Implementation of truncate is unsafe

[s-m-e]

I ran into this issue testing my Python implementation - your C++ implementation shows the same problem:

You implement truncate by calling the truncate system call. As you might have noticed, there is no truncateat system call which would be required for truncating something with a path relative to a file descriptor. So your implementation assumes that its current working directory is never changed throughout its lifetime.

I have spend a while researching it ( 1, 2 ) and it appears that the best solution is to just divert from the truncate system call to an openat-ftruncate-close-sequence. I have successfully tested this with my Python implementation.

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EDIT: Actually, this issue applies to most system calls that you use which take a path as an argument. You are always relying on the current working directory. You should use the savefd file descriptor throughout your code and specify paths relative to it.