Ext3 users - Feb 2008 - How to read group descriptor?

Hi All,
I am trying to read group descriptor from disk.
I am able to read the superblock structure successfully.
But I am not getting successful to get the group descriptor. Here is my
code. Please check out what is missing.



#include<linux/ext3_fs.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>
#include<string.h>

#define block_size 4096
#define boot_block_size 1024

int main()
{
	char *buff = (char *)malloc(block_size);
	
	struct ext3_super_block * sblock = (struct ext3_super_block
*)malloc(sizeof(struct ext3_super_block));
	
	struct ext3_group_desc * gdesc = (struct ext3_group_desc
*)malloc(sizeof(struct ext3_group_desc));
	
	int fd = open("/dev/hda3",O_RDONLY);
	
	lseek(fd,boot_block_size,SEEK_CUR); //skip the boot block
	
	read(fd,buff,block_size); //read superblock,we read one block(4096
bytes)here
	
	memcpy((void *)sblock,(void *)buff,sizeof(struct
ext3_super_block));//here I can fill sblock successfully..
	
	//read the next block..assuming this will contain group dsc for this
block..i.e. first group descriptor for first block group
	read(fd,buff,block_size); //read first block. It should contain
group description structure.
	
	memcpy((void *)gdesc,(void *)buff,sizeof(struct ext3_group_desc));
	
	//try to print some field inside structure..
	printf("\nbg_inode_table:%u",gdesc->bg_inode_table); //Bang....It
gives me zero.

	close(fd);	
	return 0;
}

On Feb 01, 2008  16:51 +0530, ashish mahamuni wrote:
> I am trying to read group descriptor from disk.
> I am able to read the superblock structure successfully.
> But I am not getting successful to get the group descriptor. Here is my
> code. Please check out what is missing.
Why not just use libext2fs?

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

--- On Tue, 5/2/08, Peter Teoh <htmldeveloper at gmail.com> wrote:
> From: Peter Teoh <htmldeveloper at gmail.com>
> Subject: Re: How to read group descriptor?
> To: "ashish mahamuni" <ashitpro at yahoo.co.in>
> Date: Tuesday, 5 February, 2008, 4:41 AM
> How about the using the command:
> 
> Using this to check for ext2/ext3 filesystem:
> file -s /dev/sdb1
> and
> /sbin/dumpe2fs /dev/sdb1?
> 
> to dump the group descriptor?
> 
> On Feb 1, 2008 7:21 PM, ashish mahamuni
> <ashitpro at yahoo.co.in> wrote:
> > Hi All,
> > I am trying to read group descriptor from disk.
> > I am able to read the superblock structure
> successfully.
> > But I am not getting successful to get the group
> descriptor. Here is my
> > code. Please check out what is missing.
> >
> >
> >
> > #include<linux/ext3_fs.h>
> > #include<sys/types.h>
> > #include<sys/stat.h>
> > #include<stdio.h>
> > #include<unistd.h>
> > #include<fcntl.h>
> > #include<stdlib.h>
> > #include<string.h>
> >
> > #define block_size 4096
> > #define boot_block_size 1024
> >
> > int main()
> > {
> >         char *buff = (char *)malloc(block_size);
> >
> >         struct ext3_super_block * sblock = (struct
> ext3_super_block
> > *)malloc(sizeof(struct ext3_super_block));
> >
> >         struct ext3_group_desc * gdesc = (struct
> ext3_group_desc
> > *)malloc(sizeof(struct ext3_group_desc));
> >
> >         int fd = open("/dev/hda3",O_RDONLY);
> >
> >         lseek(fd,boot_block_size,SEEK_CUR); //skip the
> boot block
> >
> >         read(fd,buff,block_size); //read superblock,we
> read one block(4096
> > bytes)here
> >
> >         memcpy((void *)sblock,(void
> *)buff,sizeof(struct
> > ext3_super_block));//here I can fill sblock
> successfully..
> >
> >         //read the next block..assuming this will
> contain group dsc for this
> > block..i.e. first group descriptor for first block
> group
> >         read(fd,buff,block_size); //read first block.
> It should contain
> > group description structure.
> >
> >         memcpy((void *)gdesc,(void
> *)buff,sizeof(struct ext3_group_desc));
> >
> >         //try to print some field inside structure..
> >        
> printf("\nbg_inode_table:%u",gdesc->bg_inode_table);
> //Bang....It
> > gives me zero.
> >
> >         close(fd);
> >         return 0;
> > }
> >
> >
> >
> > _______________________________________________
> > Ext3-users mailing list
> > Ext3-users at redhat.com
> > https://www.redhat.com/mailman/listinfo/ext3-users
> >
> 
> 
> 
> -- 
> Peter Teoh
> HP: 96809281
Thats fine.
But I need it in my source code for some calculations.



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