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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 LINUX

ipcs

IPCS(1) 						     Linux Programmer's Manual							   IPCS(1)

NAME

       ipcs - provide information on ipc facilities

SYNOPSIS

       ipcs [-asmq] [-tclup]
       ipcs [-smq] -i id
       ipcs -h

DESCRIPTION

       ipcs provides information on the ipc facilities for which the calling process has read access.

       The -i option allows a specific resource id to be specified.  Only information on this id will be printed.

       Resources may be specified as follows:

       -m     shared memory segments

       -q     message queues

       -s     semaphore arrays

       -a     all (this is the default)

       The output format may be specified as follows:

       -t     time

       -p     pid

       -c     creator

       -l     limits

       -u     summary

SEE ALSO

       ipcrm(1), ipcmk(1)

CONFORMING TO

       The  Linux  ipcs  utility is not fully compatible to the POSIX ipcs utility.  The Linux version does not support the -b and -o options, but
       does support the -l and -u options not defined by POSIX. The portable application shall not use the -a, -b, -o, -l, and -u options.

AUTHOR

       krishna balasubramanian (balasub@cis.ohio-state.edu)

AVAILABILITY

       The ipcs command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

Linux 0.99							  9 October 1993							   IPCS(1)
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