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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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CFDISK(8)							 GNU fdisk Manual							 CFDISK(8)

NAME
GNU fdisk, lfdisk, gfdisk - manipulate partition tables on a hard drive SYNOPSIS
fdisk [options] [device] DESCRIPTION
fdisk is a disk partition manipulation program, which allows you to create, destroy, resize, move and copy partitions on a hard drive using a menu-driven interface. It is useful for organising the disk space on a new drive, reorganising an old drive, creating space for new oper- ating systems, and copying data to new hard disks. For a list of the supported partition types, see the --list-partition-types option below. It comes in two variants, gfdisk and lfdisk. Lfdisk aims to resemble Linux fdisk 2.12, while gfdisk supports more advanced disk operations, like resizing the filesystem, moving and copying partitions. When starting fdisk, the default is to run gfdisk. OPTIONS
-h, --help displays a help message. -v, --version displays the program's version. -L, --linux-fdisk turns on Linux fdisk compatibility mode. This is the same as running lfdisk. -G, --gnu-fdisk turns off Linux fdisk compatibility mode. -i, --interactive where necessary, prompts for user intervention. -p, --script never prompts for user intervention. -l, --list lists the partition table on the specified device and exits. If there is no device specified, lists the partition tables on all detected devices. -r, --raw-list displays a hex dump of the partition table of the disk, similar to the way Linux fdisk displays the raw data in the partition table. -u, --sector-units use sectors, instead of cylinders for a default unit. -s, --size=DEVICE prints the size of the partition on DEVICE is printed on the standard output. -t, --list-partition-types displays a list of supported partition types and features. The following options are available only to lfdisk. -b, --sector-size=SIZE Specify the sector size of the disk. Valid values are 512, 1024 and 2048. Should be used only on older kernels, which don't guess the correct sector size. -C, --cylinders=CYLINDERS Specify the number of cylinders of the disk. Currently does nothing, it is left for Linux fdisk compatibility. -H, --heads=HEADS Specify the number of heads of the disk. Reasonable values are 255 or 16. -S, --sectors=SECTORS Specify the number of sectors per track. A reasonable value is 63. BUGS
Before editing a BSD disklabel, the partition with the disklabel should already exist on the disk and be detected by the OS. If you have created a BSD-type partition, you need to write the changes to the disk. If fdisk fails to notify the OS about the changes in partition ta- ble, you need to restart your computer. As fdisk tries to guess the device holding the BSD disklabel, it might fail to edit it at all, even if the OS has detected it. In this case you are adviced to simply open the device with fdisk directly. It is possible that it doesn't work on some operating systems. Getting the size of a partition with -s might fail, if fdisk fails to guess the disk device, for the same reasons as with the previous bug. SEE ALSO
mkfs(8), cfdisk(8), parted(8) The fdisk program is fully documented in the info(1) format GNU fdisk User Manual manual. fdisk 18 August, 2006 CFDISK(8)
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