Structure Conversion between VB6 and C

[jdy0803]

My VB6 program should call C DLL.
I should pass structure type as a parameter.
Original C structure is like following.

typedef struct strIniSensor
{
const char* theInitFile;
CallBackFunc IniSensorCallBack;
void * CallBackParameters;
}strIniSensor;

I made VB6 side Type Definition like following.
Type strIniSensor '
theInitFile As String
IniSensorCallBack As Long
CallBackParameters As Long
End Type

Is this correct? vensor who provide dll says I can assign NULL to all items(theInitFile,IniSensorCallBack,CallBackParameters) in the structure.

[Bonnie West]

Looks OK to me. Your API declaration should pass your structure ByRef.

[jdy0803]

Yes I do pass my structure ByRef.
Thanks!

[jdy0803]

I have trouble in other C DLL function call.
Can somebody review this VB6 code is correct?
------------------------------------------------------------------
<C DLL and calling in C>
typedef struct strImageAcq
{
const char* theHandle;
char* theImageBuf;
long* theImageBufSizePtr;
const struct timeval* theTimeout;
long ImageStatus;
CallBackFunc AcqImageCallBack;
void * CallBackParameters;
}strImageAcq;

struct timeval {
long tv_sec;
long tv_usec;
}

__declspec (dllexport) long TRI_AcqImage(strImageAcq* myImageAcq);

strImageAcq myImageAcq;
strImageAcq* ptr_myImageAcq = &myImageAcq;

myImageAcq.theHandle = theHandle;
myImageAcq.theImageBuf = theImageBuf;
myImageAcq.theImageBufSizePtr = &theImageBufSize;
myImageAcq.theTimeout = &timeout; //This is the time window in which the sensor waits for X-ray, max value 65535 seconds
myImageAcq.AcqImageCallBack = &MyCallbacFunc;//Pointer to the user defined callback function, can be NULL
myImageAcq.CallBackParameters = ptr_ImgAcqEvent;//Pointer to the callback function parameters, can be NULL
myImageAcq.ImageStatus = NULL;//Variable to retrieve image status, 0 is successful

TRI_AcqImage(ptr_myImageAcq);//This function creats a thread and returns right away
------------------------------------------------------------------
<VB6 declaration and VB6 code>
Type timeval
tv_sec As Long 'seconds(long)
tv_usec As Long 'any micro seconds(long)
End Type

Type strImageAcq
theHandle As Long
theImageBuf As Byte
theImageBufSizePtr As Long
theTimeout As timeval
ImageStatus As Long
AcqImageCallBack As Long
CallBackParameters As Long
End Type

Public Declare Function TRI_AcqImage Lib "MyDLL" (myImageAcq As strImageAcq) As Long

myImageAcq.theHandle = theHandle
myImageAcq.theImageBuf = theImageBuf(0)
myImageAcq.theImageBufSizePtr = theImageBufSize
myImageAcq.theTimeout.tv_sec = timeout.tv_sec
myImageAcq.theTimeout.tv_usec = timeout.tv_usec
myImageAcq.ImageStatus = -1
myImageAcq.AcqImageCallBack = 0
myImageAcq.CallBackParameters = 0

lRet = TRI_AcqImage(myImageAcq)

[Doogle]

Looks OK to me, What 'trouble' are you having?

[Milk]

I think you might want something more like this...

Type timeval
   tv_sec As Long 'seconds(long)
   tv_usec As Long 'any micro seconds(long)
End Type

Type strImageAcq
   theHandle As Long
   ImageBufPtr As long
   ImageBufSizePtr As Long
   TimeoutPtr As Long
   ImageStatus As Long
   AcqImageCallBack As Long
   CallBackParametersPtr As Long
End Type

'...

Dim myImageAcq As strImageAcq

myImageAcq.theHandle = theHandle
myImageAcq.ImageBufPtr = VarPtr(theImageBuf(0))
myImageAcq.ImageBufSizePtr = VarPtr(theImageBufSize)
myImageAcq.TimeoutPtr = VarPtr(timeout)
myImageAcq.ImageStatus = -1
myImageAcq.AcqImageCallBack = 0 'or AddressOf callback function
myImageAcq.CallBackParametersPtr = 0 'or VarPtr(callBackParams)

Edit: Be careful with scope, the call could crash your program if any of the pointers refer values that have gone out of scope.

[jdy0803]

I tried this but program crashed.
You said "be careful with scope". Can you explain more detail about scope?
If you have some idea, please let me know.

[Milk]

Confession; I've not used VB6 in some time.

The call is asynchronous so it will return almost straight away but it won't be finished until some time later. The pointers in the structure must refer to memory that exists for the duration of the full call. If these are dimensioned at module level (at the top) then they will remain in scope. If these are dimensioned locally (in a sub/function) then they will only remain in scope until the function returns, this might be tricky if you don't know when the async call has finished.

The callback field can be set to a function pointer which will be called presumably when the call finally completes. I think you can use this in VB6 but if I remember right you can only use AddressOf with API calls so you have to use something like PutMem4 (he's called it PutDWord in the linked page) to get the actual function address.

How does theHandle work with this dll? It seems a bit strange that is defined as a const char *. Can you show how you get theHandle?

[jdy0803]

That was typo.
The original structure in C is like this.

typedef struct strImageAcq
{
int theHandle;
unsigned char* theImageBuf;
unsigned int* theImageBufSizePtr;
const struct timeval* theTimeout;
int ImageStatus;
CallBackFunc AcqImageCallBack;
void * CallBackParameters;
}strImageAcq;

[Doogle]

If you're using Milks's version you have to define 'timeout' and set the appropriate values in that structure.

Type timeval
   tv_sec As Long 'seconds(long)
   tv_usec As Long 'any micro seconds(long)
End Type

Type strImageAcq
   theHandle As Long
   ImageBufPtr As Long
   ImageBufSizePtr As Long
   TimeoutPtr As Long
   ImageStatus As Long
   AcqImageCallBack As Long
   CallBackParametersPtr As Long
End Type

Dim timeout As timeval

Dim myImageAcq As strImageAcq
timeout.tv_sec = 'whatever
timeout.tv_usec = 'whatever


myImageAcq.theHandle = theHandle
myImageAcq.ImageBufPtr = VarPtr(theImageBuf(0))
myImageAcq.ImageBufSizePtr = VarPtr(theImageBufSize)
myImageAcq.TimeoutPtr = VarPtr(timeout)
myImageAcq.ImageStatus = -1
myImageAcq.AcqImageCallBack = 0 'or AddressOf callback function
myImageAcq.CallBackParametersPtr = 0 'or VarPtr(callBackParams)

[jdy0803]

Yes I did set timeout.tv_sec and timeout.tv_usec but result is the same.

[Milk]

How about showing us the code you are using.

If you wrap the code in [code]code goes here[/code] tags it is easier for us to read.