Access to LStrHandle in the DLL

Question

i am doing new study about LabVIEW and DLL. I created a DLL which built by LabVIEW.

This is a header file.

#include "extcode.h"
#pragma pack(push)
#pragma pack(1)

#ifdef __cplusplus
extern "C" {
#endif
typedef struct {
    int32_t numeric_input;
    LStrHandle string_input;
} Cluster;

/*!
 * Cluster_dll
 */
void __cdecl Cluster_dll(Cluster *Input, int32_t *numeric_output, 
    char string_output[], int32_t length);

MgErr __cdecl LVDLLStatus(char *errStr, int errStrLen, void *module);

#ifdef __cplusplus
} // extern "C"
#endif

#pragma pack(pop)

To call it, i created Visual C++ program.

#include "stdafx.h"
#include <Windows.h>
#include <iostream>
#include <cstdint> 
#include "C:\Program Files\National Instruments\LabVIEW 2014\cintools\extcode.h"
using namespace std;

struct Input_t{
    int32_t A;
    LStrHandle B;
} variable_t;

typedef void (*FUNC)(Input_t,int32_t *,char [],int32_t);


int _tmain(int argc, _TCHAR* argv[])
{
    HINSTANCE hDLL = LoadLibrary(TEXT("C:\\Cluster_dll.dll"));
        if(hDLL == NULL){
        cout << "error" << endl; // error check
    }


    FARPROC lpIO = GetProcAddress(HMODULE(hDLL),"Cluster_dll");

    //define type
    FUNC myFunc;
    myFunc = FUNC(lpIO);

    //define returned variable

    Input_t Cluster;
    Cluster.A = 1;
    Cluster.B;

    int32_t myValue = 0;
    char str[10];

    //call function
    myFunc(Cluster,&myValue,str,10);


    FreeLibrary(hDLL);
    return 0;
}

i built it and ran. then the application crashed. i guess i cannot treat "LStrHandle" correctly. it is an original data type defined by LabVIEW.

typedef struct { 
    int32 cnt;  /* number of bytes that follow */ 
    uChar str[1];  /* cnt bytes */ 
} LStr, *LStrPtr, **LStrHandle;

I looked for NI web page and some web resource but could not solve the crash.

Please comment if you have any advice.

Why are you not using the generated header. Also you should not pass an indeterminate pointer. At least null it. — Cheers and hth. - Alf, Dec 28 '14 at 07:36

score 1 · Answer 1 · edited Sep 08 '15 at 12:18

if you are not using the generated header, at least insert #pragma pack(1)
Your generated function accepts a pointer to Cluster, however you redefine it as the one accepting the cluster itself. This is a mistake. Likely correction:
```
typedef void (*FUNC)(Input_t*,int32_t *,char [],int32_t);
```
All internal LabVIEW data handles are allocated on the heap with LabVIEW memory manager functions. I.e. you can not allocate Input_t Cluster on a stack and pass it to a LabVIEW generated function.

You have to use something like that:

Input_t *Cluster(DSNewPtr(sizeof(Input_t));
Cluster->A = 1;
Cluster->B = DSNewHandle(sizeof(LStr));

you should also program your logic to check for memory allocation errors and proper deallocations

Access to LStrHandle in the DLL

1 Answers1