I'm can't use the function of a dll in delphi. I'm having some problems with the conversions of types.
This is the function I want to call the Delphi DLL:
function SyncFunc(var Type:string; var MaxUsers:integer; var ErrCode :Word):boolean;
C++ code:
unsigned char(WINAPI *SyncFunc)(PCHAR Type, INT *MaxUsers, WORD *ErrCode);
HMODULE hLib;
BOOL Res = FALSE;
WORD ErrCode = 0;
INT MaxUsers = 0;
CHAR Type[256];
hLib = LoadLibrary("delphi.dll");
Res = SyncFunc(Type, &MaxUsers, &ErrCode);
Someone please help.
P.S. similar question here C++ consuming delphi DLL (but my function uses string, not WideString)
function SyncFunc(var Type:string; var MaxUsers:integer; var ErrCode :Word):boolean;
There are two problems facing you here:
string is a native Delphi type that can only be created and consumed by Embarcadero tools. Further more, since it uses the Delphi runtime heap, ShareMem or similar must be used.
The function uses the default register calling convention which again is only available using Embarcadero tools.
The obvious way forward is to fix the DLL and arrange for it to use standard platform interop types and calling conventions. You might use BSTR (WideString in Delphi) for strings, and stdcall as the calling convention.
If you cannot change the DLL then your only hope is to write an adapter DLL using the same compiler as was used to compile this errant DLL. But that can only work if the errant DLL was compiled using a shared memory manager. If that is not the case then your task is next to impossible.
Related
I wrote a DLL in C++ whose functions will be called from a Delphi application.
One of the functions in the DLL takes a Pointer to a buffer where an XML string should be written. But, when I write a string into the buffer, after returning from the function the application crashes with an "Access violation at address 0048B... in module ....exe. Write of address 3030D..." error.
The calling convention of the function declarations are the same, both in the DLL and the application.
I've made a simple application in Delphi to simulate the behavior of the application and it works fine. The biggest problem is that I don't have any information about the application internals: no sources, no documents, not even logs. Just function declarations and parameter descriptions.
Function declaration in delphi:
function functionName(var Buffer: Pointer; var BuffLen: Integer): Integer; stdcall;
Function Declaration in the DLL:
extern "C" int WINAPI functionName(char*, int*);
Does someone know how to solve this?
From my tests, I have a feeling that the problem is in the application, not in the DLL. However, I'm not completely sure about this. Are there any possible tests I can do at the DLL site to either solve the problem or locate the issue?
I'd really appreciate any help in this matter.
As a side note, the DLL is compiled with Visual Studio. Can this cause the problem?
The DLL function you showed is declared wrong in your Delphi code. var Buffer: Pointer is equivilent to void** in C, or void*& in C++, but certainly not to char* like the DLL function is expecting. Using a void**/void*& parameter would be useful if the DLL were allocating memory to return to the application, but from your description that is not the case.
Use this Delphi declaration instead:
function functionName(Buffer: PAnsiChar; var BuffLen: Integer): Integer; stdcall;
PAnsiChar in Delphi is equivalent to char* in C/C++.
You should read the following blog article about the gotchas to watch out for when converting C/C++ declarations to Delphi:
Rudy's Delphi Corner: Pitfalls of converting
I am creating a DLL in C++, it would be used in a Delphi 7 project.
This question is related to this one, where I present two functions Validate and GetToken only that now they will be done in C++ and the array of strings GetToken produces would be sent back to Delphi.
The problems is I don't know how to create the function in the dll that will return the array of string in c++, and I don't know how it would be stored for further use in Delphi.
The declaration of the function is as follows:
function GetToken(Chain:string):Arrayofstring;
According to your code review, the Delphi code expects the function to have the following signature:
function GetToken(Chain: AnsiString): array of AnsiString;
You cannot write such a function in C++. C++ doesn't know what Delphi strings are, and it doesn't know what Delphi dynamic arrays are, either. Both types need to be allocated from Delphi's memory manager, which your C++ DLL won't have access to. Furthermore, C++ doesn't know how to use Delphi's register calling convention.
The DLL interface was designed poorly. DLLs should never use language-specific types unless it was the designer's intention to exclude all other languages. (And in this case, even later versions of the same language are excluded because the definition of AnsiString changed in Delphi 2009 to include more metadata that Delphi 7 won't handle properly.) The safest calling convention to choose is generally stdcall. It's what everything in the Windows API uses.
