Positions and truncation

file.tell() -> Result[u64, IoError] returns an opaque text-position cookie. file.seek(cookie: u64) -> Result[(), IoError] restores it. Both require exclusive access, allocate nothing, and report native seek errors, including nonseekable streams.

Zero rewinds. Other cookies must come from tell() on the same stream with unchanged contents. They preserve pending CRLF translation state, rather than representing a plain byte offset. Arithmetic and persistence of cookies are unsupported. A failed seek leaves decoder state unchanged; arbitrary cookies remain memory-safe but can yield invalid text or surprising positions. Relative and end-relative seeking are unsupported.

file.truncate(size: i64) -> Result[i64, IoError] changes a writable file’s length in bytes, returns that length, and leaves the cursor unchanged. Omitting size uses the current physical byte position, not a text cookie. Negative sizes fail before mutation. The operation requires exclusive access, allocates nothing, can fail at the OS, and does not synchronize storage.

Extending regular Linux files adds zero bytes. Shrinking can split UTF-8, which later reading reports as invalid data.