How do I call following C function in Pascal/Delphi?

Question

I got following situation:

I want to call a C function from my Pascal program. The C function should fill the passed pointers with values.

Here is the C function:

DLLEXPORT int dpstate_callPluginFunction(const char* plugin, const char* function, bool synchronous, const char* p0, const char* p1, const char* p2, const char* p3, const char* p4, const char* p5, const char* p6, char** o0, char** o1, char** o2, char** o3, char** o4, char** o5, char** o6)

The “p” parameters are the input params and the “o” parameters are the output params. I’m trying to call the function in my Pascal program like so:

C Functioncall declaration:

var dpstate_callPluginFunction: function(plugin, method: PAnsiChar; synchronous: boolean; p0, p1, p2, p3, p4, p5, p6: PAnsiChar; o0, o1, o2, o3, o4, o5, o7: PPAnsiChar): integer; cdecl;

C Functioncall loading:

@dpstate_callPluginFunction:= GetProcAddress(mConnectorLibrary, 'dpstate_callPluginFunction');

Functioncall declaration:

function callPluginFunction(plugin, method: PAnsiChar; synchronous :boolean; param, returnParam:array of PAnsiChar): integer;

Function which should call the function:

procedure TForm1.btn_pluginFunctionClick(Sender: TObject);
var param, returnParam: array of PAnsiChar;
begin
SetLength(param, 7);
SetLength(returnParam, 7);
param[0]:= 'Param1';
param[1]:= 'Param2';
connector.callPluginFunction('dpserverplugin', 'showconfigdialog', true, param, returnParam);

output.Append(returnParam[0]);
output.Append(returnParam[1]);
end;

Function:

function PConnect.callPluginFunction(plugin, method: PAnsiChar; synchronous :boolean; param, returnParam:array of PAnsiChar): integer;
  var i, error: integer;
  var p: array[0..6] of PAnsiChar;
  var o: array[0..6] of PPAnsiChar;
begin
  for i:=0 to 6 do
            p[i]:= param[i]; 
  dpstate_callPluginFunction(plugin, method, synchronous, p[0], p[1], p[2], p[3], p[4], p[5], p[6], @o[0], @o[1], @o[2], @o[3], @o[4], @o[5], @o[6]);

  for i:=0 to 6 do
            if o[i] <> Nil then
                returnParam[i]:= o[i]^; 
end;

My problem is now, that the output "returnParam" always contains "Adress xxxxxx out of bounds". I would be happy with a fast answer :)

I would be happy if you provided enough information for you to receive a fast answer. Here's what you did wrong in the way you asked the question: 1. You asked for a fast answer. 2. You didn't include your Pascal declaration of the imported function. 3. You didn't tell us how the parameters are passed, only their types. Which ones are IN, which ones are OUT etc. Who allocates memory? 4. You didn't tell us what error you see. The bottom line is that you need to make more effort when asking your question. Please improve the question. -1 — David Heffernan
– David Heffernan, Commented Feb 20, 2013 at 9:37
You didn't address item 1 on my original list. Don't ask for a fast answer. When you pass @o[0], that's actually a pointer to PPAnsiChar. But the function expects PPAnsiChar. So that's a mismatch. But we still don't know what the interface contract is. Describe the rules for the char** parameters. In my view you need to simplify the interface considerably for us to help. Try a version which uses just a single char** parameter and see if you can make it work. — David Heffernan
– David Heffernan, Commented Feb 20, 2013 at 10:29

Community · Accepted Answer · 2017-05-23 10:29:40Z

I think you mixed dynamic array and open array. Which are worded the same but in different contexts. http://rvelthuis.de/articles/articles-openarr.html

Try this:

type TPAnsiCharDynArray = array of PAnsiChar;

function callPluginFunction(plugin, method: PAnsiChar; 
         synchronous :boolean; param: array of PAnsiChar;
     out returnParam: TPAnsiCharDynAttay): integer;

Also this line is problematic:

for i:=0 to 6 do
         p[i]:= param[i];

Either you know for sure that your arrays are ALWAYS 0..6 - then there is no point in using dynamic arrays.

type TDLLVectorIndex = 0..6; 
     TDLLPCharArray = array [TDLLVectorIndex] of PAnsiChar;

function callPluginFunction(const plugin, method: PAnsiChar; 
         synchronous :boolean; 
         const param: TDLLPCharArray;
         out returnParam: TDLLPCharArray): integer;

Don't forget argument modifiers const/var/out to document your call contract AND to make values passed by-reference rather than cloned and passed as-value

Or you don't know exact dimensions - then you should not assume 6 magic number and iterate to the real size of array passed in.

for i:=0 to High(param) do
        p[i]:= param[i];

However this does not seems to be your case, but it was very striking that you explicitly declare parameter "god-only-knows-what-length-array" and then use it with hardcoded magic constant.

