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Need simple demo call Delphi DLL in C++
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How can I receive a string from a FPC DLL? I would like to send two pointers concat them and receive the result in another string in Delphi.
library Test;
{$mode Delphi}
uses
Classes;
function Concat(const S1, S2: PWideChar): String; cdecl;
begin
Result := S1 + S2;
end;
exports
Concat name 'Concat';
begin
end.
In Delphi, a String is a complex, structured type with many details managed for you by the compiler and RTL 'magic' that hides these details. In particular, for 'long strings' there is a reference count and a length and, depending on the Delphi version involved, possibly other information.
Any DLL cannot know the details of precisely what information is required to be returned (or may be present in) any 'string' variables (or results) that an application may require. The DLL may not even be called by a Delphi program at all, in which case the 'string' type will be quite different again.
For this reason, a DLL will usually choose to deal with strings as simple 'C'-style pointer to char types. That is, some pointer to a null terminated region of memory. The caller of the DLL must then also ensure to exchange 'string' values with the DLL accordingly.
In the case of some function returning a value, the issue is complicated by the fact that allocation of the area of memory required to hold the result must be performed by the caller, with the function in the DLL taking appropriate steps to ensure that the memory supplied is sufficient. Applying these principles in this case results in a DLL routine that might look similar to this:
function Concat(const S1, S2, DEST: PWideChar; const aMaxLen: Integer): Boolean; cdecl;
begin
// left as exercise
end;
This is a simple implementation that returns TRUE if aMaxLen is sufficient to accommodate the concatenated result. You should also consider other behaviours of the function under a variety of conditions (eg. S1 or S2 or both are NIL, aMaxLen is too big, etc).
Whatever implementation choices are made for performing the concatenation (left as an exercise for you), the result of the function call must be to place the result in the buffer pointed to by DEST.
The caller must then also ensure that a buffer of sufficient length is provided and the correct length indicated in the call:
var
a, b, ab: WideString; // Or: String or UnicodeString in Delphi 2009 and later
begin
a := 'foo';
b := 'bar';
// Make sure 'ab' is big enough to hold the concatenated result of a + b
SetLength(ab, Length(a) + Length(b));
if Concat(PWideChar(a), PWideChar(b), PWideChar(ab), Length(ab)) then
// success: ab == 'foobar'
else
// something went wrong
end;
The question has to be asked though: Why are you doing this in an FPC DLL when Delphi already handles the concatenation of strings quite comfortably ? O.o
Related
When dynamically loading a DLL where DoSomething is a method type variable we can do
DoSomething:= GetProcAddress(MyDLLHandle ,'DoSomething');
or
#DoSomething:= GetProcAddress(MyDLLHandle ,'DoSomething');
Both of these seem to behave identically in modern versions of Delphi. My question is - has the # operator always been optional and, if not, in which version of Delphi did it become optional?
The documentation states that #, when used with routine (function/procedure) types returns the entry point of the function with type Pointer. When using the variable directly it naturally has whatever specific type it was declared with but also returns the entry point to the method. GetProcAddress returns a Pointer so I'm assuming that the habit of including # when loading a DLL comes from a time when these mismatched types were not assignment compatible. Is this the case?
Are there any reasonable arguments to prefer either of these styles?
I don't believe that anything has changed since the original versions of Delphi.
The official header translation for GetProcAddress has a return type of FARPROC which is an alias to the untyped Pointer type. Because of this, you can put pretty much anything pointer-esque on the left hand side of the assignment statement, because type checking is suspended when one of the operands is Pointer.
On the other hand, consider this program:
var
Proc: procedure;
Ptr: Pointer;
begin
Ptr := Proc;
end.
This fails to compile with:
E2010 Incompatible types: 'Pointer' and 'procedure, untyped pointer or untyped parameter'
The simple fix is to use the # operator:
var
Proc: procedure;
Ptr: Pointer;
begin
Ptr := #Proc;
end.
It is certainly the case that many Delphi examples use:
#Proc := GetProcAddress(...);
rather than
Proc := GetProcAddress(...);
I rather suspect that there is nothing deep here. Just a case of a choice made by the author of one of the first online articles on the subject propagating throughout history with no good reason. Which reminds me of the terrible Rosetta code example of dynamic loading that tests whether an HMODULE is greater than 32, an erroneous check that can be seen in Stack Overflow questions on a weekly basis!
Should you use # in these circumstances or not? In my view you should not. It doesn't make much difference, and given that why bother with a needless piece of punctuation?
There is one other scenario that I know where you need to use #, and in that case you actually need ##. Consider the following program:
{$APPTYPE CONSOLE}
uses
SysUtils;
var
Proc: procedure;
Ptr: Pointer;
begin
Ptr := #Proc;
Writeln(Format('%p', [Ptr]));
Ptr := ##Proc;
Writeln(Format('%p', [Ptr]));
Readln;
end.
