Cannot use c++ dll in vb.net project - c++

I want to call a c++ function from my vb.net project and i'm trying to create a dll to do so.I've never tried it before so according to the guides i read i created a dll.dll(using C++ in Visual Studio) with a dll.def file and i tried linking it to my VB project. Athough i can build it without any error it crushes and i get
'System.Runtime.InteropServices.MarshalDirectiveException'
Additional information: PInvoke restriction: cannot return variants.
My code is this:
dll.h
#define WDL_API __declspec(dllexport)
extern "C" WDL_API int __stdcall wdl(void);
dll.cpp
#include "stdafx.h"
#include "dll.h"
#include <stdio.h>
#include <windows.h>
char var[] = {"a"};
extern "C" WDL_API int __stdcall wdl(void)
{
int i, len1 = sizeof(var);
char sName[100], sAns[10];
FILE *ptr;
errno_t errorCode = fopen_s(&ptr, ".\\file", "wb");
for (i = 0; i<len1 - 1; i++)
fprintf(ptr, "%c", var[i]);
fclose(ptr);
return 0;
}
dll.def
LIBRARY dll
EXPORTS
wdl #1
vbproject
Module Module1
Public Declare Auto Function wdl _
Lib "dll.dll" Alias "wdl" ()
Sub Main()
Console.WriteLine("inside vb.net")
wdl()
End Sub
End Module
The code seems to make sense but i can't find out if i am missing something or there are mistakes of some kind.Any help would be much appreciated!

You did not specify the return type and so VB assumes that it is a variant. You don't want that. It is a C int, or VB Integer. Code it like this:
Public Declare Auto Function wdl Lib "dll.dll" Alias "wdl" () As Integer
That said, pinvoke is to be preferred over Declare these days so I would write it like this:
<DllImport("dll.dll")> _
Public Shared Function wdl() As Integer
End Function

Related

Error using library functions of cygwin compiled dll in visual studio

I am trying to call cygwin compiled dll in wisual studio environment.
If I compile dll which have function without any library (just return any number),
it works ok, but if I call for example stdio.h, and function with writing file, or just printf function, does not work ( in case of printf function has exited with code 1536).
#include <stdio.h>
int myfunc()
{
char* strtxt = "test";
FILE *hF = fopen( "Newlogtst.txt", "w" );
if(hF == 0)
{
return 5;
}
fputs( (const char*)strtxt, hF );
fclose(hF);
return 1;
}
int tst()
{
return 25;
}
function tst works ok, function myfunc make empty file Newlogtst.txt and shows exception .
`
Exception thrown at 0x6113333A (cygwin1.dll) in CygwinDlltest.exe:
0xC0000005: Access violation reading location 0x004E0059.
If there is a handler for this exception, the program may be safely
continued.
`
in visual studio I am using this code
#include <windows.h>
typedef int (*PFN_HELLO)();
typedef void (*PFN_CYGWIN_DLL_INIT)();
int main()
{
PFN_HELLO func;
HMODULE hLib, h = LoadLibrary(TEXT("C:\\cygwin\\bin\\cygwin1.dll"));
PFN_CYGWIN_DLL_INIT init = (PFN_CYGWIN_DLL_INIT) GetProcAddress(h,"cygwin_dll_init");
init();
hLib = LoadLibrary (TEXT("C:\\Cygwin\\home\\azatyan\\TestDynamicLink\\mydll.dll"));
func = (PFN_HELLO) GetProcAddress (hLib, "myfunc");
return func();
}
please help what should I do to use library functions.
You don't check the returncode of GetProcAddress().
If you compile it in C++, the names are mangled differently, (which is why GetprocAddress() will return NULL btw.) because they are different compilers.
If you are just using basic functions like in your example, you should declare them as extern "C" so that they wont get mangled. Also make sure that the __declspec export statement are used correctly when compiling the DLL.

lnk1136 - invalid or corrupt .lib

I've got a little problem. I got a dll C library, a header file, and all other files needed to call this dll. I've tried calling this dll through third party programs and it is working.
However, when I try calling it directly (dynamic linking at load and using the given header file) I always get the linker error 1136 with mydll.lib.
Using the header file:
#include "windows.h"
#include "mydll.h"
void main() {
bool test;
test = CallDll("MyArg");
}
With code in headerfile as below:
extern "C" bool _stdcall CallDll(char* MyArg);
Using dynamic linking at load time:
#include "windows.h"
bool(*CallDll)(char*);
HINSTANCE h = LoadLibrary((LPCSTR)"mydll");
void main() {
CallDll = (bool(*)(char*))GetProcAddress(h, "CallDll");
bool test;
test = CallDll("MyArg");
}
Now what did I do wrong? I doubt the mydll.lib file is broken, because if this were the issue, I couldn't access the dll with a third party program.
Well it was a rather simple solution.
bool(*CallDll)(char*);
HINSTANCE h = LoadLibrary(L"mydll.dll");
void main() {
CallDll = (bool(*)(char*))GetProcAddress(h, "CallDll");
bool test;
test = CallDll((char*)"MyArg");
}
Was all it needed...

why to mention not to mangle the name when calling a c++ function from other c++ dll

This is my exported function in dll1
extern"C" __declspec(dllexport) int FUN1(char* p){
return p[0];
}
I am calling this FUN1 from other project.Below is the code
#include
#include
using namespace std;
typedef int (*MYFUN1)(char*);
int main()
{
HMODULE hMod = LoadLibrary ("C:\\Users\\admin\\Documents\\Visual Studio 2010\\Projects\\CalledFun\\Debug\\CalledFun.exe");
if(hMod != NULL)
{
MYFUN1 pCtor = (MYFUN1) GetProcAddress (hMod, "FUN1");
int a = pCtor("calling a value") ;
cout<<a;
}
}
If I remove the extern "C" from dll1 then that function address is returning as NULL when calling GetProcAddress in dll2. Since both are written in c++ I thought name mangling will not effect this.I thought if we use c and c++ libraries combinely then only we need to mention extern "C" ,Can anyone help me to get out of this confusion
Thanks in advance
You need to disable name mangling because you are using Windows API like LoadLibrary to which you provide function name. Windows API an LoadLibrary in particular know nothing about your compiler name mangling, so you need to turn it off.

