[RESOLVED] How to call C-Dll with char[] signature

[keltsch]

Hi all,
I tried the search (forum / google) before, but unfortunately I found no solution.
From VB.NET I have to call a very old c-dll, the methods have signatures like this:

int __stdcall getNumber();
int __stdcall getValue(char val1[256]);

I'm able to call the getNumber Method to retrieve an integer value.
With the Method getValue I have to retrieve a value into my VB app,
but everything I tried, I get an access violation within my VB app.

Using old VB6 the following worked:

Private Declare Function getMessage Lib "val.dll" (ByVal val1 As String) As Long
...
dim sTest as String*256
...
Call getMessage(sTest)
...

In VB I tried the method-declaration with IntPtr, FixedLengthString, Char(), Byte() and so on, but with no success. I now, that char* declarations can be handled with the Stringbuilder, but that seem not to work in this case, as the declaration is a char[256] and no pointer.

So how to declare the c-method above to be used in VB.NET?

Any hints are welcome!

[jmcilhinney]

That second method should have a parameter of type Char() and a return type of Integer. If it requires an array with 256 elements then you need to specify an upper bound of 255 when you create it in VB.

[keltsch]

Thanks for your reply,
that is the declaration I tried:

Private Declare Function getValue Lib "val.dll" (ByRef val1 As Char()) As Integer
...
Dim sVal as Char(255)
Dim iRes as Integer = getValue(sVal)
...

Unfortunately without success, it gives the same access violation.

[formlesstree4]

I think you have to declare the array as new:

Dim sVal As Char() = new Char(255)

[Niya]

You can simply do this:-

'
    <StructLayout(LayoutKind.Sequential)> _
    Public Structure FixedString256
 
        <MarshalAs(UnmanagedType.ByValTStr, sizeconst:=256)> _
        Public val1 As String
 
        Public Shared Widening Operator CType(ByVal s As String) As FixedString256
            Dim ret As FixedString256
 
            ret.val1 = s
            Return ret
        End Operator
 
        Public Shared Widening Operator CType(ByVal s As FixedString256) As String
            Return s.val1
        End Operator
 
 
    End Structure
 
    <DllImport("Val.Dll")> _
    Public Shared Function getValue(ByRef val1 As FixedString256) As Integer
    End Function

FixedString256 can be treated as a normal String in code because I've overriden the CTYPE operator.

[keltsch]

Thanks for your replies,
however - no luck at all.

[jmcilhinney]

Thanks for your reply,
that is the declaration I tried:

Private Declare Function getValue Lib "val.dll" (ByRef val1 As Char()) As Integer
...
Dim sVal as Char(255)
Dim iRes as Integer = getValue(sVal)
...

Unfortunately without success, it gives the same access violation.

Why ByRef? ByVal is the default so unless specified, use ByVal ByRef in managed code implies a pointer in unmanaged code. I obviously can't test it but I would expect this to work:

Private Declare Function getValue Lib "val.dll" (ByVal val1 As Char()) As Integer
...
Dim sVal(255) as Char
Dim iRes as Integer = getValue(sVal)
...

Note the proper declaration and creation of the Char array.

[keltsch]

I tried that out, also no luck.
Once again, the old style use with vb6 works.

For additional background information:
The called c-dll is a wrapper for a JNI caller which intercats with the JAVA runtime.

Might that perhaps interfere with a different threading model in VB.NET?

[Niya]

Did you try what I posted in #5 ? That definitely works.

[keltsch]

Thanks for all your hints - we localized the problem and it unfortunately isn't affected by the call signature but by the dll itself. The called dll uses an internal JVM initialization which fails in the second call as the call to the Dll comes from a different thread. That's why the call fails and lasts in an exception.

[jmcilhinney]

Chalk up another victory for multi-threading.

[keltsch]

Yes, that's quite odd!

Now we ran into another problem, perhaps anyone has an idea in chance?

The construct is:
.NET App (MultiThreaded) -> C-DLL (JNI) -> JAVA (in JVM)

After several tests the Initialization of the JVM only works, if the demanded HeapSpace is reduced to 32MB (!) - a larger size (i.e. 256MB) last in an initialization error (not enough memory on heap). System indicates free memory of 1.5GB - so it's no physical problem. What might be the best way to come around with this?

* Increasing heap size for unmanaged code called from .NET (I think it isn't possible).
* Defragmenting heap (I think it's also not possible from .NET at all ).

I tried figuring out the maximum additional available memory using System.Runtime.MemoryFailPoint() and it fails at 300MB ... so I'm suprised again ... is there any setting (compiler o.e.) to adjust this.
The System ist WinXP SP3 3GB with /3GB switch on ...

Thanks again ...