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Want to talk about files and shredding them etc

I would like to talk about the filesystem or whatver is relevant to this. 
When I delete a file, it is not actually deleted, but marked as free space, 

So, assuming I have a program that can do these things:

- issue system call to get file size in bytes
- open file for writing, preserving contents
- overwrite exact byte count with random junk, all 0s, all 1s, then random 
junk again etc etc
- close file
- 'delete' file (unlink system call?)

and that if overwriting with sys_write() or whatever for the exact number of 
bytes in the file, this will only write over those blocks the file originally 
occupied, right? So essentially I can write a user mode file 
shredder/secure deletion program in C using just the standard C library on 



"Girls are for pleasure; boys are for ecstasy."

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