LabVIEW
Allocate memory in C++ .dll for a matrix of structures
Solved!
I'm guessing here, i don't work much with DLLs, but in order to not have to allocate a huge matrix (to be sure), you can make it a 2 step process. The 1st function creates your result matrix, but only returns the size. You use this to allocate a fitting matrix and then you implement a Grab/copy result function that copies/grabs the result to your matrix.
First improvement would be this:
#include "lv_prolog.h"
// LabVIEW compatible Measurement_t
typedef struct LVMeasurement {
double value; /*!< Numerical value. */
UnitPfx_t prefix; /*!< Unit prefix in the range [femto, Peta]. */
LStrHandle unit; /*!< Unit. \note Can be any string, the library is not aware of real units meaning. */
} LVMeasurement_t;
// LabVIEW compatible vector of Measurement_t
typedef struct {
int32 cnt; /* number of measurements that follow */
LVMeasurement_t item[1]; /* cnt measurements */
} LMeas, *LMeasPtr, **LMeasHandle;
typedef struct {
int32 cnt[2]; /* size of matrix of measurements that follow */
LVMeasurement_t item[1]; /* cnt vector of measurements */
} LVecMeas, *LVecMeasPtr, **LVecMeasHandle;
#include "lv_epilog.h"
These includes around data structures that you pass from LabVIEW to your DLL, make sure that the structures use the correct alignment. This is Byte Packing for 32-bit Windows and 8-byte default alignment for all other platforms.
Next improvement:
// Allocate memory of a LStrHandle and copy the content of an std::string
void string2Output(std::string s, LStrHandle *o) {
MgErr err = NumericArrayResize(uB, 1, (UHandle *)o, s.length());
if (!err) {
MoveBlock(s.c_str(), LStrBufH(*o), s.length());
LStrLenH(*o) = s.length();
}
}
If the incoming handle is a NULL value (this is a valid value for empty arrays/strings in LabVIEW) then NumericArrayResize() will allocate a new handle and if the handle parameter is by value this memory allocation will get lost as it can't get passed out.
Of course a vector of vectors could theoretically be have different size of inner vectors! A LabVIEW 2 D array can't, as it is a consecutive area of memory.
@fcona wrote:
I'll try changing the string2Output as you suggest, maybe the strings are not NULL in my working example with the vector, but they are NULL in the other case with the vector of vectors.
Even if vectors in a c++ vector can have different sizes this is not the case for me, I have to return matrices basically.
So apart from the unhandled NULL string case, do you think the code should be working?
They will be for sure NULL since you allocate the according handle with DSSetHSzClr(). This function makes sure the whole content is initialized to 0. And you better do that in this case, a random value where a handle is expected would make the NumericArrayResize() function believe that it is a valid handle that needs to be resized, and that would for sure end in the General Protection nirvana, rather sooner than later.
Well
void vectorMeasurement2Output(std::vector <Measurement_t> v, LMeasHandle * o) {
int offset = 0;
MgErr err = DSSetHSzClr(* o, Offset(LMeas, item)+sizeof(LVMeasurement_t)*v.size());
You pass in the handle by reference. In this case LabVIEW will pass in a NULL handle if you connect an empty array (or) none. DSSetHSzClr() tries to resize that NULL handle and bam!
You need to explicitly check if the passed in handle is NULL and if so, you need to call DSNewHClr() instead to allocate a new valid handle. If it is not NULL you can resize it!
