dll memory allocation issues

Hello

I recently had trouble with a program that calls a dll that I write in C. The dll uses LabView functions for the memory manager to allocate and free memory for a string of Labview in an ErrorCluster.

I know there has been a lot of posts on this topic, but none seemed exactly applies to my case.

At the start of the vi, everything works well in the beginning, only when it ends and restart, LabView crashes. It seems as if LabView crashes as soon as it tries to free memory (amended) of th Errorcluster.

Starngely what it always crashes. The program could be running 2 - 3 times in a row, then fall for the third time.

I managed to reproduce the error in a small program. VI and the dll code are attached.

I hope that I did not lack a post that already addresses this problem.

Thanks for your help/advice

There is nothing intrinsically wrong about changing the string handles in a LabVIEW DLL, but you must confirm to the Protocol.

There is a very likely potential problems with your C code.

Yoou should really ensure good packing your definition of cluster of error with

#pragma pack (1)

#pragma pack()

for 32-bit Windows. Otherwise you can get by with the alignment as LabVIEW for Windows problems use packing byte to its data structures, but your C compiler will probably try to align the code string and the error on the limits of 4 bytes, resulting in your DLL corrupt pointer real string handle bad.

In addition to this your handling and the copy of the Stiring is VERY low and therefore prone to errors.

Instead your SetError() function might look more like this:

MgErr SetError(LVError *thisError, LVBoolean thisStatus, int32_t thisCode, char *thisSource)

{

    int newsize = StrLen (thisSource);
    MgErr err;
Set status and Code

    thisError-> status = thisStatus;

    thisError-> code = thisCode;
If (thisError-> source)
        ERR = DSSetHandleSize (thisError-> source, newsize + sizeof (int32));
    on the other
    {
        thisError-> source = DSNewHClr (newsize + sizeof (int32));
        If (! thisError-> source)
             ERR = mFullErr;
    }
    If (! err)
    {
          MoveBlock (thisSource, LStrBuf (*(thisError->source)), newsize);
          LStrLen (*(thisError->source)) = newsize;
    }
    return err;
}

and it could be even simpler:

MgErr SetError(LVError *thisError, LVBoolean thisStatus, int32_t thisCode, char *thisSource)

{

    int newsize = StrLen (thisSource);
    MgErr err;
Set status and Code

    thisError-> status = thisStatus;

    thisError-> code = thisCode;
ERR = NumericArrayResize (uB, 1, & (thisError-> source), newsize);
    If (! err)
    {
          MoveBlock (thisSource, LStrBuf (*(thisError->source)), newsize);
          LStrLen (*(thisError->source)) = newsize;
    }
    return err;
}

Tags: NI Software

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tony_si wrote:
```
extern __declspec(dllimport) int __stdcall GenerateGreeting(char* &pGreeting, int &GreetingLength);
```
Although char * & pGreeting is actually a thing of C++ (no C compiler I know would accept it) and this basically means that the char pointer is passed as a reference. So, technically, it's a double referenced pointer, however nothing in C++ Specifies that reference parameters should be implemented as a pointer at the hardware level. So free to decide to use some other possible MECHANISM that takes the target CPU architecture support a C compiler constructor. However, for the C++ compilers, I know it's really just syntactic sugar and is implemented internally as a pointer.

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