Re: [Dev-C++] Some extra bytes written to file?

[Ioannis V. <no...@ho...>]

-------Original Message-------=0D
 =0D
From: "J=F6rgen"=0D
Date: =D0=DD=EC=F0=F4=E7, 6 =CC=DC=F1=F4=E9=EF=F2 2003 5:00:15 =EC=EC=0D
To: dev...@li...=0D
Subject: [Dev-C++] Some extra bytes written to file?=0D
 =0D
Hi.=0D
 =0D
This problem puzzles me. I have the following data.=0D
-4 0 0 4 1 0 4 2 0 3 2 27 1 3 108 1 3 135 1 3 189 1 3 351 1 4 0 8 5 0 4 6=
 0
4 7 0 4 8 0 4 9=0D
 0 3 9 27 1 10 27 2 10 36 1 10 54 1 11 108 1 11 189 1=0D
 =0D
I'm not to familiar with reg.exp, but I guess that the reg.exp. for the d=
ata
would be =0D
result [value1 value2 value3]*=0D
 =0D
where =0D
  result is an integer but is always in the range -64 to 64=0D
  value1 is an integer but is always in the range 0 to 11=0D
  value2 is an integer but is always in the range 0 to 65536=0D
  value3 is an integer but is always in the range 0 to 4=0D
 =0D
let's say that we call [value1 value2 value3] for 'value' and 'nbrOfValue=
s'
the number of sets=0D
 of 'value'. In this case nbrOfvalues =3D 20=0D
 =0D
When I write the data to file, using ofstream foo("foofile") I do it in t=
he
following way=0D
 =0D
 =0D
=3D=3D> Since it is C++, why don't you use bitset<> elegantly and efficie=
ntly?=0D
 =0D
 =0D
 =0D
if(result < 0)=0D
  foo << '-';=0D
else=0D
  foo << '+';=0D
foo << ((unsigned char) (abs(result) & 0x000000FF));=0D
 =0D
for(int i=3D0; i<numberOfValues; i++){=0D
  foo << ((unsigned char) (value[i][0] & 0x000000FF));=0D
  foo << ((unsigned char) ((value[i][1] & 0x0000FF00) >> 8));=0D
  foo << ((unsigned char) (value[i][1] & 0x000000FF));=0D
  foo << ((unsigned char) (value[i][2] & 0x000000FF));=0D
 =0D
 =0D
=3D=3D> Since value2 is [0,65536]  use unsigned long. E.g.=0D
 =0D
for(int i=3D0; i<numberOfValues; i++){=0D
  foo << static_cast<unsigned long> (value[i][0] & 0x000000FF);=0D
  foo << static_cast<unsigned long>(value[i][1] & 0x0000FF00) >> 8);=0D
  foo << static_cast<unsigned long> (value[i][1] & 0x000000FF);=0D
  foo << static_cast<unsigned long> (value[i][2] & 0x000000FF);=0D
 =0D
 =0D
 =0D
 =0D
}=0D
 =0D
The problem comes when I read the data from file. If I just reverse the
procedure and prints=0D
 the decoded data to screen I get=0D
-4 0 0 4 1 0 4 2 0 3 2 27 1 3 108 1 3 135 1 3 189 1 3 351 1 4 0 8 5 0 4 6=
 0
4 7 0 4 8 0 4 9 0=0D
 3 9 27 1 10 27 2 10 36 1 10 54 1 11 108 1 11 189 1 0 80 114=0D
 =0D
It looks just the same as the original data except for the last three
numbers (0 80 114).=0D
 =0D
When looking at the filesize it shows that it is 85 bytes, but the data
written is only 82 bytes=0D
 =0D
For debugging purposes, instead of writing directly to file, I first stor=
e
everything in a=0D
string s. That is, replacing foo << ... with s +=3D ... Finally I write i=
t to
file with foo << s;=0D
 =0D
When I print the string s to screen in hex-mode=0D
 =0D
cout.setf( ios_base::hex, ios_base::basefield );=0D
for(int i=3D0; i<s.size(); i++)=0D
  cout << " " << s[i];=0D
 =0D
it gives the following output=0D
2d 4 0 0 0 4 1 0 0 4 2 0 0 3 2 0 1b 1 3 0 6c 1 3 0 87 1 3 0 bd 1 3 1 5f 1=
 4
0 0 8 5 0 0 4 6 0=0D
 0 4 7 0 0 4 8 0 0 4 9 0 0 3 9 0 1b 1 a 0 1b 2 a 0 24 1 a 0 36 1 b 0 6c 1=
 b
0 bd 1=0D
and it is the way it should be =0D
 =0D
But when I look at the datafile in a hexeditor I see=0D
2d 4 0 0 0 4 1 0 0 4 2 0 0 3 2 0 1b 1 3 0 6c 1 3 0 87 1 3 0 bd 1 3 1 5f 1=
 4
0 0 8 5 0 0 4 6 0=0D
0 4 7 0 0 4 8 0 0 4 9 0 0 3 9 0 1b 1 d a 0 1b 2 d a 0 24 1 d a 0 36 1 b 0=
 6c
1 b 0 bd 1=0D
 =0D
An extra byte 'd' emerges before every one of the three "value1"-'a' byte=
s =0D
(d a 0 1b 2 , d a 0 24 1 , d a 0 36 1).=0D
 =0D
Where did these 'd' bytes come from?=0D
 =0D
 =0D
=3D=3D> Truncation. You "bitwise" a signed value (casted to unsigned type=
) and
its bit, marking its signess floats around. Also keep in mind that the
binary representation of negative values is radically different from the
positive ones.=0D
 =0D
 =0D
Why are the only occuring in front of the 'a' byte?=0D
How come that when I print the data to screen it does not mess up the val=
ues
but instead=0D
appends (0 80 114) at the end?=0D
 =0D
 =0D
=3D=3D> Hmmm.=0D
 =0D
=0D
 =0D
Ioannis Vranos=0D
 =0D
* Programming pages: http://www.noicys.freeurl.com=0D
* Alternative URL: http://run.to/noicys=20