Zephyr message bus (zbus)

The Zephyr message bus - Zbus is a lightweight and flexible message bus enabling a simple way for threads to talk to one another.

Concepts 

Threads can broadcast messages to all interested observers using zbus. Many-to-many communication is possible. The bus implements message-passing and publish/subscribe communication paradigms that enable threads to communicate synchronously or asynchronously through shared memory. The communication through zbus is channel-based, where threads publish and read to and from using messages. Additionally, threads can observe channels and receive notifications from the bus when the channels are modified. The figure below shows an example of a typical application using zbus in which the application logic (hardware independent) talks to other threads via message bus. Note that the threads are decoupled from each other because they only use zbus’ channels and do not need to know each other to talk.

zbus usage overview — A typical zbus application architecture.

The bus comprises:

Set of channels that consists of a unique identifier, its control metadata information, and the message itself;
Virtual distributed event dispatcher (VDED), the bus logic responsible for sending notifications to the observers. The VDED logic runs inside the publishing action in the same thread context, giving the bus an idea of a distributed execution. When a thread publishes to a channel, it also propagates the notifications to the observers;
Threads (subscribers) and callbacks (listeners) publishing, reading, and receiving notifications from the bus.

The bus makes the publish, read, and subscribe actions available over channels. Publishing and reading are available in all RTOS thread contexts. However, it cannot run inside Interrupt Service Routines (ISR) because it uses mutexes to control channels access, and mutexes cannot work appropriately inside ISRs. The publish and read operations are simple and fast; the procedure is a mutex locking followed by a memory copy to and from a shared memory region and then a mutex unlocking. Another essential aspect of zbus is the observers, which can be:

Static; defined in compile time. It is not possible to remove it at runtime, but it is possible to suppress it by calling the zbus_obs_set_enable();
Dynamic; it can be added and removed to and from a channel at runtime.

For illustration purposes, suppose a usual sensor-based solution in the figure below. When the timer is triggered, it pushes an action to a work queue that publishes to the Start trigger channel. As the sensor thread subscribed to the Start trigger channel, it fetches the sensor data. Notice the VDED executes the blink callback because it also listens to the Start trigger channel. When the sensor data is ready, the sensor thread publishes it to the Sensor data channel. The core thread, as a Sensor data channel subscriber, processes the sensor data and stores it in an internal sample buffer. It repeats until the sample buffer is full; when it happens, the core thread aggregates the sample buffer information, prepares a package, and publishes that to the Payload channel. The Lora thread receives that because it is a Payload channel subscriber and sends the payload to the cloud. When it completes the transmission, the Lora thread publishes to the Transmission done channel. The VDED executes the blink callback again since it listens to the Transmission done channel.

This way of implementing the solution makes the application more flexible, enabling us to change things independently. For example, we want to change the trigger from a timer to a button press. We can do that, and the change does not affect other parts of the system. Likewise, we would like to change the communication interface from LoRa to Bluetooth; we only need to change the LoRa thread. No other change is required in order to make that work. Thus, the developer would do that for every block of the image. Based on that, there is a sign zbus promotes decoupling in the system architecture.

Another important aspect of using zbus is the reuse of system modules. If a code portion with well-defined behaviors (we call that module) only uses zbus channels and not hardware interfaces, it can easily be reused in other solutions. The new solution must implement the interfaces (set of channels) the module needs to work. That indicates zbus could improve the module reuse.

The last important note is the zbus solution reach. We can count on many ways of using zbus to enable the developer to be as free as possible to create what they need. For example, messages can be dynamic or static allocated; notifications can be synchronous or asynchronous; the developer can control the channel in so many different ways claiming the channel, developers can add their metadata information to a channel by using the user-data field, the discretionary use of a validator enables the systems to be accurate over message format, and so on. Those characteristics increase the solutions that can be done with zbus and make it a good fit as an open-source community tool.

Virtual Distributed Event Dispatcher 

The VDED execution always happens in the publishing’s (thread) context. So it cannot occur inside an Interrupt Service Routine (ISR). Therefore, the IRSs must only access channels indirectly. The basic description of the execution is as follows:

The channel mutex is acquired;
The channel receives the new message via direct copy (by a raw memcpy());
The event dispatcher logic executes the listeners and pushes the channel’s reference to the subscribers’ notification message queue in the same sequence they appear on the channel observers’ list. The listeners can perform non-copy quick access to the constant message reference directly (via the zbus_chan_const_msg() function) since the channel is still locked;
At last, the publishing function unlocks the channel.

To illustrate the VDED execution, consider the example illustrated below. We have four threads in ascending priority T1, T2, T3, and T4 (the highest priority); two listeners, L1 and L2; and channel A. Supposing L1, L2, T2, T3, and T4 observer channel A.

