Delphi: Access violation after calling a function from an external DLL (C ++)
There is a function there, written in C ++ and compiled as a DLL, that I want to use in my Delphi application.
Scraper.cpp:
SCRAPER_API bool ScraperGetWinList(SWin winList[100])
{
iCurrWin=0;
memset(winList,0,100 * sizeof(SWin));
return EnumWindows(EnumProcTopLevelWindowList, (LPARAM) winList);
}
Scraper.h:
#ifdef SCRAPER_EXPORTS
#define SCRAPER_API __declspec(dllexport)
#else
#define SCRAPER_API __declspec(dllimport)
#endif
struct SWin
{
char title[512];
HWND hwnd;
};
extern "C" {
SCRAPER_API bool ScraperGetWinList(SWin winList[100]);
}
This is how I declare a function in a Delphi application :
type
tWin = record
Title: Array [0..511] of Char;
hWnd: HWND;
end;
tWinList = Array [0..99] of tWin;
function ScraperGetWinList(var WinList: tWinList): Boolean; stdcall; external 'Scraper.dll';
The function works, but when finished, I get a debugger error: Project ... with an error with the message: "Access violation at 0x0012f773: writing address 0xffffffc0". The process has been stopped. Use Step or Run to continue.
If I add __stdcall
(after SCRAPER_API bool
) to Scraper.cpp and Scraper.h, then the Delphi application does not start at all: the entry point to the ScraperGetWinList procedure cannot be located in the Scraper .dll dynamic link library.
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You need to put __stdcall
after bool
. The complete declaration, after expanding the macros, should look like this:
extern "C"
{
__declspec(dllexport)
bool __stdcall ScraperGetWinList(SWin winList[100]);
}
EDIT: Looks like you also need a .def file. This is the file that lists all the functions exported to the DLL, in which case it only needed to force the C ++ compiler not to interfere with the exported names. The content will be as follows:
EXPORTS
ScraperGetWinList
I'm not sure which C ++ compiler you are using, but usually you just specify the .def file along with the .cpp; for example the following works for VC ++:
cl.exe foo.cpp foo.def
In addition, you will need to tell Delphi to use stdcall by adding the keyword stdcall
just before external
the Delphi function declaration.
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If you are using a packed array [1..512] of char, you do not need the ConvertToString () function.
"packed char array" is a Delphi string compatible assignment (this refers to the very early forms of pascal array from char WAS string type). You will need a scrab result for a null ($ 0) char to find the end of the C string
Also which version of Delphi are you using? if Delphi 2009+ you need to use packed array [1..512] from AnsiChar ;
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It would be good to know exactly where your access violation is happening. What variable / memory location is your work environment trying to access?
Then find out if the location should really be available, and if so, why it isn't.
My suspicion: you are accessing an array element that is not initialized correctly.
Index := 0;
S := ConvertToString(myWinList[Index].Title);
while S <> '' do
begin
WinListMemo.Lines.Add(S);
Inc(Index);
//////// Is Index pointing to a valid entry here? No check!
S := ConvertToString(myWinList[Index].Title);
end;
Or
- dll does not initialize it correctly,
- or is there another way to find out the last item.
- or you just deleted the array completely: element 101 is dereferenced as well. and 102nd if this memory location contains character 0.
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Make sure your Delphi function definition matches what you also declare a C ++ function. In particular, make sure you have a stdcall at the end and that your bool values are consistent. C ++ and Delphi use different values and sizes for bool, depending on the C ++ compiler, so it's better to use an appropriate Integer size. Since the bool size may not match the C ++ size, it can affect the stack and therefore cause access violations.
[edited to remove mixed language mixing reaction]
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