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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)
# 8  
Old 01-27-2011
These commands all have man pages accessible with 'man 2 open', 'man 2 read', 'man 2 write'. Also lseek.

Code:
#include <sys/types.h>
#include <unistd.h>
#include <sys/fcntl.h>
int main(void)
{
        char buf[512];
        ssize_t rb,wb;
        int fd=open("/path/to/dev", O_RDWR);
        if(fd < 0)
        {
                perror("Couldn't open device");
                return(1);
        }

        rb=read(fd, buf, 512);
        fprintf(stderr, "Read %d bytes\n", (int)rb);

        if(lseek(fd, 0L, SEEK_SET) != 0L)
        {
                perror("Couldn't seek");
                close(fd);
                return(1);
        }

        wb=write(fd, buf, rb);
        fprintf(stderr, "Wrote %d bytes\n", (int)wb);

        close(fd);
        return(0);
}

This User Gave Thanks to Corona688 For This Post:
shen747
# 9  
Old 01-27-2011
Question
Hi Corona688,

I used your code & it works like a charm. I organized the code into simple separate functions as shown below.


Code:
#include <sys/types.h>
#include <unistd.h>
#include <sys/fcntl.h>
#include <cstdio>
#include <iostream>

#define DISK_NAME "/dev/loop0"

using namespace std;


/**
Read a set of Blocks into the Buffer
Block_SIZE=512bytes
*/
int readFromDisk(char *buffer,int length)
{
  ssize_t readBytes;
  int fd=open(DISK_NAME, O_RDWR);

   if(fd < 0)
    {
        perror("Couldn't open device");
        return 1;
    }

    readBytes=read(fd, buffer, length);
    cout<<"Read Bytes : "<<(int)readBytes<<endl;
	close(fd);

 return 0;
}

/**
Write the Buffer Content to HDD
Block_SIZE=512bytes
*/
int writeToDisk(char *buffer,int length)
{
    ssize_t writeBytes;
    int fd=open(DISK_NAME, O_RDWR);

	writeBytes=write(fd,buffer,length);
    cout<<"Wrote Bytes : "<<(int)writeBytes<<endl;
	close(fd);

 return 0;
}

/**
Seek out a Particular Block In the Disk
Block_SIZE=512bytes
*/
int seekDisk()
{
    cout<<"Seeking the File Disk..."<<endl;
    int fd=open(DISK_NAME, O_RDWR);
	if(lseek(fd, 0L, SEEK_SET) != 0L)
        {
                perror("Couldn't seek");
                close(fd);
                return(1);
        }
    close(fd);
    return 0;
}



int main(void)
{

cout<<"\n"<<"Running the OS File Manager....Main"<<endl;
/*
Test The File Manager
*/
char buffer[1024];
readFromDisk(buffer,512);
seekDisk();
writeToDisk(buffer,300);

return 0;
}

Now I'm having trouble figuring out a way to create a file using the above code. Could you please help me out here by giving me a point to start on working please ?.

---------- Post updated at 06:54 AM ---------- Previous update was at 01:41 AM ----------

Could Anyone Please Help me with this ?. I need to use the following code I've come up so far & figure out a way to create a file inside the block device. Following is my code.

Code:
#include <sys/types.h>
#include <unistd.h>
#include <sys/fcntl.h>
#include <cstdio>
#include <iostream>

#define DISK_NAME "/dev/loop0"

using namespace std;


/**
Read a set of Blocks into the Buffer
Block_SIZE=512bytes
*/
int readFromDisk(char *buffer,int length)
{
  ssize_t readBytes;
  int fd=open(DISK_NAME, O_RDWR);

   if(fd < 0)
    {
        perror("Couldn't open device");
        return 1;
    }

    readBytes=read(fd, buffer, length);
    cout<<"Read Bytes : "<<(int)readBytes<<endl;
	close(fd);

 return 0;
}

/**
Write the Buffer Content to HDD
Block_SIZE=512bytes
*/
int writeToDisk(char *buffer,int length)
{
    ssize_t writeBytes;
    int fd=open(DISK_NAME, O_RDWR);

	writeBytes=write(fd,buffer,length);
    cout<<"Wrote Bytes : "<<(int)writeBytes<<endl;
	close(fd);

 return 0;
}

/**
Seek out a Particular Block In the Disk
Block_SIZE=512bytes
*/
int seekDisk()
{
    cout<<"Seeking the File Disk..."<<endl;
    int fd=open(DISK_NAME, O_RDWR);
	if(lseek(fd, 0L, SEEK_SET) != 0L)
        {
                perror("Couldn't seek");
                close(fd);
                return(1);
        }
    close(fd);
    return 0;
}



int main(void)
{

cout<<"\n"<<"Running the OS File Manager....Main"<<endl;
/*
Test The File Manager
*/
char buffer[1024];
readFromDisk(buffer,512);
seekDisk();
writeToDisk(buffer,300);

return 0;
}

Can anyone lend me a helping hand here please ?.
Last edited by shen747; 01-27-2011 at 07:41 AM..
# 10  
Old 01-27-2011
'man 2 creat'

creat(2) operates on file descriptors as does the rest of the stdio.h calls - give the manpage a view.

