Question

How can I write to any block on my HDD using the C programming language?

There was a question about writing the MBR but it didn’t cover the C aspects that much.

Since filedescriptors are – as the word says – for files, I guess there is no way to use them in this case. The Low-level I/O included in the C standard library is also implemented with filedescriptors.

To put it more precisely:

This question is rather about writing HDD blocks than files (OS independent).

The answers to the question mentioned above basically suggested using dd (coreutils) on UNIX-Systems. This is why I am asking for a way in C. Maybe bootloaders (GRUB) and boot sector viruses use different techniques?

I guess changing the actual pointer inside of a filedescriptor is not a legitimate way.

Problems and limitations:

I know that there are certain aspects to keep in mind, such as

• Some operating systems restrict direct access to volumes (e.g. Windows)
• Writing errors as well as writing the wrong data into certain blocks
  might result in filesystem corruption (loss of data on the HDD).
• Antivirus-Software might flag it as suspicious code.

This question is oriented more theoretical.

Answer 1

The C language has access to files with functions fopen/fclose/fread/fwrite etc. But there is no such thing as a block device in the language (not even a device, for that matter).

POSIX on the other hand has the low level functions open/close/read/write to access to files, and have the concept of block device. These functions can be used (with care) for a block device, as long as you follow a few simple rules (block alignment, mainly) and you know the name of your device (/dev/xxx).

If you are in a non-POSIX system, such as Windows, then the OS will have a specific way to handle the block device access. In Windows, for example, you can use the CreateFile function with the device name \\.\PhysicalDrive0, \\.\C: or such.