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passing (short) strings to a DLL

Can someone tell me how i can pass a couple of short strings and a record type containing a few more short strings from my delphi program to my delphi DLL ?

i know all about using ShareMem and DelphiMM DLLs with long strings.

i want to pass shortstrings (and preferably as value parameters)

The following is not working (an under statement: it crashes win 95 quite often !)
record type var is not in these funcs.

Any comments ?
in the DLL:

function AddCRToLF(Caller:HWND; infile, outfile: ShortString):boolean;

//do all i want

exports AddCRToLF index 1;

in the calling program

function calldll(caller:HWND; s1,s2:shortstring);

var  h : THandle;
     f : function(Caller:HWND; infile, outfile: ShortString):boolean;stdcall;

h := LoadLibrary('MyDLL.DLL');
if h <> 0 then
   @f := GetProcAddress(h, 'AddCRToLF');
   if @f <> nil then
      AddCRToLF := f(Caller, s1, s2);//<======crashes here !
   FreeLibrary(h);      // Free the library...
1 Solution

I tried the following, and it works fine.

In exe:
  TRec = record
    RecInt: Integer;
    RecDescrip: ShortString;

procedure DoSomething(s: ShortString; ARec: TRec); external 'TestDll.dll';

procedure TForm1.Button1Click(Sender: TObject);
  Rec: TRec;
  Rec.RecInt := 5;
  Rec.RecDescrip := 'Record Description';
  DoSomething('Hello', Rec);

In dll:
  TRec = record
    RecInt: Integer;
    RecDescrip: ShortString;

procedure DoSomething(s: ShortString; ARec: TRec);


I am using Delphi 3.

I noticed in your example that when you declared the functon in your DLL you did NOT use "stdcall", but when you declared the function type in your EXE you did use "stdcall".  You should use it or NOT use it in both places.

As JB mentioned, I also don't think that you should declare the function as stdcall!!
All the above +
Might also want to modify the function in the DLL as:

function AddCRToLF(Caller:HWND; infile, outfile: ShortString):boolean;  EXPORT;

to make it callable outside of the .DLL. Also check to see if your using SHAREMEM in your uses clause.

Good luck with your project!
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You may notice that in my original comment (!!) I put "exports DoSomething" at the bottom of my DLL section, which allows the call to be seen outside of the DLL.  Notice too that in the original question, DelphiOnly IS exporting AddCRToLF!  

One more thing:  You do NOT need ShareMem in your uses clause when passing ShortStrings, which is what DelphiOnly specifically mentioned.

DelphiOnlyAuthor Commented:
Jim Bob,
Thanks for pointing out my stupid mistake ('stdcall' in one proc).

yes after removing the stdcall, i can get the string to be passed on. But i'm still having problems with getting my record variable. i'll follow up with a detailed comment.

But a housekeeping comment first :-)
Pegasus, Sharemem/DelphiMM are needed only for large strings and JBs comment is the answer.

i want to give credit to to JimBob for the answer: so yours get rejected.

now how do i go about declaring the comment as an answer ?

DelphiOnlyAuthor Commented:
for your comments, guys. (i assume :-)

My record var is TTeam defined as follows:
TName   = Record
          first : string;
          last  : string;
TScore  = Record
          GP,pp,sh,gw,gt,pim,shots: integer;
          ppgame  : real;
TPlayer = Record
          name  : TName;
          score : TScore;
TTeam   = Record
          ID       : string[03];
          Name     : string;
          FileName : string;
          HTTPAddr : string;
          Player   : array[0..35] of TPlayer ;

in the DLL

function ParseFile(Caller:HWND; var T:TTeam):boolean;

note : T is a var parameter: i'm passing in a fielname with data
and having the DLL to fill in the array player[0..35];

in program:
     f : function(Caller:HWND; var team:TTeam):boolean;

Well, everything works fine the first time i call the function.
The second call.

Would any of you like to comment on passing such a "large" record via stack ? i guess, since var params only pass the address of a variable, it is ok.

Is there a better way to do this ? (Is  passing the address of the variable and dereferencing in DLL is better ? Will it work)

DelphiOnlyAuthor Commented:
btw, another weird thing is that if i do a freelibrary, then the record variable loses all it's string values !!

vat T is declared in main proc.
i call f(handle, T)
f return values for T.Name.last (say 'john')

now if i free library then T.Name.last becomes null !
Why ? ... why ?..... Why ???????

(talking to myself)
looks like i'm asking too many things. i should increase the points, oops i have got only 10 pts available. wth, something is better than nothing. so points increased!
DelphiOnlyAuthor Commented:
Another observation:

if i remove the 'var' in declaring the function then everything works fine (well, except ofcourse i don't get back the reqd. data).

Also if i use a seperate record type variable, each time, it works. i.e

This works:

f(Caller, team1);
f(Caller, team2);

but this doesn't !
f(Caller, team1);
f(Caller, team1); <====== crash here !

Looks like passing as var parameter fiddles with the memory space somehow ??

(team1, team2 are type TTeam and passed as var parameters)

Does this ring any bells ?
Hehe...  Change all your "string"s (in TName you have First and Last, & in TTeam you have Name, FileName and HTTPAddr) to "ShortString"s and it should be fine.  (You can leave "ID : string[03]", because by fixing the length to 3 it's automatically a Short String.)

I'm making this an answer because of the "stdcall" thing, but if you have any more questions...

DelphiOnlyAuthor Commented:

actually i have turned off the huge strings by using
{$LONGSTRINGS OFF }. that should work i guess.

and remember that the first time i call my func in the dll, i get all proper values.

i consider the stdcall thing an "excellent" point by the way, but i'll grade your answers once i get some more insight into this.
DelphiOnlyAuthor Commented:
OK. i learnt a couple of things :-)

1. compiler options {$H-} are local ! (holy grace, they were global in TP)

2.  {$A-} is needed when passing records to DLLs

Thx for all your comments.
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