Polling API
The polling API is used to wait concurrently for any one of multiple conditions to be fulfilled.
Concepts
The polling API’s main function is k_poll(), which is very similar
in concept to the POSIX poll() function, except that it operates on
kernel objects rather than on file descriptors.
The polling API allows a single thread to wait concurrently for one or more conditions to be fulfilled without actively looking at each one individually.
There is a limited set of such conditions:
a semaphore becomes available
a kernel FIFO contains data ready to be retrieved
a kernel message queue contains data ready to be retrieved
a kernel pipe contains data ready to be retrieved
a poll signal is raised
A thread that wants to wait on multiple conditions must define an array of poll events, one for each condition.
All events in the array must be initialized before the array can be polled on.
Each event must specify which type of condition must be satisfied so that its state is changed to signal the requested condition has been met.
Each event must specify what kernel object it wants the condition to be satisfied.
Each event must specify which mode of operation is used when the condition is satisfied.
Each event can optionally specify a tag to group multiple events together, to the user’s discretion.
Apart from the kernel objects, there is also a poll signal pseudo-object type that be directly signaled.
The k_poll() function returns as soon as one of the conditions it
is waiting for is fulfilled. It is possible for more than one to be fulfilled
when k_poll() returns, if they were fulfilled before
k_poll() was called, or due to the preemptive multi-threading
nature of the kernel. The caller must look at the state of all the poll events
in the array to figure out which ones were fulfilled and what actions to take.
Currently, there is only one mode of operation available: the object is not
acquired. As an example, this means that when k_poll() returns and
the poll event states that the semaphore is available, the caller of
k_poll() must then invoke k_sem_take() to take
ownership of the semaphore. If the semaphore is contested, there is no
guarantee that it will be still available when k_sem_take() is
called.
Implementation
Using k_poll()
The main API is k_poll(), which operates on an array of poll events
of type k_poll_event. Each entry in the array represents one
event a call to k_poll() will wait for its condition to be
fulfilled.
Poll events can be initialized using either the runtime initializers
K_POLL_EVENT_INITIALIZER() or k_poll_event_init(), or
the static initializer K_POLL_EVENT_STATIC_INITIALIZER(). An object
that matches the type specified must be passed to the initializers. The
mode must be set to K_POLL_MODE_NOTIFY_ONLY. The state
must be set to K_POLL_STATE_NOT_READY (the initializers take care
of this). The user tag is optional and completely opaque to the API: it is
there to help a user to group similar events together. Being optional, it is
passed to the static initializer, but not the runtime ones for performance
reasons. If using runtime initializers, the user must set it separately in the
k_poll_event data structure. If an event in the array is to be
ignored, most likely temporarily, its type can be set to
K_POLL_TYPE_IGNORE.
struct k_poll_event events[4] = {
K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_SEM_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_sem, 0),
K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_FIFO_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_fifo, 0),
K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_MSGQ_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_msgq, 0),
K_POLL_EVENT_STATIC_INITIALIZER(K_POLL_TYPE_PIPE_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_pipe, 0),
};
or at runtime
struct k_poll_event events[4];
void some_init(void)
{
k_poll_event_init(&events[0],
K_POLL_TYPE_SEM_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_sem);
k_poll_event_init(&events[1],
K_POLL_TYPE_FIFO_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_fifo);
k_poll_event_init(&events[2],
K_POLL_TYPE_MSGQ_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_msgq);
k_poll_event_init(&events[3],
K_POLL_TYPE_PIPE_DATA_AVAILABLE,
K_POLL_MODE_NOTIFY_ONLY,
&my_pipe);
// tags are left uninitialized if unused
}
After the events are initialized, the array can be passed to
k_poll(). A timeout can be specified to wait only for a specified
amount of time, or the special values K_NO_WAIT and
K_FOREVER to either not wait or wait until an event condition is
satisfied and not sooner.
A list of pollers is offered on each semaphore or FIFO and as many events can wait in it as the app wants. Notice that the waiters will be served in first-come-first-serve order, not in priority order.
In case of success, k_poll() returns 0. If it times out, it returns
-EAGAIN.
