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You clearly have not much C programming experience. Which of course is a very bad starting point to try to write C code that should then interoperate with LabVIEW. First this: // +++++ I try to add this, Gues this will not work .. see extcode.h LStrHandle textString[TEXT_STRING_SIZE]; }structSample; You are basically defining a fixed size array of TEXT_STRING_SIZE LabVIEW string handles, not a LabVIEW string handle of TEXT_STRING_SIZE length. LabVIEW string handles are never fixed size but instead dynamically allocated memory blocks with an extra pointer reference to it. And that dynamic allocation (and deallocation) ABSOLUTELY and SURELY must be done by using the LabVIEW memory manager functions. Anything else is nothing more than a crash site. What you have built there as datatype would look like an array of structs and each of these structs would contain three integers followed by 256 LabVIEW string handles, which is not only pretty weird but absolutely NOT compatible with any possible LabVIEW structure. And after allocating all these things you eventually only send the actual string handle to the event and leak everything else and the handle itself too! typedef struct { int32_t firstInt; int32_t secondInt; int32_t thirdInt; LStrHandle lvString; } MyStruct, *MyStructPtr; MgErr CreateStringHandle(LStrHandle *lvStringHandle, char* stringData) { MgErr err; size_t len = strlen(stringData); if (*lvStringHandle) { err = DSSetHandleSize(*lvStringHandle, sizeof(int32_t) + len); } else { *lvStringHandle = DSNewHandle(sizeof(int32_t) + len); if (!*lvStringHandle) err = mFullErr; } if (!err) { MoveBlock(stringData, LStrBuf(**lvStringHandle), len); LStrLen(**lvStringHandle) = (int32_t)len; } return err; } MgErr SendStringInSructToLV(LVUserEventRef *userEvent) { MyStruct structure = {1, 2, 3, NULL); MgErr err = CreateStringHandle(&structure.lvString, "Some C String!"); if (!err) { err = PostLVUserEvent(*userEvent, &structure); DSDisposeHandle(structure.lvString); } return err; }