[SOLVED ]Detect if a file is a typed file
lazarus / 21-General

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torbente · 2021-01-04 06:22

Hello;
i have the following

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Mydata=
Packed Record     Data1: string[15];     Data2: int64;     Data3:
boolean;     end;   myfile: file of mydata;
How i can detect if a file is of this type when i open it?
I used this approach, but txt files are opened anyway

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
assignfile(myfile,Filename);tryreset(myfile)...except on E:Exception
do   ...end;

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Peter H · 2021-01-04 08:07

I can only think about a heuristic approach.
Data1 must probably be plain text.
Data3 must be 0 or 1.
This must be true for all records in the file.
Filesize must be a multiple of the record size.

It would be better to start the file with a magic number, maybe a
special record that has unique content.

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ASerge · 2021-01-04 09:18

Quote from: torbente on January 04, 2021, 07:22:48 amHow i can detect
if a file is of this type when i open it?
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i {$MODE
OBJFPC}{$APPTYPE CONSOLE}{$LONGSTRINGS ON} uses SysUtils; type 
TMyData = packed record    Data1: string[15];    Data2: Int64;   
Data3: Boolean8;  end;   TMyDataFile = file of TMyData; const 
CItemsToStoreInFile = 10; var  CHeader: TMyData = (Data1:
'MyFileSignature'; Data2: $8BADF00DDEFEC8ED; Data3: False); procedure
MakeDataFile(const FileName: string);var  F: TMyDataFile;  R:
TMyData;  i: SizeInt;begin  AssignFile(F, FileName);  Rewrite(F); 
try    Write(F, CHeader);    R.Data1 := 'Data1';    R.Data2 := 2;   
R.Data3 := True;    for i := 1 to CItemsToStoreInFile - 1 do     
Write(F, R);  finally    CloseFile(F);  end;end; procedure
MakeTextFile(const FileName: string; FileSize: SizeInt);var  F:
TextFile;  C: AnsiChar;  i: SizeInt;begin  AssignFile(F, FileName); 
Rewrite(F);  try    for i := 1 to FileSize do    begin      C :=
Chr(Random(64) + Ord(' '));      Write(F, C);    end;  finally   
CloseFile(F);  end;end; function TextFileIsOpenedAsDataFile(const
FileName: string; CheckHeader: Boolean): Boolean;var  F: TMyDataFile; 
R: TMyData;begin  AssignFile(F, FileName);  try    Reset(F);    try  
  Result := True;      if CheckHeader then      begin        Read(F,
R);        Result := (R.Data1 = CHeader.Data1) and (R.Data2 =
CHeader.Data2) and          (R.Data3 = CHeader.Data3);      end;   
finally      Close(F);    end;  except    on E: Exception do    begin 
    Writeln('Error: ', E.Message);      Result := False;    end; 
end;end; const  CDataFileName = 'file.data';  CTextFileName =
'file.txt'; begin  MakeDataFile(CDataFileName); 
MakeTextFile(CTextFileName, SizeOf(TMyData) * CItemsToStoreInFile); 
if TextFileIsOpenedAsDataFile(CTextFileName, False) then   
Writeln('It is possible')  else    Writeln('Strange');  if
TextFileIsOpenedAsDataFile(CTextFileName, True) then    Writeln('Very
strange')  else    Writeln('Yes!');  Readln;end.

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Warfley · 2021-01-04 13:45

Well, you can't, bytes are bytes.

Besides a simple heuristic as suggested above, you could check if the
filesize is a multiple of the record size but thats about it.

Usually thats why most custom filetypes have a header containing
custom information like a magic number.
The reading of the file is than as follows:
1. Read the header
2. Check if the magic number matches
3. check additional header information for parsing (e.g. version
number)
4.a. If 2 and 3 do not match what is expected (unknown version number,
wrong magic number), throw an error
4.b. Otherwise read file in the format indicated by the header

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Bart · 2021-01-04 14:03

With one of my old applications I used the following approach:
The filesize must be N times the sizeof(recordstructure), where N is
an integer.Like ASerge suggested, the firs record I write to that file
was a special record. Just check for that if you read the file.
In the text field of the header record, you can put some special
characters like #0, ^D and ^Z.
This has the advantage that when you do "type filename" on the
commandprompt, it will stop displaying information after it encounterd
those characters: ^Z is EOF for text files, ^D maybe has the same
meaning on *nix (I'm not really sure).
It also prevents writing such a header using a simple text-editor
(notpead and the like).

Bart

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torbente · 2021-01-05 16:45

Thanks. I will use the magic number method.