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Joined 2026-09-21 Message
Incoming links: Message passing
Message-passing mutual exclusion server 2026-10-25
Maintain an owner identity, initially empty, and a FIFO queue of accepted client requests. A request appends its sender to the FIFO queue. A release from the owner clears the owner. Whenever there is no owner and the FIFO queue is nonempty, remove its oldest request, record that client as owner, and send a grant. The server processes these state changes sequentially.
Safety proof. At most one grant is outstanding: a new grant requires an empty owner field, and a valid release clears precisely the preceding ownership. Compliant clients access the resource only while holding their grant, so mutual exclusion follows. With reliable eventual message passing, continued server execution and finite resource-use durations, the FIFO queue also gives starvation freedom for accepted requests, since later requests cannot overtake them.
Give the hardware to a single device driver process. Other processes request a conversion through message passing and never touch its memory-mapped input-output registers. The following is protocol pseudocode for Phōs: its operation names specify their required semantics, rather than asserting an unverified spelling of a particular 2019 C interface. The hardware actions and the request/reply design are fully specified.
Use a FIFO queue of accepted requests, each containing its sender and a per-call token. Bind the device's interrupt to a persistent notification addressed to the driver before any conversion is started. A notification must remain pending if it arrives before the driver next waits; this is the lost-wakeup avoidance requirement. The driver receives both client requests and interrupt notifications in its event loop.
configure sensor and connect its interrupt notification to this process
TEMP_DATARDY := 0
clear stale pending interrupt notification
enable sensor interrupt and interrupt-controller delivery
pending := empty FIFO
active := none
forever:
if active is none and pending is not empty:
active := remove oldest request from pending
TEMP_DATARDY := 0
clear stale pending interrupt notification
TEMP_START := 1
event := wait for a request or persistent interrupt notification
if event is REQUEST(sender, token):
append (sender, token) to pending
if event is sensor interrupt and active is not none:
if TEMP_DATARDY != 0:
value := TEMP_TEMP
TEMP_DATARDY := 0
acknowledge the interrupt through the OS interrupt mechanism
send RESULT(active.token, value) to active.sender
active := noneAfter receiving an idle-time request, the next loop iteration starts its conversion before waiting again. While a conversion is in progress, additional requests join the queue without starting another conversion. A completed reading is copied before its ready flag is cleared, and the next request cannot start until the current result has been delivered. Spurious or stale notifications do not complete a request unless the device's ready register confirms completion.
Clearing old status and notification precedes
TEMP_START; doing so afterwards could erase a very fast completion. The operating system interrupt binding and persistent notification cover completion in the interval between starting the device and entering the wait. Register accesses must use the platform's device-access semantics, including required volatile-qualified objects in C or ordering primitives. Exact initialization and interrupt-connection call names remain an implementation binding, not invented Phōs API calls. The later teaching site's named system is documented separately; it is not used as proof of an identical historical API.[ref]Digital Systems course archive of the University of Oxford. 2018–2019 course record and linked teaching-site description of process/message-based device drivers
Maintain the latest five temperature readings in a circular buffer, together with their running sum. The following C programming language code spells out the message passing interface rather than assuming a particular library:
send(dst,&m) is a blocking synchronous message passing send; receive(src,&m) receives from the specified sender or ANY and returns the sender's identity; sendrec(dst,&m) atomically sends the request and waits for that sender's reply in the same message object. Successful, reliable operations are assumed. Message contents are copied by the interface.enum { ANY = -1, TIMER = 1, TEMP = 2 };
enum { PULSE, PING, REQUEST, REPLY };
typedef struct {
int type;
int period_ms;
double value;
} Message;
extern void send(int dst, const Message *m);
extern int receive(int src, Message *m);
extern void sendrec(int dst, Message *m);
void smooth(void) {
double history[5] = {0};
double sum = 0;
int count = 0, next = 0;
Message m = { .type = PULSE, .period_ms = 1000 };
send(TIMER, &m); /* subscribe once */
for (;;) {
int sender = receive(count < 5 ? TIMER : ANY, &m);