A better interface would use types that are common to all languages, and it would dictate the use of memory management that's accessible universally. There are several common ways to do that. For example:
The strings are returned as simple nul-terminated arrays of characters — PAnsiChar in Delphi; char* in C++. The DLL allocates buffers for the strings, and also allocates a buffer for the array of those strings. When the host application is finished using the array and the strings, it calls another function exported by the DLL wherein the DLL frees the memory it allocated. This is the model used by, for example, FormatMessage; when the host program is finished the with message string, it calls LocalFree.
type
PStringArray = ^TStringArray;
TStringArray = array[0..Pred(MaxInt) div SizeOf(PAnsiChar)] of PAnsiChar;
function GetToken(Char: PAnsiChar): PStringArray; stdcall;
procedure FreeStringArray(StringArray: PStringArray); stdcall;
char** __stdcall GetToken(char const* Chain);
void __stdcall FreeStringArray(char** StringArray);
Use COM to return a safearray of BStr objects. It's similar to the previous technique, but the memory management is defined by COM instead of by your DLL, so there's less stuff that needs to be defined by either party of the interface.
Pass a callback function to the DLL, so instead of returning an array of strings, the DLL just calls the function once for each string it identifies. Then you don't have to define what any array looks like, and the lifetime of each string can be just the lifetime of the callback call — if the host application wants a copy, it can do so. The new function signature would look something like this:
type
TTokenCallback = procedure(Token: PAnsiChar); stdcall;
procedure GetToken(Chain: PAnsiChar; ProcessToken: TTokenCallback); stdcall;
typedef void (__stdcall* TokenCallback)(char const* Token);
void __stdcall GetToken(char const* Chain, TokenCallback ProcessToken);
If you're not the one who designed the DLL interface, then you need to lean on the folks who did and get it changed to be more accessible to non-Delphi code. If you can't do that, then the final alternative is to write a DLL in Delphi that wraps your DLL to massage the parameters into something each side understands.
I have a DLL written in Delphi 7 that I need to use in Visual C++ 2008.
From documentation that came with DLL, I can see that function is declared as (Delphi 7):
function ReadInfo(pCOM, pBuf, pErr: Pointer):boolean;
where pCom is pointer to data structure:
TCOM = record
dwBaudRate: Longword;
nCom,
nErr,
nLang : Byte;
end;
pBuf is pointer to "array of byte" (as it is written in DLL's documentation).
pErr - not used.
So now in c++ (after successfully loading DLL with LoadLibrary), I call:
myFunc = (MY_FUNC_POINTER)GetProcAddress(dllHandle, "ReadInfo");
which also doesn't return any errors.
MY_FUNC_POINTER is defined as:
typedef bool (*MY_FUNC_POINTER)(TCOM*, BYTE*, void*);
where TCOM is:
struct TCOM
{
unsigned long dwBaudRate;
BYTE nComm;
BYTE nError;
BYTE nLanguage;
};
I defined:
TCOM MyCom;
BYTE *myRes;
myRes = new BYTE[1024*1024];
But after calling
myFunc(&MyCom, myRes, NULL)
I get “The value of ESP was not properly saved across a function call.” error.
There would appear to be a calling convention mismatch. On the face of it, the function declares no calling convention in the Delphi, so the default Borland register convention is used. Your C++ code does not declare a calling convention for the import so the default of cdecl is used. But it is plausible that the Delphi code and documentation are not aligned and the Delphi code actually uses a different calling convention. Check the Delphi code, or contact the vendor. No matter what, the error message that you report does indicate a binary mis-match across the boundary between your module and the other module.
If the function really does use the Borland register calling convention (but see below for more), then you cannot readily call the function from languages other than Delphi. In that case you'd need a bridge to adapt that to a standard calling convention such as stdcall. By that I mean a Delphi DLL that can call the original DLL and expose it's functionality a way suited to interop. A better solution would be to fix the root problem and build the DLL again using standard calling conventions.
In fact, I now suspect that all the other commentators are correct. I suspect that the Delphi documentation does not match the Delphi code. I suspect that the function really is stdcall. So you can, probably, solve your problem by changing the function pointer typedef to be as follows:
typedef bool (__stdcall *MY_FUNC_POINTER)(TCOM*, BYTE*, void*);
My reasoning for this is that in stdcall the callee is responsible for cleaning the stack. That's not the case for cdecl, and since all the parameters, and the return value, fit in registers, it's not the case for Delphi register calling convention, for this function. Since there is a stack pointer mis-match, it follows that the most likely explanation is that the Delphi function is stdcall.
All the same, it's not comfortable to be working out calling conventions this way. If you cannot get any help from the DLL vendor then I'd be inclined to dig a little deeper by looking at the DLL function's code under a disassembler.
I have a dll with exporting function
extern "C" __declspec(dllexport) IDriver * __stdcall GetDriver()
There is a programm which is written on Delphi. It can't see the function GetDriver().
It's double awful because I can not get and modify sources of this program.
What may be the reason of successful loading my dll (according log file) and failed call exporting function? Thank you.
