I have a few questions about fixed record files. I have the following:

Type
   DrawCMD = Record
                GC: Byte;
                PX,PY,PZ,GX,GY,GZ,GI,GJ,R,SA,EA:Double;
             End;
Var
  DrawFile                    : File Of DrawCMD;
  Drawinfo                    : DrawCMD;

I wish to get to the end of the file and read the last record. Currently I am doing it like this:

         Reset(DrawFile);
         while not eof(DrawFile) do
            begin
               Read(DrawFile,DrawInfo);
            End;

When this finishes Drawinfo contains the last record of the file. But I'm wondering if there is a better way since it's a file of fixed records.

I'm also wondering if there is a way to insert a record at the beginning of the file, or in the middle of the file without doing something like copying the entire file to a temp file, then re-writing the entire new file in the desired order?

James

Hello,

I'll try something like

If filesize(drawfile)>0 then
    Seek(drawfile, sizeof(drawfile)-1)
else
    "empty file";

For portability of you file and software, I recommand you to use a "packed record" with an internal forced word-align of the double data. If size matters, no need to align.

Type
      DrawCMD = Record
                   GC: Byte;
                   ______: byte; { no use axcept to force align on words for speed on certain systems }
                   PX,PY,PZ,GX,GY,GZ,GI,GJ,R,SA,EA:Double;
                End;

This can also be uses for memory access, as on some processors, word aligning data can save some cpu cycles with reference to non aligned data for data longer than one byte/char.

Chers, Ched

I prefer performance over size, The only reason 'GC' was a byte was because it didn't really need to be a word, however it wouldn't hut to be a word anyway, so I could just use this:

Type
      DrawCMD = Record
                   GC: Word;
                   PX,PY,PZ,GX,GY,GZ,GI,GJ,R,SA,EA:Double;
                End;

I don't understand "packed record" could you elaborate?

James

Hi,
 
"packed record" is for compatibility. Its guaranteed that inner alignment and size of record will be always the same and will not change in future versions of compiler, unlike the plain "record".  
https://www.freepascal.org/docs-html/ref/refsu15.html  
V.
______________________________________________________________

I don't understand "packed record" could you elaborate?

James

Thanks for the link. For best compatibility should I make it:

Type
      DrawCMD = Packed Record
                   GC: Word;
                   PX,PY,PZ,GX,GY,GZ,GI,GJ,R,SA,EA:Double;
                End;

Or

Type
     {$PackRecords 2}
      DrawCMD =  Record
                   GC: Word;
                   PX,PY,PZ,GX,GY,GZ,GI,GJ,R,SA,EA:Double;
                End;
     {$PackRecords default}

Or am I not understanding this correctly?

James