Read blocks from an ext3 filesystem?

Question

What's the easiest way to access an ext3 file system at the block level? I don't care for the files, or raw bytes, I just have to read the FS one block at a time. Is there a simple way to do this (in C)? Or maybe a simple app whose source I could look into for inspiration? I found no usable tutorials on the net, and I'm a bit scared to dive into the kernel source to find out how to do it.

Answer 1

Yes, see e2fsprogs. This provides tools you can use to do anything(!) with ext2, ext3, and ext4 filesystems. It also contains a library interface so you can do anything else.

See the included debugfs, it might be enough for you to start. Otherwise, check out the headers and write some code.

Answer 2

If you want a simple app then I suggest you can take a look at "dd" utility. I comes as part of GNU Core Utility. Its source is available for download. Take a look at its home page, here.
If you want to achieve same from a C code, then please refer to following code. Hope this helps you. :)

#include <stdio.h>
#include <linux/fs.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define SECTOR_NO 10 /*read 10th sector*/

int main()
{
        int sector_size;
        char *buf;
        int n = SECTOR_NO;

        int fd = open("/dev/sda1", O_RDONLY|O_NONBLOCK);
        ioctl(fd, BLKSSZGET, &sector_size);
        printf("%d\n", sector_size);
        lseek(fd, n*sector_size, SEEK_SET);

        buf = malloc(sector_size);
        read(fd, buf, sector_size);

        return 0;
}

Answer 3

Disk devices, and partitions within them, behave just like regular files that you can read from (and write to), e.g.:

head -c 2048 /dev/sda1 > first_2048_bytes

You'll need to be root of course.