Window 7 x64, Visual Studio 2010, C++ project for x86 target
The most likely explanation is that the function will have been exported with a decorated name. I'd expect it to have been exported with the name GetDriver#0. So you could import it like this:
function GetDriver: IDriver; stdcall; external 'DllName.dll' name 'GetDriver#0';
Use a tool like Dependency Walker to check the exact name used to export the function.
If you cannot modify the Delphi code, then you'll need to make your C++ DLL match. Do that by using a .def file which allows you control over the exported name.
The other problem you will face is that Delphi's ABI for return values differs from that used by most other tools on the Windows platform. Specifically a return value is semantically a var parameter. On the other hand, your C++ compiler will regard the return value as an out parameter. My question on Delphi WideString return values covers exactly this issue.
Because of this, I'd expect the function declaration above to lead to access violations. Instead you should declare the return value to be a Pointer and cast it to an interface reference in your Delphi code. You'll need to double check and make sure that the reference counting is handled appropriately.
Again, if you cannot modify the Delphi code, you need to make the C++ code match. A Delphi interface return value is implemented as an additional var parameter following the other parameters. So, to make your C++ function match, declare it like this:
void __stdcall GetDriver(IDriver* &retVal);
I've compiled a DLL in Visual Studio (the source code is in C++, which I barely understand). Here's a piece of Scraper.h:
struct SWin
{
char title[512];
HWND hwnd;
};
SCRAPER_API bool ScraperGetWinList(SWin winList[100]);
Now I'm trying to use the above function in my Delphi application:
type
tWin = record
title: String;
hwnd: HWND;
end;
function ScraperGetWinList(var WinList: Array of tWin): Boolean; external 'Scraper.dll';
var
myWinList: Array [1..100] of tWin;
procedure TMainForm.GetWinListButtonClick(Sender: TObject);
begin
ScraperGetWinList(myWinList);
...
The project doesn't compile, and I get the following message: The procedure entry point ScraperGetWinList could not be located in the dynamic link library: Scraper.dll.
What am I doing wrong?
From my Linux experience, I'd say that you've encountered so-called "name-mangling" issue. The entry point of your procedure is not called "ScraperGetWinList", but something like "_ZN18ScraperGetWinListEpN4SWin".
The thing is that, Unlike in C, in C++ language the name of entry point is not the same as the function name. No wonder: assume, you have a set of overloaded functions; they should have different entry points in your DLL. That's where name mangling comes into play.
The most common solution to this problem is to define interface of your library in such a way that it will use C calling convention. No name mangling will happen with the interface functions then.
Note that you don't have to write the whole library in C, you only should declare functions for them to emit C-like entry points.
Usually it's written like this:
extern "C" {
SCRAPER_API bool ScraperGetWinList(SWin winList[100]);
// More functions
}
Recompile your library and use it in Delphi without problems.
Note, that you should also adjust calling conventions (stdcall or cdecl) for them to match in your C++ header and Delphi code. However, that's best explained in another question.
Name mangling is most likely the problem. Name mangling is usually done is C++ code,
and when writing a DLL in C++ that should be used by code in an other langauge,
you should use the Extern "C" construction as Pavel Shved already suggested.
When using DLLs, especially when writtin in other languages, you should also keep
an eye on calling conventions. I suggest that you specify in both delphi and c++ to use the stdcall calling convenstion. This is the calling convention also used by the windows api, so it guarantees the best interoperatability between different compilers.
This would mean
extern "C" {
SCRAPER_API __stdcall bool ScraperGetWinList(SWin winList[100]);
}
and
function ScraperGetWinList(var WinList: Array of tWin): Boolean; external 'Scraper.dll';
But that's not all, the stdcall calling convention has an impact on the name mangling, and it would turn out to be something like _ScraperGetWinList#4 (Where 4 is the size of the parameter, where an array would have a pointer to the first element, so 4 bytes)
To confirm the correct symbols to use, I suggest Dependency Walker
( http://www.dependencywalker.com/ ) this program shows that exactly the function names are exported by the dll. Having confirmed the name to be '_ScraperGetWinList#4' then you add this in delpgi like this:
function ScraperGetWinList(var WinList: Array of tWin): Boolean; external 'Scraper.dll' name '_ScraperGetWinList#4';
Have you actually exported the entry point function in the c++ code? This really stumped me the first time I compiled a C++ dll in Visual Studio for use in a dotnet app.
For example, I needed to expose a print driver in unmanaged code so some other developers could access it in VB.net. This is what I did.
In foo.cpp:
extern "c" {
___declspec(dllexport) bool FooBar()
{
// Call some code on my cpp objects to implement foobar
}
}
Then in a file called foo.def:
LIBRARY "mylib"
EXPORTS
FooBar
AnyOtherFunctionsItExports
This is how I got it to work. I might not be doing things the best possiable way. I am a little light on C++ experience and also mainly don't work on windows.