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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 DEBIAN

set_tid_address

SET_TID_ADDRESS(2)					     Linux Programmer's Manual						SET_TID_ADDRESS(2)

NAME

       set_tid_address - set pointer to thread ID

SYNOPSIS

       #include <linux/unistd.h>

       long set_tid_address(int *tidptr);

DESCRIPTION

       For  each  process, the kernel maintains two attributes (addresses) called set_child_tid and clear_child_tid.  These two attributes contain
       the value NULL by default.

       set_child_tid
	      If a process is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to the value passed in the ctid  argu-
	      ment of that system call.

	      When set_child_tid is set, the very first thing the new process does is writing its PID at this address.

       clear_child_tid
	      If  a  process  is started using clone(2) with the CLONE_CHILD_CLEARTID flag, clear_child_tid is set to the value passed in the ctid
	      argument of that system call.

       The system call set_tid_address() sets the clear_child_tid value for the calling process to tidptr.

       When a process whose clear_child_tid is not NULL terminates, then, if the process is sharing memory with other processes or threads, then 0
       is written at the address specified in clear_child_tid and the kernel performs the following operation:

	   futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);

       The  effect  of	this  operation is to wake a single process that is performing a futex wait on the memory location.  Errors from the futex
       wake operation are ignored.

RETURN VALUE

       set_tid_address() always returns the PID of the calling process.

ERRORS

       set_tid_address() always succeeds.

VERSIONS

       This call is present since Linux 2.5.48.  Details as given here are valid since Linux 2.5.49.

CONFORMING TO

       This system call is Linux-specific.

SEE ALSO

       clone(2), futex(2)

COLOPHON

       This page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-07-19							SET_TID_ADDRESS(2)
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