Output
00000000
00423EBC
The first assignment gets the value held in the Proc variable, which because it is a default initialised global, is zero. The second assignment gets the address of the Proc variable. And for that you need ##. It's pretty unusual to need this much indirection, but it tends to crop up when writing code related to dynamic linking.
I am getting this error will debugging a project, which used to be in Delphi 7 and I have been upgrading to Delphi XE2, the same error happens in several methods.
First chance exception at $006DC660. Exception class $C0000005 with message 'access violation at 0x006dc660 read of address 0xffffffff'
This is one of the methods:
PFI = ^TFI;
TFI = record
Id : TToken;
Name : TName;
Parameters : string;
end;
function TListFI.IsIn(S: PChar): PFI;
function SearchName2(Item: PFI):Boolean;
var N1, N2: PChar;
begin
N1:= StrNew(Item^.Name);
N2:= StrNew(S); //Here is the issue
SearchName2:= (StrComp(StrUpper(N1), StrUpper(N2)) = 0);
StrDispose(N1);
StrDispose(N2);
end;
begin
IsIn:= PFI(FirstThat(#SearchName2));
end;
I have googled and I found someone describing a similar problem, and he affirms that when the incremental linker is disabled it works, can someone tell me what and where is it or give some advice to solve this situation.
[EDIT]
Removing the # now gives me the following error in IsIn:= PFI(FirstThat(SearchName2));
E2010 Incompatible types: 'TObject' and 'PFI'
I am adding the FirstThat procedure to see if it may help.
TFuncionColeccion = function (Elemento: TObject): Boolean;
function TColeccion.FirstThat (Rutina: TFuncionColeccion): TObject;
var
i: Integer;
begin
For i:=0 to Count-1 do
if Rutina(Items[i]) then
begin
FirstThat:=Items[i];
exit;
end;
FirstThat:=nil;
end;
It is (and always has been) an error to call local (nested) procedures by pointer, which is clearly what your FirstThat function does. The compiler has to do special things with the stack to call local functions and give them access to the parent scope's variables (S in your code), but the compiler can only know to do those special things when the local function is called directly. The compiler cannot know that the argument to FirstThat will be a local function, so it doesn't include the special code when FirstThat invokes the pointed-to function.
The bottom line is that the stack inside the function doesn't get set up the way it's supposed to, and that means any number of strange symptoms may appear. You'll have to use some other way. Maybe make SearchName2 be a two-argument function, and then write FirstThat to accept S as a parameter that it can forward to the function argument.
You shouldn't need to use the # operator when constructing a function pointer. When you do, the compiler tends to skip type checking, which is what allowed you to pass a local function pointer to FirstThat in the first place. When the function you're passing really matches the required prototype, the compiler will allow you to pass it without the # operator.
You are reporting an access violation in
StrNew(S)
where S is of type PChar. The explanation for that, with probability very close to 1, is that S is not in fact a pointer to null terminated array of WideChar.
In Delphi 7, PChar is an alias for PAnsiChar. That is a pointer to null terminated array of AnsiChar, i.e. 8 bit characters. In Delphi XE2, PChar is an alias for PWideChar, a pointer to null terminated array of WideChar, i.e. 16 bit characters.
It helps to understand what StrNew does. It walks the array until it finds a null character. For 8 bit text that is a single zero byte. For 16 bit text, the null is a zero 16 bit word. Then it allocates a new block of memory of the same length as the input string, and makes a copy into that new memory. The source code is:
function StrNew(const Str: PWideChar): PWideChar;
var
Size: Cardinal;
begin
if Str = nil then Result := nil else
begin
Size := StrLen(Str) + 1;
Result := StrMove(WideStrAlloc(Size), Str, Size);
end;
end;
The only plausible failure mode is that when StrLen walks the array, it attempts an invalid memory read. And that can only happen if your input parameter is invalid. In other words, this must be a programming error on your part.
One possible explanation is that you are in fact passing 8 bit text to this function despite promising to pass 16 bit text. An easy mistake to make, especially if you are not yet fully familiar with the Unicode change. The 8 bit text has a zero terminator, but the byte that follows happens not to be zero. Or the zero byte falls at an odd numbered offset from the start. And then StrNew continues walking the buffer, but now it is off the end and it so happens that it doesn't find a zero word before overrunning into an address that has not been allocated. And that is an access violation.
If that is so then solution will be either:
Change the function's parameter to be of type PAnsiChar, and fix the dubious casting at the call site.
Pass the function 16 bit text as it requires.
In your update you include the address which cannot be read, 0xffffffff. This is -1 in hex. And that would seem to be the most prosaic of errors. Your pointer is completely bogus! Your exact error message can be reproduced with this code: StrNew(PChar(-1)).
