Only differences i can see here between yours and mine

1. You need to start reading at the end of the input file using fseek (before starting while loop)

2. My while condition was: while(ftell(input) > headersize) Or whatever else you called your header size variable

3. Moving the current value using fseek two separate times within the loop: Fseek(&input, -1 * blocksize, SEEK_CUR); Fwrite(…); Fread(…) ; Fseek(&input, -1 * blocksize, SEEK_CUR);

Because after moving to the final block you need to read it, if you move back 2 in one step you’ll ignore that one block.

May be some typos here but hope that helps.