// assume there is no contention on this semaphore and FIFO
// -EADDRINUSE will not occur; the semaphore and/or data will be available
void do_stuff(void)
{
rc = k_poll(events, ARRAY_SIZE(events), K_MSEC(1000));
if (rc == 0) {
if (events[0].state == K_POLL_STATE_SEM_AVAILABLE) {
k_sem_take(events[0].sem, 0);
} else if (events[1].state == K_POLL_STATE_FIFO_DATA_AVAILABLE) {
data = k_fifo_get(events[1].fifo, 0);
// handle data
} else if (events[2].state == K_POLL_STATE_MSGQ_DATA_AVAILABLE) {
ret = k_msgq_get(events[2].msgq, buf, K_NO_WAIT);
// handle data
} else if (events[3].state == K_POLL_STATE_PIPE_DATA_AVAILABLE) {
ret = k_pipe_get(events[3].pipe, buf, bytes_to_read, &bytes_read, min_xfer, K_NO_WAIT);
// handle data
}
} else {
// handle timeout
}
}
When k_poll() is called in a loop, the events state must be reset
to K_POLL_STATE_NOT_READY by the user.
void do_stuff(void)
{
for(;;) {
rc = k_poll(events, ARRAY_SIZE(events), K_FOREVER);
if (events[0].state == K_POLL_STATE_SEM_AVAILABLE) {
k_sem_take(events[0].sem, 0);
}
if (events[1].state == K_POLL_STATE_FIFO_DATA_AVAILABLE) {
data = k_fifo_get(events[1].fifo, 0);
// handle data
}
if (events[2].state == K_POLL_STATE_MSGQ_DATA_AVAILABLE) {
ret = k_msgq_get(events[2].msgq, buf, K_NO_WAIT);
// handle data
}
if (events[3].state == K_POLL_STATE_PIPE_DATA_AVAILABLE) {
ret = k_pipe_get(events[3].pipe, buf, bytes_to_read, &bytes_read, min_xfer, K_NO_WAIT);
// handle data
}
events[0].state = K_POLL_STATE_NOT_READY;
events[1].state = K_POLL_STATE_NOT_READY;
events[2].state = K_POLL_STATE_NOT_READY;
events[3].state = K_POLL_STATE_NOT_READY;
}
}
Using k_poll_signal_raise()
One of the types of events is K_POLL_TYPE_SIGNAL: this is a “direct”
signal to a poll event. This can be seen as a lightweight binary semaphore only
one thread can wait for.
A poll signal is a separate object of type k_poll_signal that
must be attached to a k_poll_event, similar to a semaphore or FIFO. It must
first be initialized either via K_POLL_SIGNAL_INITIALIZER() or
k_poll_signal_init().
struct k_poll_signal signal;
void do_stuff(void)
{
k_poll_signal_init(&signal);
}
It is signaled via the k_poll_signal_raise() function. This function
takes a user result parameter that is opaque to the API and can be used to
pass extra information to the thread waiting on the event.
struct k_poll_signal signal;
// thread A
void do_stuff(void)
{
k_poll_signal_init(&signal);
struct k_poll_event events[1] = {
K_POLL_EVENT_INITIALIZER(K_POLL_TYPE_SIGNAL,
K_POLL_MODE_NOTIFY_ONLY,
&signal),
};
k_poll(events, 1, K_FOREVER);
int signaled, result;
k_poll_signal_check(&signal, &signaled, &result);
if (signaled && (result == 0x1337)) {
// A-OK!
} else {
// weird error
}
}
// thread B
void signal_do_stuff(void)
{
k_poll_signal_raise(&signal, 0x1337);
}
If the signal is to be polled in a loop, both its event state must be
reset to K_POLL_STATE_NOT_READY and its result must be
reset using k_poll_signal_reset() on each iteration if it has
been signaled.
struct k_poll_signal signal;
void do_stuff(void)
{
k_poll_signal_init(&signal);
struct k_poll_event events[1] = {
K_POLL_EVENT_INITIALIZER(K_POLL_TYPE_SIGNAL,
K_POLL_MODE_NOTIFY_ONLY,
&signal),
};
for (;;) {
k_poll(events, 1, K_FOREVER);
int signaled, result;
k_poll_signal_check(&signal, &signaled, &result);
if (signaled && (result == 0x1337)) {
// A-OK!
} else {
// weird error
}
k_poll_signal_reset(signal);
events[0].state = K_POLL_STATE_NOT_READY;
}
}
Note that poll signals are not internally synchronized. A k_poll() call
that is passed a signal will return after any code in the system calls
k_poll_signal_raise(). But if the signal is being
externally managed and reset via k_poll_signal_init(), it is
possible that by the time the application checks, the event state may
no longer be equal to K_POLL_STATE_SIGNALED, and a (naive)
application will miss events. Best practice is always to reset the
signal only from within the thread invoking the k_poll() loop, or else
to use some other event type which tracks event counts: semaphores and
FIFOs are more error-proof in this sense because they can’t “miss”
events, architecturally.
Suggested Uses
Use k_poll() to consolidate multiple threads that would be pending
on one object each, saving possibly large amounts of stack space.
Use a poll signal as a lightweight binary semaphore if only one thread pends on it.
Note
Because objects are only signaled if no other thread is waiting for them to become available and only one thread can poll on a specific object, polling is best used when objects are not subject of contention between multiple threads, basically when a single thread operates as a main “server” or “dispatcher” for multiple objects and is the only one trying to acquire these objects.
Configuration Options
Related configuration options: