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Full Discussion: C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)
Operating Systems Linux C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk) Post 302490908 by shen747 on Wednesday 26th of January 2011 05:23:56 AM
Old 01-26-2011
Question C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)
Hi all,

I'm kind of new to programming in Linux & c/c++. I'm currently writing a FileManager using Ubuntu Linux(10.10) for Learning Purposes. I've got started on this project by creating a loopback device to be used as my virtual hard disk. After creating the loop back hard disk and mounting it has the following configuration.

Code:
$> sudo fdisk -l /dev/loop0

Disk /dev/loop0: 10 MB, 10977280 bytes
255 heads, 63 sectors/track, 1 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/loop0 doesn't contain a valid partition table

Now what I want to do is develop a c++ program to read & write files to this loop back device,which I'm using to simulate an actual hard disk,at the blocks & sectors level. So far I've come up with the following code. But I'm still unable to read files from the hard disk one block at a time.

Code:
#include <iostream>
#include <stdio.h>

using namespace std;

int main()
{

    char block[512];
    int length=0;
    cout<<"Implementation of the File Handler Read Method..."<<endl;

    FILE *f = fopen("/dev/loop0", "r");
    if(f == NULL)
    {
        cout<<"Error In Opening the HardDisk File Retuning Error..."<<endl;
        return -1;
    }

    //Read One Block of Data to Buffer
    length = fread(block, 1, sizeof(block), f);

    /* Do something with the data */
    cout<<"Length : "<<length<<endl;

    return 0;
}


When I run this Program All what I get is the message for NULL.
"Error In Opening the HardDisk File Retuning Error...".
Could you please help me by pointing what am I doing wrong here ?. So I could open the loopback device as a file an access it at the sectors & block level.
 

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LEARN ABOUT BSD

disktab

DISKTAB(5)							File Formats Manual							DISKTAB(5)

NAME

       disktab - disk description file

SYNOPSIS

       #include <disktab.h>

DESCRIPTION

       Disktab	is a simple date base which describes disk geometries and disk partition characteristics.  The format is patterned after the term-
       cap(5) terminal data base.  Entries in disktab consist of a number of `:' separated fields.  The first entry for each disk gives the  names
       which are known for the disk, separated by `|' characters.  The last name given should be a long name fully identifying the disk.

       The following list indicates the normal values stored for each disk entry.

       Name  Type  Description
       ns    num   Number of sectors per track
       nt    num   Number of tracks per cylinder
       nc    num   Total number of cylinders on the disk
       b0    str   Filename of block zero primary bootstrap for device
       ba    num   Block size for partition `a' (bytes)
       bd    num   Block size for partition `d' (bytes)
       be    num   Block size for partition `e' (bytes)
       bf    num   Block size for partition `f' (bytes)
       bg    num   Block size for partition `g' (bytes)
       bh    num   Block size for partition `h' (bytes)
       fa    num   Fragment size for partition `a' (bytes)
       fd    num   Fragment size for partition `d' (bytes)
       fe    num   Fragment size for partition `e' (bytes)
       ff    num   Fragment size for partition `f' (bytes)
       fg    num   Fragment size for partition `g' (bytes)
       fh    num   Fragment size for partition `h' (bytes)
       pa    num   Size of partition `a' in sectors
       pb    num   Size of partition `b' in sectors
       pc    num   Size of partition `c' in sectors
       pd    num   Size of partition `d' in sectors
       pe    num   Size of partition `e' in sectors
       pf    num   Size of partition `f' in sectors
       pg    num   Size of partition `g' in sectors
       ph    num   Size of partition `h' in sectors
       se    num   Sector size in bytes (default 512)
       sf    bool  supports bad144-style bad sector forwarding
       so    bool  partition offsets in sectors
       ty    str   Type of disk (e.g. removable, winchester)

       Disktab entries may be automatically generated with the diskpart program.

FILES

       /etc/disktab

SEE ALSO

       newfs(8), diskpart(8), getdiskbyname(3)

BUGS

       This file shouldn't exist, the information should be stored on each disk pack.

4.2 Berkeley Distribution					   May 17, 1986 							DISKTAB(5)
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