-or-

you can OR the O_CREAT flag with O_RDWR in open(2) to create the file if it doesn't already exist.

Code:
O_RDWR | O_CREAT

Note the omitted trailing 'e' to creat(2).
This User Gave Thanks to dsw For This Post:
shen747
# 11  
Old 01-27-2011
Why are you opening and closing the file every time you do something?

This especially becomes useless:
Code:
/**
Seek out a Particular Block In the Disk
Block_SIZE=512bytes
*/
int seekDisk()
{
    cout<<"Seeking the File Disk..."<<endl;
    int fd=open(DISK_NAME, O_RDWR);
	if(lseek(fd, 0L, SEEK_SET) != 0L)
        {
                perror("Couldn't seek");
                close(fd);
                return(1);
        }
    close(fd);
    return 0;
}

...because the seek position gets reset every time you open it anyway. You should open it once and keep using the number it gives you. Your read code would end up looking like int readFromDisk(int fd, char *buffer,int length) { ... } iow exactly like normal read...so you might as well get used to ordinary read call instead.

You should also be doing something more with the number of bytes written and read than just printing it. read() and write() will occasionally surprise you with less than you asked for.
# 12  
Old 01-27-2011
Hi Corona688,

I just reorganized my code & came up with the following structure & it works like a charm. But I'm still having trouble creating a file inside the block device using the
Code:
int writeToDisk(int fileDisId,char *buffer,int length)

method. Could you please point me from where I could get started. I want to simulate the working of the real os File Manager,which does it at the sector's level when writing a file to the disk. So I was wondering how could I use the above write method to achieve this. Could you please advice me on this as how may I proceed ?. Will I need to maintain a FAT like table when creating files so I could know in which sectors of the virtual hard disk the file is stored ?.


Code:
#include <sys/types.h>
#include <unistd.h>
#include <sys/fcntl.h>
#include <cstdio>
#include <iostream>
#include <string>

#define DISK_NAME "/dev/loop0"

using namespace std;


/**
Initialize the Virtual Disk File
*/
int initDisk()
{
    int fd=open(DISK_NAME, O_RDWR);

    if(fd < 0)
    {
        perror("Couldn't open device");
        return 1;
    }
    return fd;
}


/**
Read a set of Blocks into the Buffer
*/
int readFromDisk(int fileDisId,char *buffer,int length)
{
    ssize_t readBytes;
    readBytes=read(fileDisId, buffer, length);
    close(fileDisId);
    return (int)readBytes;
}

/**
Write the Buffer Content to HDD Blocks
*/
int writeToDisk(int fileDisId,char *buffer,int length)
{
    ssize_t writeBytes;
    writeBytes=write(fileDisId,buffer,length);
    close(fileDisId);
    return (int)writeBytes;
}

/**
Seek out a Particular Block In the Disk
Block_SIZE=512bytes
*/
int seekDisk(int fileDisId)
{
    cout<<"Seeking the File Disk..."<<endl;

    if(lseek(fileDisId, 0L, SEEK_SET) != 0L)
    {
        perror("Couldn't seek");
        close(fileDisId);
        return(1);
    }
    close(fileDisId);
    return 0;
}



int main(void)
{

    cout<<"\n"<<"Running the OS File Manager....Main"<<endl;

    /*
    Test The File Manager
    */

    int diskId;


    char buffer[1024];
     diskId = initDisk();
    cout<<"No of Bytes Read : "<<readFromDisk(diskId,buffer,1024)<<endl;

     diskId = initDisk();
     seekDisk(diskId);

    diskId = initDisk();
    cout<<"No of Bytes Written : "<<writeToDisk(diskId,buffer,1024)<<endl;

    return 0;
}

---------- Post updated at 11:48 AM ---------- Previous update was at 11:23 AM ----------

Hi dsw,

Thank you very much for your response. But what I need is to use the following method.

Code:
int writeToDisk(int fileDisId,char *buffer,int length)

So I could simulate the working of the read file manager by creating a file @ the sector's level. I know I might sound ridiculous. But this is for a learning purpose & I would really appreciate if you could correct my approach if I'm wrong & guide me here Smilie.
Last edited by shen747; 01-27-2011 at 12:39 PM..
# 13  
Old 01-27-2011
The way you're using it, it contains no "files". A block device is nothing but a giant pile of undifferentiated blocks, ordered from first to last.

If you want to create a file in it, you have to know what type of partition it is, and follow that partition system's rules for how blocks are arranged into directories and files.
This User Gave Thanks to Corona688 For This Post:
shen747
# 14  
Old 01-27-2011
Question
Hi Corona688,

So you mean I should find how the ext3 (as I've formatted my loop back disk with ext3 with mkfs tool) creates files & directories on the hard disk drive ?. Is that approach correct ?. If yes where do you suggest I start reading on programming material for me to get started please ?.
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