[fpc-pascal] Using TFileStream class

[Howard Page-Clark]

On Wed, 4 Mar 2009 20:42:14 +0100
Aurélie de LUCA <aureliedeluca at gmail.com> wrote:

> Hy everybody,
> 
> I'm trying to use the TFileStream class to read a file with a record,
> and I obtain this message when I execute my application : Access
> violation. You can find my code in the following :
> 
> type
>     DmatLine= record
>           ID1: string;
>           ID2: string;
>           C: integer;//classe différence 1 if reactions aren't of same
> classe and 0, on the contrary.
>           Euclide: double;//Euclidian distance between two reactions.
>           Tanimoto: double;//Tanimoto coefficient between two
> reactions. Dice: double;//Dice coefficient between two reactions.
>     end;
> 
> ....
> 
> procedure TReadDmat.ReadDmat(input: string; nbCpd : integer);
> var
>    i, j, k, l, m, sizeOfArray, test: integer;
>    DmatFile: TFileStream;
>    LineRecord1, LineRecord2: DmatLine;
> begin
>      if (FileExists(input) )then
>      begin
>           DmatFile:=TFileStream.Create(input, fmOpenRead);
>           try
>              DmatFile.Position := 0;
>              while (DmatFile.position < DmatFile.size) {NB ^ see
> above..} do begin
>                    with LineRecord1 do
>                    begin
>                         DmatFile.Read(ID1, sizeOf(string));
>                         DmatFile.Read(ID2, sizeOf(string));
(snip)

One problem is your use of long strings in a record structure. Long
strings can be any size, and the size can change at runtime.
Longstring variables are pointers to dynamically allocated storage, so 
Sizeof(string) will always return 4 on 32bit systems.
If DMatLine.ID1 and DmatLine.ID2 are <= 255 characters, you could use
shortstrings, declared e. g. as
type
 string40 = string[40];

DmatLine= record
>           ID1: string40;
>           ID2: string40;
>           C: integer;
		...
	   end;

Then the compiler knows exactly how big each record is, and does the
TFileStream.


Howard