VLSI DV Interview Puzzles · All levels

wait fork Synchronization Point

Order these prints by time and explain what `wait fork` waits for.

Puzzle

Difficulty: Easy · Puzzle 3 of 6 · Topic: Fork Join Puzzles

Order these prints by time and explain what `wait fork` waits for.

Code

systemverilog

module p3;
  initial begin
    fork
      #3 $display("%0t A", $time);
      #7 $display("%0t B", $time);
    join_none
    $display("%0t parent_after_join_none", $time);
    wait fork;
    $display("%0t parent_after_wait_fork", $time);
  end
endmodule

Hint

`join_none` returns immediately, but `wait fork` blocks until all currently active children from this process finish.

Step-by-step solution

diagram

1) Parent prints `parent_after_join_none` at t=0 immediately after spawning children.
2) Child A prints at t=3.
3) Child B prints at t=7.
4) `wait fork` unblocks only after both children complete, so final parent print is at t=7.

Answer

Answer: parent_after_join_none@0, A@3, B@7, parent_after_wait_fork@7

Why candidates get it wrong

A frequent miss is assuming `wait fork` only waits for the most recently spawned child.

Interviewer follow-up

What if another nested fork starts a forever loop before `wait fork`?

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