Puzzle

Difficulty: Hard · Puzzle 5 of 6 · Topic: Semaphore & Mailbox Puzzles

Producer sends two items after both consumers are blocked on get. Is consumer wake-up order guaranteed?

Code

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module tb;
  mailbox #(int) mb = new();

  task consumer(string name);
    int v;
    mb.get(v);
    $display("[%0t] %s got %0d", $time, name, v);
  endtask

  initial fork
    consumer("C1");
    consumer("C2");
  join_none

  initial begin
    #5 mb.put(10);
    #0 mb.put(20);
    #1 $finish;
  end
endmodule

Hint

Mailbox preserves message FIFO, but waiter scheduling fairness is not strict across processes.

Step-by-step solution

1) Message order in mailbox is FIFO: 10 then 20.
2) Which blocked consumer gets first wake-up is not a robust fairness contract to rely on.
3) One run may print C1->10, C2->20; another may swap consumer names with same data order.

Answer

Answer: Data order is FIFO (10 then 20), but consumer-to-item mapping is not a portability-safe fairness guarantee.

Why candidates get it wrong

Assuming blocked getter wake-up order is deterministic across all tools can break regressions.

Interviewer follow-up

How would you enforce deterministic ownership (per-agent mailbox or tagged routing)?

Related topics

• Back to Semaphore & Mailbox Puzzles

• Concurrency & Timing Puzzles section