Prep: Errors, and Your First Command — 08r · 10c

Session: Fri Sep 18, 1h30 · Exercises: 08r_errors, 10c_echo · Prep time: ~20 min · Lecture: Week 4 · Collections, Traits, Errors, and Your First Command

What you will build

First, a chain of fallible steps over a toy directory: a scan that reports absence with Option, a call that turns absence and an over-long name into failure with your own error enum, steps glued together with ?, and a boundary where the Result collapses into one Unix-style integer — a byte count, or minus an errno number. Second, echo, your first command, against the ulib façade: arguments in as byte slices, bytes out through write_all on fd 1, exit status 0. The tests check that each bad name yields the right error variant (never a panic), and that echo emits byte-exact output for zero, one, several, and empty arguments.

Concepts you need

• Absence vs failure: Option vs Result — Week 4 · Absence is not failure, A sentinel or an Option · Rust for Systems: Enums, Option, exhaustive match
• Your own error enum; .ok_or() turns absence into failure — Week 4 · An error enum, and where absence becomes failure
• ?, and where a Result becomes a number — Week 4 · ?, and where a Result becomes a number
• A command: argv, fd 1, exit status, one façade — Week 4 · A command: words in, bytes out, a number back, One file, two targets: the ulib seam
• Arguments are bytes, not strings — Week 4 · A command: words in, bytes out, a number back · ulib guide: Portability rules a command must follow
• write_all, never bare write — Week 4 · write_all, and the short write · ulib guide: The complete API surface
• A separator is not a terminator — Week 4 · A command: words in, bytes out, a number back, its "one thing to get right" box

Read before class

What	Time
Week 4 · Friday · 08r Errors as values, all three sections	5 min
Week 4 · Friday · 10c Your first command, all three sections	4 min
Week 4 · For the exam: A sentinel or an Option. Not needed today; on Midterm 1	1 min
ulib guide: The complete API surface and Portability rules a command must follow	5 min

Mental model

Waking a process by pid — not a filesystem, same three layers:

enum ProcError { NoSuchPid, NotSleeping }

fn slot_of(table: &[Proc], pid: u32) -> Option<usize> { /* scan */ }

fn wakeup(table: &mut [Proc], pid: u32) -> Result<(), ProcError> {
    let i = slot_of(table, pid).ok_or(ProcError::NoSuchPid)?;   // absence becomes failure HERE
    if table[i].state != State::Sleeping { return Err(ProcError::NotSleeping); }
    table[i].state = State::Runnable;
    Ok(())
}

fn sys_wakeup(table: &mut [Proc], pid: u32) -> i64 {           // the boundary: one integer
    match wakeup(table, pid) { Ok(()) => 0, Err(ProcError::NoSuchPid) => -3, Err(ProcError::NotSleeping) => -22 }
}

The scan states a fact; the middle layer decides, and may use ? only because it returns Result; the edge collapses everything into the one register a user program can receive. A kernel has no exception to throw and nothing to unwind into, so every system call you write from 50k on has this shape.

Check yourself

1. A directory scan finds no entry for a name. Option or Result, and where is "missing means the call failed" decided?
   Answer

   The scan returns Option — absence is not an error there. The caller that promised to open something converts with .ok_or(SomeVariant); that one line is where the policy lives.
2. Printing items x, an empty item, and y with a space separator: how many spaces, and what would "a space after every item" give instead?
   Answer

   Two, giving x y — the empty item still counts. "After every item" is a terminator and leaves a trailing space. n items need n − 1 separators; the newline goes once, after the loop.
3. Why write_all and never write, when the tests pass either way?
   Answer

   write may accept fewer bytes than offered and only says so in its count. The host harness accepts everything, so bare write is green on your laptop and loses bytes the first time a real device takes fewer. write_all loops until every byte is out.

What "done" looks like

oslings run is green, then oslings submit before you leave. Not green? Submit anyway — the tests that pass earn their share — then finish it at a make-up session (office hours, on the class network) for three quarters, or anywhere else for half, and submit again.

If you finish early

Work Week 4 · Problem 3: Trace the exits and Problem 4: Short writes by the numbers on paper. Then Rustlings error_handling and options groups: https://github.com/rust-lang/rustlings. 100 Exercises To Learn Rust, chapter 5 (enums, fallibility, Result, ?): https://rust-exercises.com/100-exercises/. Or start Thursday's prep page, Prep: cat.