File I/o questions

[Neo]

I am writing an encryption program. I am using fopen , fgetc and fput c to
open, read a block of data and write a block of encrypted.

Thus all the blocks are overwritten with the encrypted.
I use win 2000 / Xp and file sys FAT and NTFS.
My program will add a 512 byte header to the beginning.. so the end file
will be (original size + 512) rounded to the nearest 128 bit multiple

My question are :

*Is it guaranteed that the functions mentioned above will over write the
same sectors in the hard disk, and not wrie to a new block and change the
allocaton table?
*Also are there any other functions i can use to perform direct changes to
blocks of a file diretly onto the hard disk.?

*Are there any tools on the net that will tell me the exact ?

[Thomas J. Gritzan]

Neo schrieb:

I am writing an encryption program. I am using fopen , fgetc and fput c to
open, read a block of data and write a block of encrypted.

Thus all the blocks are overwritten with the encrypted.
I use win 2000 / Xp and file sys FAT and NTFS.
My program will add a 512 byte header to the beginning.. so the end file
will be (original size + 512) rounded to the nearest 128 bit multiple

My question are :

*Is it guaranteed that the functions mentioned above will over write the
same sectors in the hard disk, and not wrie to a new block and change the
allocaton table?
No. C++ does not know anything about sectors or hard disks, so C++ does
not guarantee something like that. You should ask in a Newsgroup about
your OS (Windows).

(And I think it is not guaranteed by Windows either.)

*Also are there any other functions i can use to perform direct changes to
blocks of a file diretly onto the hard disk.?

There may be platform specific functions, known to the people in the
right newsgroups.

--
Thomas

[osmium]

"Neo" writes:

I am writing an encryption program. I am using fopen , fgetc and fput c to
open, read a block of data and write a block of encrypted.

Thus all the blocks are overwritten with the encrypted.
I use win 2000 / Xp and file sys FAT and NTFS.
My program will add a 512 byte header to the beginning.. so the end file
will be (original size + 512) rounded to the nearest 128 bit multiple

My question are :

*Is it guaranteed that the functions mentioned above will over write the
same sectors in the hard disk, and not wrie to a new block and change the
allocaton table?
*Also are there any other functions i can use to perform direct changes to
blocks of a file diretly onto the hard disk.?

*Are there any tools on the net that will tell me the exact ?

You alluded to what is basically C code in a C++ group. I would open the
file for input and output in binary mode and use istream::read() to capture
the data. Immediately write all zeros to the entire file using
ostream::write().. Then compute your results and store them in the same
file. If the system crashes while your program is running, there is still
no problem. This is not a place to save a few milliseconds. I can't
foresee any problems in the world of actual computers as opposed to some
gedanken experiment.

I assume you are using C code for I/O because it is easier to comprehend.
Bite the bullet and learn iostreams, it's worth the effort.

[Neo]

"osmium" <r1********@comcast.net> wrote in
news:44***********@individual.net:

"Neo" writes:

I am writing an encryption program. I am using fopen , fgetc and fput

I assume you are using C code for I/O because it is easier to
comprehend. Bite the bullet and learn iostreams, it's worth the
effort.

Oh i know them alright :)... my basic question is will the same sectors in
the HD be used.. becoz i dont wnt the encrypted stuff elsewhere on
unallocated space.. i want the same sectors to be used for the encrypted
file.

[Jacek Dziedzic]

Neo wrote:

I am writing an encryption program. I am using fopen , fgetc and fput c to
open, read a block of data and write a block of encrypted.

Thus all the blocks are overwritten with the encrypted.
What if there is power shortage when you are in the middle
of doing that? Your file is screwed.
I use win 2000 / Xp and file sys FAT and NTFS.
My program will add a 512 byte header to the beginning.. so the end file
will be (original size + 512) rounded to the nearest 128 bit multiple

My question are :

*Is it guaranteed that the functions mentioned above will over write the
same sectors in the hard disk, and not wrie to a new block and change the
allocaton table?
The answer is easily 'no'. Imagine an original file that has
a size of exactly n*sector_size. If you extend it by 512 bytes
it will occupy at least (n+1) sectors. Therefore you have a
guarantee that a new block will be written and hence the sectors
will not be the same.
*Also are there any other functions i can use to perform direct changes to
blocks of a file diretly onto the hard disk.?
Possibly, but not in C++ as the standard defines it.
*Are there any tools on the net that will tell me the exact ?

Possibly, but this is beyond the scope of this newsgroup.

- J.