFromDll(DWORD frequency, DWORD duration); }; Writing a custom P/Invoke function in C++/CLI is often much easier than writing an equivalent function in other languages. Since native types can seamlessly be used in C++/CLI, you can simply copy the native function declaration and replace some native declaration aspects with equivalent managed aspects. As an example, have a look at the declaration of Beep in winbase.h: WINBASEAPI BOOL WINAPI Beep( __in DWORD dwFreq, __in DWORD dwDuration ); In this code, the macro WINBASEAPI evaluates to __declspec(dllimport). The managed equivalent of __declspec(dllimport) is the DllImportAttribute. Therefore, you must remove the WINBASEAPI macro and apply the DllImportAttribute instead. The WINAPI macro evaluates to __stdcall. It is used to specify the calling convention of Beep. Instead of using this native calling convention, your P/Invoke function must apply the DllImportAttribute as shown here: namespace Win32Native { using namespace System::Runtime::InteropServices; [System::Security::SuppressUnmanagedCodeSecurity] [DllImport("kernel32.dll", SetLastError=false, CallingConvention = CallingConvention::StdCall)] void Beep(DWORD frequency, DWORD duration); }; The default setting for the CallingConvention property of the DllImportAttribute is CallingConvention::WinApi. This setting is used to specify that the default calling convention of the platform s system APIs should be chosen. For the Win32 API, this is __stdcall. Since Beep is a function of the system API, it is also possible to keep this default setting instead of specifying CallingConvention = CallingConvention::StdCall.
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Be Aware of Implicit GetLastError-Caching Optimizations
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There are two scenarios that can result in wrong GetLastError values due to the GetLastErrorcaching optimizations that are done by C++/CLI and the CLR. Both scenarios are unlikely, but according to Murphy s Law, unlikely means that they will surely occur at some time. Therefore, you should be aware of them. The first scenario is related to the optimizations done for native functions that are not imported from a DLL, but reside in the same project. As mentioned before, for these native local functions, C++/CLI automatically generates P/Invoke metadata without the lasterror flag, because it is very uncommon to use the GetLastError value to communicate error codes within a project. However, the MSDN documentation on GetLastError allows you to use SetLastError and GetLastError for your own functions. Therefore, this optimization can theoretically cause wrong GetLastError values. As an example, the output of the following application depends on the compilation model: // GLECachingTrouble1.cpp // build with "CL /clr GLECachingTrouble1.cpp" // or with "CL GLECachingTrouble1.cpp" #include <windows.h> #include <stdio.h> void managedFunc() { SetLastError(0x42); } // remember that you usually should not use #pragma [un]managed. // It is used here only to avoid discussing two different source files. #pragma managed(push, off) void nativeFunc() { SetLastError(0x12345678); } #pragma managed(pop) int main() { managedFunc(); nativeFunc(); // if app is built with /clr, the next line writes "0x42" instead of "0x12345678"! printf("0x%X", GetLastError()); }
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This simple program first calls the managed function managedFunc, which internally calls SetLastError. Since SetLastError is an imported function, it is called by a thunk that supports GetLastError-caching. This means that after the call to SetLastError, the current error value (0x42) is cached by the thunk. After that, managedFunc returns to main and main calls nativeFunc. Notice that nativeFunc is a native function is the same assembly as main. If you compile the application to native code, nativeFunc will set the GetLastError code to 0x12345678. If you compile with /clr, nativeFunc will be called via a thunk. Since nativeFunc is a function from the same project, the P/Invoke metadata generated for it does not have the lasterr modifier, and therefore its thunk does not support GetLastError-caching. Because of that, the cached error value is not modified when nativeFunc returns. The call to GetLastError inside of the printf method is redirected to mscorwks!FalseGetLastError, which returns the cached error value. As a result, the error value 0x42, which was set in managedFunc, is returned by the GetLastError call in main, even though nativeFunc has called SetLastError to modify this value to 0x12345678. If you compile this application without /clr, the value 0x12345678 will be written to the console instead of 0x42. The second potential for trouble with wrong GetLastError values is related to indirect function calls. As discussed before, when a function pointer is used to call a native function from managed code, the IL instruction CALLI is emitted by the compiler, and the JIT compiler generates the thunk. As with thunks for native local functions, thunks generated from CALLI instructions are inlined and do not perform GetLastError-caching. On the one hand, this results in fast thunks. On the other hand, this can also result in lost GetLastError values. Like the application shown before, the following application produces different outputs depending on the compilation model used: // GLECachingTrouble2.cpp // build with "CL /clr GLECachingTrouble2.cpp" // or with "CL GLECachingTrouble2.cpp" #include <windows.h> #include <stdio.h> int main() { // since Beep is called with an illegal frequency here, it will fail if (!Beep(12345678, 100)) // the GetLastError code is 0x57: ERROR_INVALID_PARAMETER printf("Direct call caused error code 0x%X\n", GetLastError()); // set the lasterror value to a value other than before SetLastError(0); // now let's call Beep via a function pointer typedef BOOL (WINAPI* PFNBEEP)(DWORD, DWORD); PFNBEEP pfn = &Beep; if (!pfn(12345678, 100))
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