I have a simple Labview dll that takes a PascalString then returns the pascal string with no changes. This is just testing what we can do. The header is as follows:

void __stdcall Read_String_In_Write_String_Out(PStr String_input, 
    PStr String_output);

the Delphi code is as follows:

var
  hbar : thandle;
  str, str2 : PChar;
  StringFunction : function (TestString: PChar): PChar; stdcall;

begin
  hbar := LoadLibrary('C:\Interface.dll');
  if hbar >= 32 then begin
    StringFunction := getprocaddress(hbar, 'Read_String_In_Write_String_Out');
    str := 'test';
    str2 := StringFunction(str);
  end;
end;

When running the program i get an Access Violation. I have no issues when doing simple math functions using dll's, but when it comes to strings everything breaks.

Can anyone help?

CodePudding user response：

You say that the DLL function is taking in a Pascal String. According to Labview's documentation:

Pascal String Pointer is a pointer to the string, preceded by a length byte.

Pascal-Style Strings (PStr)
A Pascal-style string (PStr) is a series of unsigned characters. The value of the first character indicates the length of the string. A PStr can have a range of 0 to 255 characters. The following code is the type definition for a Pascal string.

typedef uChar Str255[256], Str31[32], *StringPtr, **StringHandle;
typedef uChar *PStr;

This would be equivalent to Delphi's ShortString type (well, more accurately, PShortString, ie a pointer to a ShortString).

Based on the DLL function's declaration, its 2nd parameter is not a return value, it is an input parameter taking in a pointer by value. So your use of StringFunction is wrong on 2 counts:

• Getting the output in the wrong place. StringFunction should be a procedure with 2 parameters. However, the function can't modify the pointer in the 2nd parameter, all it can do is read/write data from/to whatever memory the pointer is pointing at. So, for output, you will have to pre-allocate memory for the function to write to.

• Passing around the wrong kind of string data. PChar is PWideChar in Delphi 2009 , but "Pascal strings" use AnsiChar instead. And your test data is not even a Pascal string, as it lacks the leading length byte.

So, try something more like this instead:

var
  hbar : THandle;
  str1, str2 : ShortString;
  StringFunction : procedure (String_input, String_output: PShortString); stdcall;
begin
  hbar := LoadLibrary('C:\Interface.dll');
  if hbar >= 32 then
  begin
    StringFunction := GetProcAddress(hbar, 'Read_String_In_Write_String_Out');
    str1 := 'test';
    StringFunction(@str1, @str2);
  end;
end;