Zbus example scenario — Zbus VDED execution example scenario.

The following code implements channel A. Note the struct a_msg is illustrative only.

ZBUS_CHAN_DEFINE(a_chan,                     /* Name */
         struct a_msg,                       /* Message type */

         NULL,                               /* Validator */
         NULL,                               /* User Data */
         ZBUS_OBSERVERS(L1, L2, T2, T3, T4), /* observers */
         ZBUS_MSG_INIT(0)                    /* Initial value {0} */
);

In the figure below, the letters indicate some action related to the VDED execution. The X-axis represents the time, and the Y-axis represents the priority of threads. Channel A’s message, represented by a voice balloon, is only one memory portion (shared memory). It appears several times only as an illustration of the message at that point in time.

Zbus publish processing detail — Zbus VDED execution detail.

The figure above illustrates the actions performed during the VDED execution when T1 publishes to channel A. Thus, the figure below describes the actions (represented by a letter) of the VDED execution.

VDED execution steps in detail.
Actions	Description
a	T1 starts and at some point, publishes to channel A.
b	The publishing (VDED) process starts. The VDED locks the channel A’s mutex.
c	The VDED copies the T1 message to the channel A message.
d, e	The VDED executes L1 and L2 in the respective sequence. Inside the listeners, usually, there is a call to the `zbus_chan_const_msg()` function, which provides a direct constant reference to channel A’s message. It is quick, and no copy is needed here.
f, g, h	The VDED pushes the notification message to queues of T2, T3, and T4 sequentially. Notice the threads get ready to execute right after receiving the notification. However, they go to a pending state because they cannot access the channel since it is still locked. At that moment, the T1 thread gets its priority elevated (priority inheritance due to the mutex) to the highest pending thread (caused by channel A unavailability). In that case, T4’s priority. It ensures the T1 will finish the VDED execution as quickly as possible without preemption from threads with priority below the engaged ones.
i	VDED finishes the publishing by unlocking channel A.
j, k	The T4 leaves the pending state since channel A is not locked. It gets in the CPU again and starts executing. As it did receive a notification from channel A, it performs a channel read (as simple as lock, memory copy, unlock), continues its execution, and goes out the CPU.
l,m, n	Now, T3 can access the channel. It repeats the same steps from T4 (j and k). T2 does the same. That is the end of the VDED execution!

Limitations 

Based on the fact that developers can use zbus to solve many different problems, some challenges arise. Zbus will not solve every problem, so it is necessary to analyze the situation to be sure zbus is applicable. For instance, based on the zbus benchmark, it would not be well suited to a high-speed stream of bytes between threads. The Pipe kernel object solves this kind of need.

Delivery guarantees

Zbus always delivers the messages to the listeners. However, there are no message delivery guarantees for subscribers because zbus only sends the notification, but the message reading depends on the subscriber’s implementation. This is because channels have a mutex protected singleton objects for which message transfer is used. In other words, it can be seen as a single size queue where publishers always overwrite if queue is full. It is possible to increase the delivery rate by following design tips:

Keep the listeners quick-as-possible (deal with them as ISRs). If some processing is needed, consider submitting a work to a work-queue;
Try to give producers a high priority to avoid losses;
Leave spare CPU for observers to consume data produced;
Consider using message queues or pipes for intensive byte transfers.

Message delivery sequence

The listeners (synchronous observers) will follow the channel definition sequence as the notification and message consumption sequence. However, the subscribers, as they have an asynchronous nature, will all receive the notification as the channel definition sequence but only will consume the data when they execute again. Hence, the delivery respects the order, but the priority assigned to the subscribers will define the reaction sequence. The delivery sequence is first the static observers and then the runtime ones.

Usage 

Zbus operation depends on channels and observers. Therefore, it is necessary to determine its message and observers list during the channel definition. A message is a regular C struct; the observer can be a subscriber (asynchronous) or a listener (synchronous).

The following code defines and initializes a regular channel and its dependencies. This channel exchanges accelerometer data, for example.

struct acc_msg {
        int x;
        int y;
        int z;
};

ZBUS_CHAN_DEFINE(acc_chan,                           /* Name */
         struct acc_msg,                             /* Message type */

         NULL,                                       /* Validator */
         NULL,                                       /* User Data */
         ZBUS_OBSERVERS(my_listener, my_subscriber), /* observers */
         ZBUS_MSG_INIT(.x = 0, .y = 0, .z = 0)       /* Initial value */
);

void listener_callback_example(const struct zbus_channel *chan)
{
        const struct acc_msg *acc;
        if (&acc_chan == chan) {
                acc = zbus_chan_const_msg(chan); // Direct message access
                LOG_DBG("From listener -> Acc x=%d, y=%d, z=%d", acc->x, acc->y, acc->z);
        }
}