if (sender == TIMER && m.type == PING) {
Message t = { .type = REQUEST };
sendrec(TEMP, &t); /* matching REPLY contains t.value */
if (count == 5)
sum -= history[next];
else
++count;
history[next] = t.value;
sum += t.value;
next = (next + 1) % 5;
} else if (count == 5 && m.type == REQUEST) {
Message answer = { .type = REPLY, .value = sum / 5.0 };
send(sender, &answer);
}
}
}Before the fifth reading, selective reception from
TIMER prevents premature client requests from being accepted. Such clients wait at their synchronous sends; they are not given a partial arithmetic mean. Thereafter the operating-system process receives both timer messages and client requests. Each accepted PING initiates one measurement; each accepted client request returns the arithmetic mean of the five most recently completed measurements, without initiating another measurement or changing the circular buffer.The loop invariant is that
count is the number of valid readings, capped at five, sum is their sum, and next is the next unused slot during startup or the oldest slot afterwards. Removing the overwritten reading and adding the new one preserves this loop invariant. Each update and reply uses time complexity and storage, since the window length is fixed. Under timely sensor replies, timer delivery and process scheduling, measurements are initiated once per second. The next parts explain failures of these timing assumptions; the code does not promise one-second sampling when the sensor itself cannot sustain it.With a synchronous reply,
send(client,&answer) blocks until the client receives. An indefinitely unready client therefore stalls the smoothing operating-system process, which may in turn stall the timer service and its other clients as in the preceding part. If the client is merely delayed, this is blocking and delayed service; a deadlock requires a circular wait or a client that will never receive.Requiring correctly used synchronous send-and-receive helps. A client calling
sendrec(SMOOTH,&m) is committed to waiting for the matching reply as soon as its request has been accepted. This closes the scheduling window between a separate send and receive and prevents unrelated client computation from postponing that receive. It does not by itself fix slow measurements, arbitrary failures, circular dependencies between other services, or lack of CPU scheduling. With buffered replies the initial effect may instead be queue growth, depending on the message passing contract.In a cooperative scheduler, a permanently runnable background operating-system process that never yields can monopolize the CPU. The timer service, smoothing operating-system process, and display may then fail to run even when their messages are ready. Correct message passing alone does not supply process scheduling.
Use timer-driven scheduling preemption, so an interrupt can force a context switch without cooperation. To give the measurement and display work bounded response despite background computation, use priority scheduling with the background operating-system process below the responsive services, or an explicit reservation policy. Round-robin scheduling gives CPU access among a fixed finite runnable set, but meeting a particular deadline additionally requires bounded service execution and sufficient CPU capacity.
Use a message-passing mutual exclusion server with an A client sends
owner field and a FIFO queue. A request includes its client's identity. The server handles one received message at a time, and a client has at most one outstanding request. The following pseudocode assumes reliable message passing and compliant clients:owner = NONE
waiting = empty FIFO queue
forever:
(kind, client) = receive_from_any_client()
if kind == REQUEST:
waiting.push_back(client)
else if kind == RELEASE:
assert client == owner
owner = NONE
if owner == NONE and not waiting.empty():
owner = waiting.pop_front()
send(owner, GRANT)REQUEST, waits for GRANT, runs its complete animation, and sends RELEASE before making another request.The invariant proving mutual exclusion is: at most one client has received an unreturned grant, and that client is
owner. Only the final block issues a grant, and it first requires no current owner. A valid release removes that ownership. Since clients use the display only while holding a grant, simultaneous use is impossible. The FIFO queue prevents accepted requests from being overtaken by later ones. With eventual message delivery, continued server execution, and finite animations, each accepted request is eventually served; these progress assumptions are separate from the safety invariant. Synchronous message passing 2026-10-25
A message passing discipline in which a send completes only when a matching receive accepts the message. This rendezvous transfers both data and control dependencies: an unready recipient can block a service and thereby delay its other clients. A reply is a separate send unless the interface supplies synchronous send-and-receive.