I've got my file open using r+ mode. I've made some alteration to it, and I would like to remove the contents of it starting from X to the end of the file.
Unfortunately I have no clue how to do that. I've been browsing through the docs, but there's no mention of that.
I don't want to write :space: to the file so it "looks" like it's cleared, I would like to make it smaller as well.
Opening the file using w is not an option in this case.
Opening a file with r+ mode preserves the current content; as the result, I don't think there is a way to truncate the rest of the file using the functions available in Lua. This SO answer indicates that it can be done using reopen with w+, but since you indicated that you can't open it for writing, I don't think there is a way to do what you want.
Testing on this script produces new contentext (where ext is the leftover from the earlier content):
local f = io.open("somefile", "w")
f:write("some long text")
f:close()
local f = io.open("somefile", "r+")
f:write("new content")
f:close()