Direct access to hard disk with no FS from C program on Linux

As mentioned elsewhere, under *NIX systems, block devices like /dev/sda can be accessed as plain files. Note that if file system is mounted from the device, opening it as file for writing would fail.

If you want to play with block devices, I would advise to first use the loop device, which presents a plain file as a block device. For example:

dd if=/dev/zero of=./loop_file_10MB bs=1024 count=10K
losetup /dev/loop0 $PWD/loop_file_10MB

After that, /dev/loop0 would behave as if it was a block device, but all information written would be stored in the file.

As device files for drives (e.g. /dev/sda) are block devices, this means you can open, seek and use the file almost like a normal file.

Yes, as others have noted, you can simply open the block device.

However, it's a really good idea to do IO (writes anyway) on block boundaries and whole blocks. You can use something like pread() and pwrite() to do these IO, or mmap some or all of the device.

There are a bunch of ioctls which can be used, see "man sd" for some more info. They don't seem to all be documented in the same place.

In linux/fs.h BLKROSET and a bunch of other ioctls are defined, you have to look around to find out how to use them. You can do useful things like find out how big the device is, and what the block size is.

The source code of the util-linux-ng package is your friend, it contains examples.