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

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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.

$> 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.

#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

ls -l /dev/loop0

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

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

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 :

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 :

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

Last edited by shen747; 01-26-2011 at 08:25 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 ??.
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