Saving and Stack Overflows

Question

I'm having a problem saving a vary large database type in Delphi. It contains an array[1..3500] of TItem, which in turn has two arrays[1..50] and [1..20]. I get a stack overflow unless I set the variable as a Pointer and use the GetMem, FreeMem commands below, but then I can't save it. Code is below.

procedure TDatabase.SaveDB;  
var  
 TempDB: ^TSaveDB;  
 K, X: integer;  
 sComment, sTitle, sComposer, sISDN, sCategory: string;  
begin  
GetMem(TempDB, SizeOf(TSaveDB));  

TempDB.CatCount := fCategoryCount;  
TempDB.ItemCount := fItemCount;  

for K := 1 to fCategoryCount do  
 TempDB.Categories[K] := fCategories[K];  

for K := 1 to fItemCount do  
 begin  
  fItems[K].ReturnSet(sTitle, sComposer, sCategory, sISDN, sComment);  
  with TempDB.Items[K] do  
   begin  
    Title := sTitle;  
    Composer := sComposer;  
    Category := sCategory;  
    ISDN := sISDN;  
   end;  

  TempDB.Items[K].Comments[1] := Copy(sComment, 1, 255);  
   Delete(sComment, 1, 255);  
  TempDB.Items[K].Comments[2] := Copy(sComment, 1, 255);  
   Delete(sComment, 1, 255);  
  TempDB.Items[K].Comments[3] := Copy(sComment, 1, 255);  
   Delete(sComment, 1, 255);  
  TempDB.Items[K].Comments[4] := Copy(sComment, 1, 255);  
   Delete(sComment, 1, 255);  

  TempDB.Items[K].KeyWCount := fItems[K].GetKeyCount;  

  for X := 1 to fItems[K].GetKeyCount do  
   TempDB.Items[K].Keywords[X] := fItems[K].GetKeywords(X);  
 end;

AssignFile(DBSave, fSaveName);  
 Rewrite(DBSave);  
  Write(DBSave, TempDB);  
Closefile(dBSave);  

FreeMem(TempDB, sizeof(TSaveDB));  
end;

Answer 1

Your problem is in the "write" statement. Doing things with arbitrary pointers leads to all sorts of strange behavior. You'd have it a lot easier if you rewrote this using a TFileStream instead of the current approach.

Answer 2

Use GetMem or SetLength or TList/TObjectList and write to the file one TSaveDB at a time. Or change the file type and use BlockWrite to write it all at once. Or even better: use TFileStream.

Answer 3

To expand on Mason's answer:

NEVER read or write a pointer, period. It would take a major stroke of luck to get anything reasonable out of doing it and in the real world when you're not just running your program again the odds of success go from infinitesimal to zero.

Rather, you need to read and write what the pointer points to.

Note, also, that any string whose length isn't named in the declaration is a pointer unless you're running in the compatibility mode that makes "string" into "string[255]"--this mode exists only for compatibility with very old code that was written when this was the only strings we had.

Since you appear to be simply writing the whole thing out there's no reason to be playing games with fixed size records. Simply write each field to a stream, write the length of a string before writing the string itself so you can load it back in properly. The file will be smaller and nothing gets truncated.

Also, as he says, use tFileStream. The old format has it's uses for a file of records that is left on disk, there's no reason to use it in a case like this.