I don't have enough information here to tell you why your pointer is bogus. Hopefully you've learnt some debugging and diagnostic techniques that will enable you to solve the problem. At least you now know that the error is in your code.
Assuming that BuscaName2 and SearchName2 are one and the same thing, then you need look no further. Local procedures can only be called from a containing function. As #Rob correctly says, the use of # with procedures is almost always incorrect and is a warning sign of serious problems with your code.
I am experimenting with the ability to dynamically invoke procedures or functions that reside in a function table. The specific application is a DLL that exports a pointer to a function table together with information on the number of arguments and types. The host application then has the ability to interrogate the DLL and call the functions. If they were object methods I could use Rtti to invoke them but they are normal procedures and functions. The DLL has to export normal function pointers not objects because the DLL could be written in any language including C, Delphi etc.
For example, I have a record declared and filled out in a DLL:
TAPI = record
add : function (var a, b : double) : double;
mult : function (var a, b : double) : double;
end;
PAPI = ^TAPI;
I retrieve the pointer to this record, declared as:
apiPtr : PAPI;
Assume I also have access to the names of the procedures, number of arguments and argument types for each entry in the record.
Assume I want to call the add function. The function pointer to add will be:
#apiPtr^.add // I assume this will give me a pointer to the add function
I assume there is no other way other than to use some asm to push the necessary arguments on the stack and retrieve the result?
First question, what is the best calling convention to declare the procedure as, cdecl? Seems easiest for assembling the stack before the call.
Second question, are there any examples online that actually do this? I came across http://www.swissdelphicenter.ch/torry/showcode.php?id=1745 (DynamicDllCall) which is close to what I want but I simplified as below, it now returns a pointer (EAX) to the result:
function DynamicDllCall(proc : pointer; const Parameters: array of Pointer): pointer;
var x, n: Integer;
p: Pointer;
begin
n := High(Parameters);
if n > -1 then begin
x := n;
repeat
p := Parameters[x];
asm
PUSH p
end;
Dec(x);
until x = -1;
end;
asm
CALL proc
MOV p, EAX <- must be changed to "FST result" if return value is double
end;
result := p;
end;
but I can't get it to work, it returns a value for the first parameters instead of the result. Maybe I have the calling convention wrong or maybe I misunderstand how to retrieve the result in EAX.
I call DynamicDllCall as follows:
var proc : pointer;
parameters: array of Pointer;
x, y, z : double;
p : pointer;
begin
x:= 2.3; y := 6.7;
SetLength(parameters, 2);
parameters[0] := #x; parameters[1] := #y;
proc := #apiPtr^.add;
p := DynamicDllCall(proc, Parameters);
z := double (p^);
Any advice gratefully received. I appreciate that some may feel this isn't the way one should go about doing this but I am still curious if it is at least possible.
Update 1 I can confirm that the add function is getting the correct values to do the addition.
Update 2 If I change the signature of add to:
add : function (var a, b, c : double) : double;
and I assign the result to c inside add, then I can retrieve the correct answer in the parameters array (assuming I add one more element to it, 3 instead of 2). The problem therefore is that I misunderstand how values are returned from functions. Can anyone explain how functions return values and how best to retrieve them?
Update 3 I have my answer. I should have guessed. Delphi returns different types via different registers. eg integers return via EAX, double on the other hand returns via ST(0). To copy ST(0) to the result variable I must use "FST result" rather than "MOV p, EAX". I least I now know it is possible in principle to do this. Whether it is a sensible thing to do is another matter I must now think about.
This is an XY problem: You want to do X, and, for whatever reason, you've decided Y is the solution, but you're having trouble making Y work. In your case, X is call external functions via pointers and Y is manually push parameters on the stack. But to accomplish X, you don't really need to do Y.
The expression #apiPtr^.add will not give you a pointer to the function. It will give you a pointer to the add field of the TAPI record. (Since add is the first member of the record, the address of that field will be equal to the address held in apiPtr; in code, Assert(CompareMem(#apiPtr, #apiPtr^.add, SizeOf(Pointer)).) The add field holds a pointer to the function, so if that's what you want, just use apiPtr^.add (and note that the ^ is optional in Delphi).
The best calling convention to use is stdcall. Any language that supports exporting DLL functions will support that calling convention.
You don't need assembler or any other tricky stack manipulation to call your functions. You already know the function's type because you used it to declare add. To call the function pointed to by that field, simply use the same syntax as for calling an ordinary function:
z := apiPtr.add(x, y);
The compiler knows the declared type of the add field, so it will arrange the stack for you.
This is a hard problem to solve. One way to dynamically access methods in a DLL at runtime would be to use a foreign function interface library such as libffi, dyncall or DynaCall(). None of these however have yet been ported to the Delphi environment.
