Pass two dimension array into C DLL from VB issue!

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Ben Ben - 28 Dec 2003 05:20 GMT

I tried to pass two dimension array into C DLL, but the initilized data is
misplaced in the array!

Type AUTOCAL_INST_HANDLER

defaultRM As Long
viGSMTestSet As Long
Voltage As Double
ExpectedPower(0 To 1, 0 To 1) As Double
End Type

Dim b As AUTOCAL_INST_HANDLER

b.defaultRM = 202
b.viGSMTestSet = 110
b.ExpectedPower(0, 0) = 1.23
b.ExpectedPower(0, 1) = 2.34
b.ExpectedPower(1, 0) = 3.45
b.ExpectedPower(1, 1) = 4.56

In C DLL file, it is defined as follows:

typedef struct
{
int defaultRM;
int viGSMTestSet;
double Voltage;
double ExpectedPower[2][2];
}AUTOCAL_INST_HANDLER;

double __stdcall TestSendWithStructure(AUTOCAL_INST_HANDLER *b)
{
double cout1,cout2,cout3,cout4;
cout1 = b->ExpectedPower[0][0];
cout2 = b->ExpectedPower [0][1];
cout3 = b->ExpectedPower [1][0];
cout4 = b->ExpectedPower [1][1];
}

The return result shows that b->ExpectedPower [0][1] = 3.45 while cout3 =
b->ExpectedPower [1][0] = 2.34,

Why the result is like this? How to ajust it? Thanks a lot!

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Mark Alexander Bertenshaw - 28 Dec 2003 13:54 GMT

> I tried to pass two dimension array into C DLL, but the initilized data is
> misplaced in the array!

This is because basic C arrays' dimensions are allocated in the opposite
order to VB. If this is a problem, then use a SAFEARRAY rather than a C
array, because this is what VB uses internally. In fact, you could do the
same with strings by using a BSTR rather than a C string. It is a bit of
extra work, and slightly slower, but it makes for easier interop.

--
Mark Alexander Bertenshaw
Kingston upon Thames
UK

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Ben Ben - 29 Dec 2003 03:17 GMT

I use VC6.0 and VB 6.0 to develop the code, could VC use SAFEARRAY? I can
not find any information about SAFEARRAY in MSDN library. Because I have to
use two dimension array to include many digital numbers rather than string.
Is there any other way to deal wiht the allocation issue?

> > I tried to pass two dimension array into C DLL, but the initilized data is
> > misplaced in the array!
[quoted text clipped - 9 lines]
> Kingston upon Thames
> UK

Reply to this Message

Mark Alexander Bertenshaw - 30 Dec 2003 12:50 GMT

> I use VC6.0 and VB 6.0 to develop the code, could VC use SAFEARRAY?

I would have thought so, since the documentation is all written assuming a
C/C++ programmer.

> I can
> not find any information about SAFEARRAY in MSDN library.

It's definitely there. Look under:

Platform SDK Documentation -> Component Services -> COM -> Automation ->
Conversion and Manipulation Functions -> Array Manipulation API Functions ->
SAFEARRAY Data Type

> Because I have to use two dimension array to include many digital numbers

rather than string.

> Is there any other way to deal wiht the allocation issue?

Not really - there are no other simple data types in C/C++ which would do
this, and you would have to create a COM wrapper for any classes you might
create to do the same thing.

HTH

Mark Alexander Bertenshaw
Kingston upon Thames
UK

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Ben Ben - 29 Dec 2003 06:24 GMT

Now I enlarge the size of the array as follows:

Type AUTOCAL_SYSTEM_PARAMETERS
displayFlag As Long
... ...
ExpectedPower(0 To 2, 0 To 16) As Double
End Type

Dim b As AUTOCAL_SYSTEM_PARAMETERS

b.ExpectedPower(0, 0) = 1.5
b.ExpectedPower(0, 1) = 2.3
b.ExpectedPower(0, 2) = 3.4
b.ExpectedPower(0, 3) = 4.5
b.ExpectedPower(0, 4) = 5.6

I pass the parameter into C DLL:

#pragma pack(4)
typedef struct
{
int displayFlag;
... ...
double ExpectedPower[3][17];
}AUTOCAL_SYSTEM_PARAMETERS;
#pragma pack(8) // Back to the default

double __stdcall TestSendWithStructure(AUTOCAL_SYSTEM_PARAMETERS *b)
{
double cout1,cout2,cout3,cout4;
cout1 = b->ExpectedPower[0][0];
cout2 = b->ExpectedPower[0][1];
cout3 = b->ExpectedPower[0][2];
cout4 = b->ExpectedPower[0][3];
return (cout1);
}

The VC debug result shows as follows:
- ExpectedPower 0x0012f178
- [0] 0x0012f178
|- [0] 1.5000000000000
|- [1] 0.00000000000000
|- [2] 0.00000000000000
|- [3] 2.3000000000000
|- [4] 0.00000000000000
|- [5] 0.00000000000000
|- [6] 3.4000000000000
|- [7] 0.00000000000000
|- [8] 0.00000000000000
|- [9] 4.5000000000000
|- [10] 0.00000000000000
|- [11] 0.00000000000000
|- [12] 5.6000000000000
|- [13] 0.00000000000000
|- [14] 0.00000000000000
|- [15] 0.00000000000000
|- [16] 0.00000000000000

How to explain this result and fix the issue?

Reply to this Message

Mark Alexander Bertenshaw - 30 Dec 2003 12:56 GMT

Ben -

I've already explained the issue: VB packs arrays in the opposite way to the
way C expects them. Your results confirm this. Either reverse the indexes
when you access the array in C, or use SAFEARRAYs.

--
Mark Alexander Bertenshaw
Kingston upon Thames
UK

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Ben Ben - 29 Dec 2003 08:22 GMT

I got the VB two dimenstion array store order and C two dimension array
store order!
VB:

b(0 To 2, 0 To 16) As Double

(0,0) (1,0) (2,0)
(0,1) (1,1) (2,1)
(0,2) (1,2) (2,2)
... ...
(0,16) (1,16) (2,16)

VC:

double B[3][17];

(0,0) (0,1) (0,2) (0,3) (0,4) ... (0,16)
(1,0) (1,1) .... .... (1,16)
(2,0) (2,0) ... ... (2,16)

So when VC program try to read the array in above order, it will read the
array from VB as follows
(0,0) (1,0) (2,0) (0,1) (1,1) (2,1) (0,2) (1,2) (2,2) ...

The conversion fomula could be like this B(m,n) :m and n is the size of the
array. When trying to read/write the data
B(i,j) of VB, which is responding to B(i',j'), then i' = (i+j*m)/n, j' =
(i+j*m)%n

How to use SAFEARRAY?

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VB Forum / COM / December 2003

Pass two dimension array into C DLL from VB issue!