Binary files._(re)

Mon, 7 Apr 1997 17:06:36 +0200 (MET DST)

According to Berend De Schouwer:
> 
> I'd like to read bytes from a file, so I looked at some of the examples.
> I found an assign(file of byte...).  Cool.  It doesn't seem to
> work to great tho... How big does it read?  Are you sure its just 1 byte?

Yes.  See below for an example program which I just tested to work
- with the alpha version gpc-970401.

> How big is a byte anyway?

8 bits.  :-)

> Looking at the examples and the info file,
> I get confused.  Is an integer 16bit or 32bit?

32.

> What happens with
> type byte=0..255;

It will take 4 bytes.  :-(  Sorry - this is a known bug.

>  I tried defining them like in the info file (using
> __short__, etc.) but it doesn't compile.  When I use the example's stuff
> it does compile.

See below for something which does compile.

> Hmmm... I am converting some stuff from borland, and some from fpk,
> so I would like to know how to get 8, 16 and 32 bit signed and unsinged
> integers.  I don't mind doing a search/replace on the source, but I do
> need to know how big each thing is.

The alpha version gpc-970401 has the following built-in types:

    Byte		unsigned	 8 bits
    ByteInt		signed		 8 bits
    ShortWord		unsigned	16 bits
    ShortInt		signed		16 bits
    Word		unsigned	32 bits
    Integer		signed		32 bits
    LongWord		unsigned	64 bits
    LongInt		signed		64 bits

    Single   = ShortReal	32 Bits
    Double   = Real		64 Bits
    Extended = LongReal		80 Bits on the PC, maybe 128 elsewhere

However there is one known problem with these types:  You cannot
`write' or `writeln' them to text file.  (Binary files are okay.)

If you

    writeln ( a [ 7 ] );

with `a' being an array of bytes, you might get some very strange
result because `writeln' outputs the integer consisting of `a [ 7 ]'
and the following three bytes.

To work around, cast these types to their base type:

    writeln ( Integer ( a [ 7 ] ) );

(The same holds for an `Extended' which you must cast to `Real' in
order to `writeln' it.)

> AND how to apply them properly to files ;)

I hope the example below helps you.
If you want to upgrade to gpc-970401, the URL is

    ftp://agnes.dida.physik.uni-essen.de/gnu-pascal/alpha/

Greetings,

    Peter

  Dipl.-Phys. Peter Gerwinski, Essen, Germany, free physicist and programmer
peter.gerwinski@uni-essen.de - http://home.pages.de/~peter.gerwinski/ [970201]
 maintainer GNU Pascal [970401] - http://home.pages.de/~gnu-pascal/ [970125]

8< ---- test.pas -------------------------------------------------------------

{OK}  (* This program will read its own source and output 'OK'. *)

Program Test;

Type
  ByteInt = __byte__ Integer;
  Byte = __unsigned__ ByteInt;
  ByteFile = file of Byte;

Var
  F: ByteFile;
  C: Char;


Procedure Assign ( Var T: ByteFile; Name: String );

Var
  B: BindingType;

begin (* Assign *)
  unbind ( T );
  B:= binding ( T );
  B.Name:= Name;
  bind ( T, B );
  B:= binding ( T );
end (* Assign *);


begin
  Assign ( F, 'test.pas' );
  reset ( F );
  read ( F, C );
  read ( F, C );
  write ( C );
  read ( F, C );
  writeln ( C );
  close ( F );
end.


Peter Gerwinski (peter@agnes.dida.physik.uni-essen.de)