ZBUS_LISTENER_DEFINE(my_listener, listener_callback_example);

ZBUS_LISTENER_DEFINE(my_listener2, listener_callback_example);

ZBUS_CHAN_ADD_OBS(acc_chan, my_listener2, 3);

ZBUS_SUBSCRIBER_DEFINE(my_subscriber, 4);
void subscriber_task(void)
{
        const struct zbus_channel *chan;

        while (!zbus_sub_wait(&my_subscriber, &chan, K_FOREVER)) {
                struct acc_msg acc = {0};

                if (&acc_chan == chan) {
                        // Indirect message access
                        zbus_chan_read(&acc_chan, &acc, K_NO_WAIT);
                        LOG_DBG("From subscriber -> Acc x=%d, y=%d, z=%d", acc.x, acc.y, acc.z);
                }
        }
}
K_THREAD_DEFINE(subscriber_task_id, 512, subscriber_task, NULL, NULL, NULL, 3, 0, 0);

It is possible to add static observers to a channel using the ZBUS_CHAN_ADD_OBS. We call that a post-definition static observer. The command enables us to indicate an initialization priority that affects the observers’ initialization order. The priority param only affects the post-definition static observers. There is no possibility to overwrite the execution sequence of the static observers.

Note

It is unnecessary to claim/lock a channel before accessing the message inside the listener since the event dispatcher calls listeners with the notifying channel already locked. Subscribers, however, must claim/lock that or use regular read operations to access the message after being notified.

Channels can have a validator function that enables a channel to accept only valid messages. Publish attempts invalidated by hard channels will return immediately with an error code. This allows original creators of a channel to exert some authority over other developers/publishers who may want to piggy-back on their channels. The following code defines and initializes a hard channel and its dependencies. Only valid messages can be published to a hard channel. It is possible because a validator function was passed to the channel’s definition. In this example, only messages with move equal to 0, -1, and 1 are valid. Publish function will discard all other values to move.

struct control_msg {
        int move;
};

bool control_validator(const void* msg, size_t msg_size) {
        const struct control_msg* cm = msg;
        bool is_valid = (cm->move == -1) || (cm->move == 0) || (cm->move == 1);
        return is_valid;
}

static int message_count = 0;

ZBUS_CHAN_DEFINE(control_chan,    /* Name */
         struct control_msg,      /* Message type */

         control_validator,       /* Validator */
         &message_count,          /* User data */
         ZBUS_OBSERVERS_EMPTY,    /* observers */
         ZBUS_MSG_INIT(.move = 0) /* Initial value */
);

The following sections describe in detail how to use zbus features.

Publishing to a channel 

Messages are published to a channel in zbus by calling zbus_chan_pub(). For example, the following code builds on the examples above and publishes to channel acc_chan. The code is trying to publish the message acc1 to channel acc_chan, and it will wait up to one second for the message to be published. Otherwise, the operation fails. As can be inferred from the code sample, it’s OK to use stack allocated messages since VDED copies the data internally.

struct acc_msg acc1 = {.x = 1, .y = 1, .z = 1};
zbus_chan_pub(&acc_chan, &acc1, K_SECONDS(1));

Warning

Do not use this function inside an ISR.

Reading from a channel 

Messages are read from a channel in zbus by calling zbus_chan_read(). So, for example, the following code tries to read the channel acc_chan, which will wait up to 500 milliseconds to read the message. Otherwise, the operation fails.

struct acc_msg acc = {0};
zbus_chan_read(&acc_chan, &acc, K_MSEC(500));

Warning

Do not use this function inside an ISR.

Warning

Choose the timeout of zbus_chan_read() after receiving a notification from zbus_sub_wait() carefully because the channel will always be unavailable during the VDED execution. Using K_NO_WAIT for reading is highly likely to return a timeout error if there are more than one subscriber. For example, consider the VDED illustration again and notice how T3 and T4's read attempts would definitely fail with K_NO_WAIT. For more details, check the Virtual Distributed Event Dispatcher section.

Forcing channel notification 

It is possible to force zbus to notify a channel’s observers by calling zbus_chan_notify(). For example, the following code builds on the examples above and forces a notification for the channel acc_chan. Note this can send events with no message, which does not require any data exchange. See the code example under Claim and finish a channel where this may become useful.

zbus_chan_notify(&acc_chan, K_NO_WAIT);

Warning

Do not use this function inside an ISR.

