mechanical HDD content access and erase with c programming

I have a mechanical hard disk. My data is in D: and E: drive only. I wanted to securely wipe the content of the D: and E: drive and therefore I wanted to write a c code which read each data of the drive and replace it with a random value. How I can do this in C?

(I know few tools are available, but I wanted to learn)

Currently, I am generating lots of text file using a C program to fill the HDD, but it is very time-consuming (very time consuming).