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Set file size without writing in it

Posted on 2004-03-24
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Last Modified: 2010-04-04
It is possible to set file size when using TFileStream:

var f : tfilestream;
begin
f := tfilestream.create('somefile.dat',fmcreate or fmopenwrite);
f.size := $1000000;
f.free
end;

Is it possible to do the same thing without using TFileStream? Example with using untyped file would be the best.
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Question by:ZhaawZ
7 Comments
 
LVL 26

Expert Comment

by:Russell Libby
ID: 10672405
Yes, you can set the file size, even beyond what is actually there...

Some code from a PAQ that I participated in. You can modify the function to just take a file handle, the real key to this are the calls to SetFilePointer / SetEndOfFile.

Hope this helps,
Russell


function HiLong(Value: Int64): LongWord;
begin
  result:=((Value shr 32) and $FFFFFFFF);
end;

function LoLong(Value: Int64): LongWord;
begin
  result:=LongWord(Value);
end;

function CreateJunkFile(FileName: String; Size: Int64): Boolean;
var  hJunk:      THandle;
     dwLow:      Integer;
     dwHigh:     Integer;
begin

  // Get high/low
  dwLow:=LoLong(Size);
  dwHigh:=HiLong(Size);

  // Create the new junk file
  hJunk:=CreateFile(PChar(FileName), GENERIC_WRITE, 0, nil, CREATE_NEW, FILE_ATTRIBUTE_NORMAL, 0);
  if (hJunk <> INVALID_HANDLE_VALUE) then
  begin
     // Set the file pointer
     if (SetFilePointer(hJunk, dwLow, @dwHigh, FILE_BEGIN) = LoLong(Size)) then
        // Set end of file
        result:=SetEndOfFile(hJunk)
     else
        // Failed to set file pointer
        result:=False;
     // Close the file
     CloseHandle(hJunk);
  end
  else
     // Failure
     result:=False;

end;
0
 
LVL 11

Author Comment

by:ZhaawZ
ID: 10672735
mm .. found it ..

var
  f : file;
begin
assignfile(f,'d:\somefile.dat');
rewrite(f,1);
seek(f,$10000000);
truncate(f);
closefile(f);
end;

rllibby, thanx for the help, but it was without using untyped files ;)

No help needed anymore...
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LVL 26

Expert Comment

by:Russell Libby
ID: 10672742

What do you thing seek and truncate call behind the scenes???

Russell



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LVL 11

Author Comment

by:ZhaawZ
ID: 10672920
Is there anything wrong?
0
 
LVL 1

Expert Comment

by:IrishFBall32
ID: 10695371
It appears that you second example would work just fine, in fact i once wrote a similar program to test that my computer could actually address the full capacity of my harddrive.

The only thing to keep in mind though, if you take a file who's size is say 1MB, and change its size to 10MB there will be 9MB worth of "garbage" in the file from whatever was on the spots on the drive that the new size takes up. In other words it is similar to an un-initialized variable being full of the garbage from whatever was in RAM before it.
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LVL 11

Author Comment

by:ZhaawZ
ID: 10695798
Yes, I know ;) Actually I just reserve the necessary space on disk before writing the file and then write data to it.
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