Compiler Warning (level 1 and 3) C4793

'function' : function is compiled as native code: 'reason'

Remarks

The compiler cannot compile function into managed code, even though the /clr compiler option is specified. Instead, the compiler emits warning C4793 and an explanatory continuation message, and then compiles function into native code. The continuation message contains the reason text that explains why function cannot be compiled to MSIL.

This is a level 1 warning when you specify the /clr:pure compiler option. The /clr:pure compiler option is deprecated in Visual Studio 2015 and unsupported in Visual Studio 2017.

The following table lists all possible continuation messages.

Reason message	Remarks
Aligned data types are not supported in managed code	The CLR must be able to allocate data as needed, which might not be possible if the data is aligned with declarations such as __m128 or align.
Functions that use '__ImageBase' are not supported in managed code	__ImageBase is a special linker symbol that is typically used only by low-level native code to load a DLL.
varargs are not supported by the '/clr' compiler option	Native functions cannot call managed functions that have variable argument lists (varargs) because the functions have different stack layout requirements. However, if you specify the /clr:pure compiler option, variable argument lists are supported because the assembly can contain only managed functions. For more information, see Pure and Verifiable Code (C++/CLI).
The 64-bit CLR does not support data declared with the __ptr32 modifier	A pointer must be the same size as a native pointer on the current platform. For more information, see __ptr32, __ptr64.
The 32-bit CLR does not support data declared with the __ptr64 modifier	A pointer must be the same size as a native pointer on the current platform. For more information, see __ptr32, __ptr64.
One or more intrinsics is not supported in managed code	The name of the intrinsic is not available at the time the message is emitted. However, an intrinsic that causes this message typically represents a low-level machine instruction.
Inline native assembly ('__asm') is not supported in managed code	Inline assembly code can contain arbitrary native code, which cannot be managed.
A non-__clrcall virtual function thunk must be compiled as native	A non-__clrcall virtual function thunk must use an unmanaged address.
A function using '_setjmp' must be compiled as native	The CLR must be able to control program execution. However, the setjmp function bypasses regular program execution by saving and restoring low-level information such as registers and execution state.

Example

The following sample generates C4793.

// C4793.cpp
// compile with: /c /clr /W3
// processor: x86
int asmfunc(void) {   // C4793, compiled as unmanaged, native code
   __asm {
      mov eax, 0
   }
}

warning C4793: 'asmfunc' : function is compiled as native code:
        Inline native assembly ('__asm') is not supported in managed code

The following sample generates C4793.

// C4793_b.cpp
// compile with: /c /clr /W3
#include <setjmp.h>
jmp_buf test_buf;

void f() {
   setjmp(test_buf);   // C4793 warning
}

warning C4793: 'f' : function is compiled as native code:
        A function using '_setjmp' must be compiled as native