user
46 lines
1.7 KiB
Markdown
46 lines
1.7 KiB
Markdown
# K12 — Semaphores and task notifications
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## Position
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- Series: FreeRTOS C++
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- Lesson: L11, card K12
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- Duration: 30 minutes
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- Waiter priority 3, signaler priority 2
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- Static semaphore controls; notification state in waiter TCB
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## Outcome
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The student selects a binary semaphore, counting semaphore or direct task
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notification from the communication cardinality and state semantics, then
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proves blocked/wake transitions and clear/value policy.
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## Lesson plan
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| Time | Mode | Evidence |
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| --- | --- | --- |
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| 0–5 | binary | empty 0, give 1, second give false, take 0 |
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| 5–10 | counting | max/initial 2, take to 1 then 0 |
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| 10–15 | blocked waiter | third take blocks; signaler observes eBlocked |
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| 15–20 | release | one give wakes exactly one waiter; token consumed |
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| 20–25 | notification count | give/take returns 1, state lives in TCB |
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| 25–30 | notification value | explicit overwrite/wait yields 0xA5A55A5A |
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## Acceptance
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- binary and counting bounds are enforced;
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- both handles equal caller static controls and heap delta is zero;
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- waiter is blocked at all three intended waits;
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- counting release is consumed immediately, leaving count 0;
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- notification give/take returns exactly 1;
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- explicit `overwrite` action transfers 0xA5A55A5A;
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- event order connects counting block/release and both notification phases;
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- target exits with PASS.
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## Main traps
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1. A semaphore signals availability; it does not carry a data record.
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2. Notification targets one specific task/slot, not arbitrary consumers.
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3. Clear-on-exit versus decrement changes binary/counting notification meaning.
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4. Wrong action or clear mask can silently lose bits/value.
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5. Identical wake behavior does not imply identical storage/cardinality.
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