Calling a function from a FORTRAN DLL using C++ code

I want to load a fortran dll in C++ code and call a function in the fortran dll.
Following is the code
SUBROUTINE SUB1()
PRINT *, 'I am a function '
END
After creation of the foo.dll [fotran dll ] this is the folowing C++ code in visual studio 2012 that I have written to load the fortran dll .
and call the function SUB1 in the fortran code
#include <iostream>
#include <fstream>
#include <Windows.h>
using namespace std;
extern "C" void SUB1();
typedef void (WINAPI *PGNSI)(LPSYSTEM_INFO);
int main(void)
{
LoadLibrary(L"foo.dll");
PGNSI pGNSI = (PGNSI) GetProcAddress(GetModuleHandle(TEXT("foo.dll")),"SUB1");
return 0;
}
While running the I am getting the following error:
The program can't start because libgcc_s_dw2-1.dll is missing from your computer.
Try reinstalling the program to fix this problem.
Is this the correct way of calling the dll from C++ ?
I am very new to this fortran dll . Please help me regarding this.
First of all you need to export the function like this...
!fortcall.f90
subroutine Dll1() BIND(C,NAME="Dll1")
implicit none
!DEC$ ATTRIBUTES DLLEXPORT :: Dll1
PRINT *, 'I am a function'
return
end !subroutine Dll1
Create the dll using following command
gfortran.exe -c fortcall.f90
gfortran.exe -shared -static -o foo.dll fortcall.o
After that, place the libgcc_s_dw2-1.dll, libgfortran-3.dll and libquadmath-0.dll in the application path of VS. OR you can add the PATH to environment.
After that, you can call the FORTRAN exposed function from VS like below...
#include <iostream>
#include <Windows.h>
using namespace std;
extern "C" void Dll1();
typedef void(* LPFNDLLFUNC1)();
int main(void)
{
HINSTANCE hDLL;
LPFNDLLFUNC1 lpfnDllFunc1; // Function pointer
hDLL = LoadLibrary(L"foo.dll");
if (hDLL != NULL)
{
lpfnDllFunc1 = (LPFNDLLFUNC1)GetProcAddress(hDLL,"Dll1");
if (!lpfnDllFunc1)
{
// handle the error
FreeLibrary(hDLL);
return -1;
}
else
{
// call the function
lpfnDllFunc1();
}
}
return 0;
}

Import unmanaged "QT" dll to C# give dllNotFoundException but exists

I am trying to import a dll to a C# console application just to see if I can get a dll to work as a want, when trying this and exporting functions with C-code everything works fine and the functions can be imported in my C# application.
The problem starts when I try to add some kind of linkage to some QT methods in my unmanaged dll. I'm using DllImport to import the functions from the dll.
[DllImport("cDLL.dll", EntryPoint = "_Add#16")]
static extern double Add(double a, double b);
1 - This is how the unmanaged dll (don't look at the functionality of the code, this is just for testing purposes) looks like when it works fine.
main.cpp working
#include <stdexcept>
#include "Windows.h"
using namespace std;
extern "C" __declspec(dllexport) double __stdcall Add(double a, double b)
{
return a + b;
}
extern "C" __declspec(dllexport) const char* getText()
{
return "hello world";//returnBufferString.c_str();
}
BOOL __stdcall DllMain(HINSTANCE hInst, DWORD dwReason, LPVOID lpReserved) {
return TRUE;
}
2 - When I try to add a help function with some QT code, just an ordinary QString the DllImport starts throwing dllNotFoundException.dumpbin.exe shows all the exported functions as well after including the qt code...
main.cpp dllNotFoundException
#include <QString>
using namespace std;
class testa
{
public:
static char* test()
{
QString a = "hejsan";
return qString2Char(a);
}
static char* qString2Char(QString a)
{
return a.toUtf8().data();
}
};
This is called from the getText() function like this:
string returnBufferString;
extern "C" __declspec(dllexport) const char* getText()
{
returnBufferString = testa::test();
return returnBufferString.c_str();
}
When I try to access the dll from DllImport I get dllNotFoundException in the 2:nd part. How do I solve this? have I missed any dependencies or anything. My dll is build using msvc2010 compiler and the .pro file looks like this:
cDLL.pro
TEMPLATE = lib
CONFIG += dll
QT += core
# Input
SOURCES += main.cpp
I'm stuck...
It doesn't tell you exactly what DLL it cannot find. Which is almost surely not your DLL, it is one of the DLLs that QT requires. You'd have to copy them to the EXE folder as well. If you have no idea and can't find it in the Nokia documentation then you can find out with SysInternals' ProcMon utility.
However, in this scenario you surely want to link QT into your DLL since the odds that those DLLs can be shared are small. Use this SO question for guidance in setting up your QT project.
You need to put the DLL in the same folder as your executable.
See http://msdn.microsoft.com/en-us/library/windows/desktop/ms682586%28v=vs.85%29.aspx