Again, if you make PAscal bridge, then using PAscal string is better than using C char pointers.

function callPluginFunction(const plugin, method: AnsiString; 
         synchronous :boolean; 
         .....

Track down what is C bool type. Is it really single-byte boolean ? Or is it some Windows bools, that can take 1,2 or even 4 bytes ? it also sometimes has slight binary incompatibilities of what is true actually, +1 or -1.

But without that bool uncertainty, you fn declaration after applying argument qualifiers would better look like.

type fn_dpstate_callPluginFunction = 
   function(const plugin, method: PAnsiChar; synchronous: boolean;
   const  p0, p1, p2, p3, p4, p5, p6: PAnsiChar; 
   var o0, o1, o2, o3, o4, o5, o7: PAnsiChar): integer; cdecl;
var dpstate_callPluginFunction: fn_dpstate_callPluginFunction;

In C++ passing parameters by-ref is not popular, and in C it maybe does not exist. But in average Pascal code passing parameters by-ref considered to be more usual and more safe, than passing pointers.

This comes to bridge re-coding:

type TDLLVectorIndex = 0..6; 
     TDLLPCharArray = array [TDLLVectorIndex] of PAnsiChar;

function PConnect.callPluginFunction(const plugin, method: AnsiString; 
         synchronous :boolean; 
         const p: TDLLPCharArray;
         out o: TDLLPCharArray): integer;

  var Error: integer;
begin
  Error :=
    dpstate_callPluginFunction( PAnsiChar(plugin), PAnsiChar(method), synchronous, 
        p[0], p[1], p[2], p[3], p[4], p[5], p[6],
        @o[0], @o[1], @o[2], @o[3], @o[4], @o[5], @o[6]);
  Result := Error + 10;
// or something like that - you did had the reason
// to declare the var and declare function return type afterall
end;

procedure TForm1.btn_pluginFunctionClick(Sender: TObject);
var param, returnParam: TDLLPCharArray;
begin
  FillChar(param, 0, SizeOf(TDLLPCharArray)); // maybe redundant, but to be on safe side
  param[0]:= 'Param1';
  param[1]:= 'Param2';
  connector.callPluginFunction('dpserverplugin', 'showconfigdialog', true, param, returnParam);
....

PS. Last but not least, does your connector have any real member variables inside ? i mean non-static ones, those that be different among several connector instances ? If not you'd probably make callPluginFunction a class function and not create instances of connector class at all.

  param[0]:= 'Param1';
  param[1]:= 'Param2';
  PConnect.callPluginFunction('dpserverplugin', 'showconfigdialog', true, param, returnParam);

PPS. David is very right about contract. Especially about "Who allocates memory?" Even if the DLL just returns string constants, there is a neat gotcha: Load DLL, get pointer to string, Unload DLL, try to use the pointer - access violation. So while the bridge above is what i believe the correct interface translation, it is not necessarily working in larger picture, due to the issues pointed by David.

PPPS. array datatype declaration may seem edundant, but it overcomes what i consider a stupid limitation of Delphi novadays: why two aliases to "array of string" treated differently?

PPPPS. using out parameters is controversial. For example David thinks that since Delphi does not actually implements them for most of data types, one should almost always use var parameters instead. Personally i think that using out parameters is good for documenting your contract and is good for compatibility with FPC and other compilers.

Marco van de Voort · Accepted Answer · 2013-02-20 12:21:12Z

2

Another small gotcha is the "boolean" type. Pascal booleans typically assume false=0 true=1 and rest undefined, pretty much like GTK gbooleans.

C booleans generally assume 0=false, anything else= true.

Recent Free Pascals have a complete set of both in various sizes. Boolean8..boolean64 for the pascal side, bytebool,wordbool,longbool etc for the C side.

answered Feb 20, 2013 at 12:21

Marco van de Voort

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4 Comments

Arioch 'The Over a year ago

i mentioned that. Though "rest undefined" seems a bit too broad state. But yes, Windows/OLE Bool typestend to have -1 as canonical true, while i386 asm and PAscal tend to have +1 as canonical true. Yet in thpoery they should treat as true any non-zero number...

Marco van de Voort Over a year ago

No that isn't overly broad. Pascal treats exactly 1 as true.

Arioch 'The Over a year ago

But does it treat 2,3,4... as undefined ?

Marco van de Voort Over a year ago

yes. It depends on the asm generated (and thus on arch and optimizationlevel) what the behaviour is

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