If the application is to interface a set of methods in a DLL together with Rtti information provided by the DLL and expose them to a scripting language such as Python, one option is to write Delphi code that inspects the DLL and writes out a ctypes compatible script which can be loaded into an embedded Python interpreter at runtime. So long as one defines before hand a limited but sufficient set of types that the DLL methods can handle, this is a practical solution.
System.TypInfo.TPropInfo has two function members (at least in D-XE3):
function NameFld: TTypeInfoFieldAccessor; inline;
function Tail: PPropInfo; inline;
I cannot find any documentation for them or any examples of their use. What are they for and how can they be used? (Hope that qualifies as one question.)
The NameFld function returns the name of a property as a TTypeInfoFieldAccessor.
This allows you to do the following:
MyPropertyName:= MyPropInfo.NameFld.ToString;
if (PropInfoA.NameFld = PropInfoB.NameFld) then begin
writeln('property names are the same');
end;
The TTypeInfoFieldAccessor stores the name of a property in a shortstring internally.
Because the NextGen compiler does not support shortstrings, a PByte type is used.
(I guess the author did not want to litter the source with ifdefs and ripped out the PShortstring references)
The input of Tail is a PByte pointing to length field of the internal shortstring.
Here's the source code for tail.
function TTypeInfoFieldAccessor.Tail: PByte;
begin
Result:=
FData //Start of the shortstring
+ FData^ + //Length of the stringData
+ 1; //Add one for the length byte itself
end;
Because shortstrings are not null terminated, you cannot do a simple "loop until the null char is found" kind of loop.
Therefore a loop from start to tail can employed to transfer the shortstring into a normal string.
Strangely enough in the actual RTL sourcecode the length byte is used everywhere instead of the tail function; so it looks like a leftover.
It would have made more sense to include a size function and rip out the tail.
I inherited a Delphi application and I know nothing about object pascal.
It's a BPL that I need to compile into the new version of C++ Builder XE.
When I run a make I get the error:
E2064 left side cannot be assigned to.
I've learned enough obj pascal to know I have a constant that is trying to be assigned a value.
But, apparently, you can over ride this behanvior; essentially turning constants into vars by going into Build options under the Delphi compiler and turning on "Assignable Typed constants".
I did that and I continue to get the same error.
I tried surrounding my code with {$J+} and {$J-} and still it will not compile.
procedure TChunkIDAT.CopyInterlacedRGB8(const Pass: Byte;
Src, Dest, Trans{$IFDEF Store16bits}, Extra{$ENDIF}: pChar );
var
Col: Integer;
begin
{Get first column and enter in loop}
Col := ColumnStart[Pass];
Dest := pChar(Longint(Dest) + Col * 3);
repeat
{Copy this row}
Byte(Dest^) := fOwner.GammaTable[pByte(Longint(Src) + 2)^]; inc(Dest);
Get the error on last line. If I change the const to a var, I then get the error that the declaration differs from the previous declaration but I have no idea where the previous declaration is....
You're type-casting a two-byte thing (Char) into a one-byte thing (Byte). Reading that value is easy to define, but making that value writable is tricky, probably for the same reason the types of formal and actual "var" parameters need to be identical.
Maybe you wanted to type-cast it to a two-byte thing, such as Word. Or maybe you want GammaTable to be an array of Char so you don't have to type-cast at all. Or maybe, if this code was originally written for a Delphi version earlier than 2009, you want those PChar declarations to be PAnsiChar — character types have gotten wider. Another option is to type-cast Dest to PByte, and then dereference the result. That's probably a bad idea, though, because you'll only be overwriting every other byte of the buffer.
Based on the name of the function, it sounds like PChar was never the right data type to use. That type is for character data, but I think this code is dealing with bytes. The correct thing to do is probably to change PChar to PByte, and then you don't need to type-cast Dest at all.
The $J directive is irrelevant; it controls whether the compiler will allow you to assign values to typed constants. You don't have any of those in this code.
The reason is that as of Delphi 2009, Char, PChar, and String are Unicode, and store more than one byte per character.
You should not cast those pointers to bytes, and the compiler prevents you from assigning them if you cast the left side of an assignment to a byte.
This compiles:
procedure CopyInterlacedRGB8(const Pass: Byte; Dest: pAnsiChar); overload;
begin
Byte(Dest^) := Pass;
end;
This doesn't:
procedure CopyInterlacedRGB8(const Pass: Byte; Dest: pChar); overload;
begin
Byte(Dest^) := Pass;
end;
Instead of pChar, you should use pByte, which makes the code simpler:
procedure CopyInterlacedRGB8(const Pass: Byte; Dest: PByte); overload;
begin
Dest^ := Pass;
end;
--jeroen
That looks like you're working with Gustavo Daud's TPngImage library. You don't need that code in an external BPL because it's been included in the RTL since D2009. Remove that unit from the BPL and you should be able to get at the updated version via the PngImage unit.