MANAGED AND UNMANAGED CODE INTEROPERATION in Visual Basic .NET

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CHAPTER 18 MANAGED AND UNMANAGED CODE INTEROPERATION
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// Note the modopt modifier of the Invoke signature -- it's very // important. Without it, the calling convention of the callback // function is marshaled as stdcall (callee cleans the stack). // But qsort expects the callback function to have the cdecl // calling convention (caller clears the stack). If we supply the // callback with the stdcall calling convention, qsort blows // the stack away and causes a memory access violation. You are // welcome to comment out the modopt line and see what happens. // Note also that the modopt modifier is placed on the delegate's // Invoke signature, not on the signature of the delegated method. .method public virtual int32 modopt([mscorlib]System.Runtime.CompilerServices.CallConvCdecl) Invoke(void*, void*) runtime {} // Well, I don't really need asynchronous invocation here, // but, you know, dura lex sed lex. .method public newslot virtual class [mscorlib]System.IAsyncResult BeginInvoke(object, class [mscorlib]System.AsyncCallback, object) runtime {} .method public newslot virtual void EndInvoke(class [mscorlib]System.IAsyncResult) runtime {} } // The hero of the occasion: the qsort function. .method public static pinvokeimpl("msvcrt.dll" ansi cdecl) void qsort(void*,int32,int32,class CompareDelegate) preservesig {} // This is the comparison method I'm going to offer as // a callback to qsort. What can be simpler than comparing // two integers .method public static int32 compInt32(void* arg1,void* arg2) { // return(*arg1 - *arg2); ldarg.0 ldind.i4 ldarg.1 ldind.i4 sub ret }
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CHAPTER 18 MANAGED AND UNMANAGED CODE INTEROPERATION
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// And now, let's get this show on the road. .method public static void Exec() { .entrypoint .locals init(class CompareDelegate) // Print the unsorted values. ldstr "Before Sorting:\n" call vararg int32 printf(string) pop ldsflda valuetype SixtyBytes DataToSort ldc.i4.s 15 call void printInt32(void*, int32) // Create the delegate. // Null object ref indicates the global method. ldnull ldftn int32 compInt32(void*,void*) newobj instance void CompareDelegate::.ctor(object,native uint) stloc.0 // Invoke qsort. ldsflda valuetype SixtyBytes DataToSort // Pointer to data ldc.i4.s 15 // Number of items to sort ldc.i4.4 // Size of an individual item ldloc.0 // Callback function pointer (delegate) call void qsort(void*,int32,int32,class CompareDelegate) // Print the sorted values. ldstr "After Sorting:\n" call vararg int32 printf(string) pop ldsflda valuetype SixtyBytes DataToSort ldc.i4.s 15 call void printInt32(void*, int32) ret }
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Exposing managed methods as unmanaged exports provides a way for unmanaged, non-COM clients to consume managed services. In fact, this technique opens the managed world in all its glory with its secure and type-safe computing and with all the wealth of its class libraries to unmanaged clients. Of course, the managed methods are not exposed as such. Instead, inverse P/Invoke thunks, automatically created by the common language runtime, are exported. These thunks provide the same marshaling functions as conventional P/Invoke thunks, but in the opposite direction. In order to expose managed methods as unmanaged exports, the IL assembler builds a v-table, a v-table fixup (VTableFixup) table, and a group of unmanaged export tables, which include the Export Address table, the Name Pointer table, the Ordinal table, the Export Name table, and the Export Directory table. 4 discusses all of these tables, their structures, and their positioning within a managed PE file. Now let s see how it all is done. The VTableFixup table is an array of VTableFixup descriptors, with each descriptor carrying the RVA of a v-table entry, the number of slots in the entry, and the binary flags indicating the size of each slot (32-bit or 64-bit) and any special features of the entry. One special feature is the creation of the marshaling thunk to be exposed to the unmanaged client. The v-table and the VTableFixup table of a managed module serve two purposes. One purpose relevant only to the VC++ compiler, the only compiler that produces mixed-code modules is to provide the intramodule managed/unmanaged code interoperation. Another purpose is to provide the means for the unmanaged export of managed methods. Each slot of a v-table in a PE file carries the token of the managed method the slot represents. At run time, after respective methods have been compiled to native code, the v-table fixups are executed, replacing the method tokens with actual addresses of the compiled methods. The ILAsm syntax for a v-table fixup definition is as follows: .vtfixup [<num_slots>] <flags> at <data_label> where square brackets are part of the definition and do not mean that <num_slots> is optional. <num_slots> is an integer constant, indicating the number of v-table slots grouped into one entry because their flags are identical. This grouping has no effect other than saving some space you can emit a single slot per entry, but then you ll have to emit as many v-table fixups as there are slots. The flags specified in the definition can be those that are described in the following list: int32. Each slot in this v-table entry is 4 bytes wide (32-bit target platform). int64. Each slot in this v-table entry is 8 bytes wide (64-bit target platform). The int32 and int64 flags are mutually exclusive. fromunmanaged. The entry is to be called from the unmanaged code, so the marshaling thunk must be created by the runtime. callmostderived. This flag is not currently used. The order of appearance of .vtfixup declarations defines the order of the respective VTableFixup descriptors in the VTableFixup table. The v-table entries are defined simply as data entries. Note that the v-table must be contiguous in other words, the data definitions for the v-table entries must immediately follow one another.
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