C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)


Login or Register for Dates, Times and to Reply

 
Thread Tools Search this Thread
Operating Systems Linux C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk)
# 1  
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.
# 2  
Check the perms on /dev/loop0

Code:
ls -l /dev/loop0

This User Gave Thanks to dsw For This Post:
# 3  
Question

Quote:
Originally Posted by dsw
Check the perms on /dev/loop0

Code:
ls -l /dev/loop0

HI dsw,

I changed the permissions of the /dev/loop0 to chmod 777 & then ran the command you gave and got the following results :

Code:
brw-rw---- 1 root disk 7, 0 2011-01-26 16:39 /dev/loop0

But now when I run the above Program I've written using the command prompt the following output appears :

Code:
Implementation of the File Handler Read Method...
Segmentation fault


Last edited by shen747; 01-26-2011 at 07:25 AM..
# 4  
Well it looks like you've got past the initial problem of the fopen() by changing perms.

I suspect you need to setup the fd first by doing an ftell() & fseek() as per the following example which should help you.

fread - C++ Reference

Failing that, add the -g flag to your g++ line and run it through gdb.
# 5  
Hi dsw,

Thanks for the reply. I will check that.
# 6  
Personally I'd use the open()/read()/write() calls instead of the stdio fopen()/fread()/fwrite() calls. You'll get more direct, unbuffered access; more useful error messages via perror() or errno; and a real file descriptor to call ioctl() on should you end up needing it. (which you might, dealing with block devices.)
# 7  
Hi Corona688,

Could you please give me an example code sample using the above 03 methods. A pointer to get me started on them ?. Because I feel I'll have to deal with the file descriptor at some point & I feel it wouldn't be optimum to do so using the <stdio.h> methods. could you give me some help for me to get started with the : open()/read()/write() please ??.
Login or Register for Dates, Times and to Reply

Previous Thread | Next Thread
Thread Tools Search this Thread
Search this Thread:
Advanced Search

Test Your Knowledge in Computers #258
Difficulty: Medium
Iannis Xenakis wrote programs in the FORTRAN language that generated numeric data that he transcribed into scores to be played by traditional musical instruments.
True or False?

10 More Discussions You Might Find Interesting

1. UNIX for Advanced & Expert Users

Need help for getting hard-disk traces

When we write a programme,we declare variables and compiler allocates memory to them.I want to get access to the physical block number of hard-disk where actually the data is stored by the programme " Some one help me out... (3 Replies)
Discussion started by: nagraz007
3 Replies

2. Red Hat

Need help for getting hard-disk traces

When we write a programme,we declare variables and compiler allocates memory to them.I want to get access to the physical block number of hard-disk where actually the data is stored by the programme " Some one help me out... (1 Reply)
Discussion started by: nagraz007
1 Replies

3. SCO

declare disk driver for IDE hard disk

hi I've a fresh installation of SCO 5.0.7 on the IDE hard disk. For SCSI hard disk I can declare, for example blc disk driver using: # mkdev hd 0 SCSI-0 0 blc 0but it works for IDE hard disk? (3 Replies)
Discussion started by: ccc
3 Replies

4. UNIX for Dummies Questions & Answers

How to increase hard disk in linux

Hi guys i have created a linux machine in virtual box now i want to add some hard disk space into it. How would i do this. Please help. Machine details are as below # lsb_release -a LSB Version: :core-3.1-ia32:core-3.1-noarch:graphics-3.1-ia32:graphics-3.1-noarch Distributor ID:... (7 Replies)
Discussion started by: pinga123
7 Replies

5. UNIX for Dummies Questions & Answers

Hard Disk at 99% Help!

:eek: I use this Solaris to run CMS a call acounting software package for my job. No one could run reports today because it said the this when you logged on "The following file systems are low, and could adversely affect server performance: File system /: 99%full" Can some one please explain... (9 Replies)
Discussion started by: mannyisme
9 Replies

6. UNIX for Dummies Questions & Answers

Hard Disk Check

How can we check the number of hard disks (both internal & external) in a server, their capacity and serial number (5 Replies)
Discussion started by: muneebr
5 Replies

7. UNIX Desktop Questions & Answers

Hard Disk

I have a cuestion. How Can I to add other hard disk to my computer? I need to configurate anyone? (4 Replies)
Discussion started by: hmaraver
4 Replies

8. Filesystems, Disks and Memory

Adding hard Disk

Hi all, I am using SCO Openserver V and I want to add one more harddisk (/dev/hd1) Hw can I do it? (1 Reply)
Discussion started by: skant
1 Replies

9. Filesystems, Disks and Memory

hard disk meltdown

I had an issue with a second hard disk in my machine. I have a sparc station running solaris 7. It was working fine but now it wont mount on boot up and when you try to mount it manually it gives an I/O error. I tried a different disk as a control which was fine. What I want to know is if my... (3 Replies)
Discussion started by: Henrik
3 Replies

10. UNIX for Dummies Questions & Answers

hard disk problems

Hi all I am facing a strange problem. I am using a sun ultra10 spark machine. first i took a 20gb IDE hard disk and installed solaris 5.8. But due to some requirement i have to reinstall the OS but this time solaris 2.6. and now the hard disk capacity is only showing 8gb. Where the 12gb... (3 Replies)
Discussion started by: Prafulla
3 Replies

Featured Tech Videos