Working out Internal Fragmentation using C Programming

Hi I have recently been trying to work out how to work out the internal fragmentation of a file. I have read a lot into this and understand that the internal fragmentation on MINIX using ext2 only occurs on the last block. I understand that every file/dir has an inode number and that you SHOULD be able to get to the last block from the inode number however I am having issues with it. How would I go about getting to this final block? Thanks C

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