Declaring channels and observers 

For accessing channels or observers from files other than its defining files, it is necessary to declare them by calling ZBUS_CHAN_DECLARE and ZBUS_OBS_DECLARE. In other words, zbus channel definitions and declarations with the same channel names in different files would point to the same (global) channel. Thus, developers should be careful about existing channels, and naming new channels or linking will fail. It is possible to declare more than one channel or observer on the same call. The following code builds on the examples above and displays the defined channels and observers.

ZBUS_OBS_DECLARE(my_listener, my_subscriber);
ZBUS_CHAN_DECLARE(acc_chan, version_chan);

Iterating over channels and observers 

Zbus subsystem also implements Iterable Sections for channels and observers, for which there are supporting APIs like zbus_iterate_over_channels(), zbus_iterate_over_channels_with_user_data(), zbus_iterate_over_observers() and zbus_iterate_over_observers_with_user_data(). This feature enables developers to call a procedure over all declared channels, where the procedure parameter is a zbus_channel. The execution sequence is in the alphabetical name order of the channels (see Iterable Sections documentation for details). Zbus also implements this feature for zbus_observer.

static bool print_channel_data_iterator(const struct zbus_channel *chan, void *user_data)
{
      int *count = user_data;

      LOG_INF("%d - Channel %s:", *count, zbus_chan_name(chan));
      LOG_INF("      Message size: %d", zbus_chan_msg_size(chan));
      LOG_INF("      Observers:");

      ++(*count);

      struct zbus_channel_observation *observation;

      for (int16_t i = *chan->observers_start_idx, limit = *chan->observers_end_idx; i < limit;
            ++i) {
            STRUCT_SECTION_GET(zbus_channel_observation, i, &observation);

            LOG_INF("      - %s", observation->obs->name);
      }

      struct zbus_observer_node *obs_nd, *tmp;

      SYS_SLIST_FOR_EACH_CONTAINER_SAFE(chan->observers, obs_nd, tmp, node) {
            LOG_INF("      - %s", obs_nd->obs->name);
      }

      return true;
}

static bool print_observer_data_iterator(const struct zbus_observer *obs, void *user_data)
{
      int *count = user_data;

      LOG_INF("%d - %s %s", *count, obs->queue ? "Subscriber" : "Listener", zbus_obs_name(obs));

      ++(*count);

      return true;
}

int main(void)
{
      int count = 0;

      LOG_INF("Channel list:");

      zbus_iterate_over_channels_with_user_data(print_channel_data_iterator, &count);

      count = 0;

      LOG_INF("Observers list:");

      zbus_iterate_over_observers_with_user_data(print_observer_data_iterator, &count);

      return 0;
}

The code will log the following output:

D: Channel list:
D: 0 - Channel acc_chan:
D:       Message size: 12
D:       Observers:
D:       - my_listener
D:       - my_subscriber
D: 1 - Channel version_chan:
D:       Message size: 4
D:       Observers:
D: Observers list:
D: 0 - Listener my_listener
D: 1 - Subscriber my_subscriber

Advanced channel control 

Zbus was designed to be as flexible and extensible as possible. Thus, there are some features designed to provide some control and extensibility to the bus.

Listeners message access

For performance purposes, listeners can access the receiving channel message directly since they already have the mutex lock for it. To access the channel’s message, the listener should use the zbus_chan_const_msg() because the channel passed as an argument to the listener function is a constant pointer to the channel. The const pointer return type tells developers not to modify the message.

void listener_callback_example(const struct zbus_channel *chan)
{
        const struct acc_msg *acc;
        if (&acc_chan == chan) {
                acc = zbus_chan_const_msg(chan); // Use this
                // instead of zbus_chan_read(chan, &acc, K_MSEC(200))
                // or zbus_chan_msg(chan)

                LOG_DBG("From listener -> Acc x=%d, y=%d, z=%d", acc->x, acc->y, acc->z);
        }
}

User Data

It is possible to pass custom data into the channel’s user_data for various purposes, such as writing channel metadata. That can be achieved by passing a pointer to the channel definition macro’s user_data field, which will then be accessible by others. Note that user_data is individual for each channel. Also, note that user_data access is not thread-safe. For thread-safe access to user_data, see the next section.

Claim and finish a channel

To take more control over channels, two functions were added zbus_chan_claim() and zbus_chan_finish(). With these functions, it is possible to access the channel’s metadata safely. When a channel is claimed, no actions are available to that channel. After finishing the channel, all the actions are available again.

Warning

Never change the fields of the channel struct directly. It may cause zbus behavior inconsistencies and scheduling issues.

Warning

Do not use these functions inside an ISR.

The following code builds on the examples above and claims the acc_chan to set the user_data to the channel. Suppose we would like to count how many times the channels exchange messages. We defined the user_data to have the 32 bits integer. This code could be added to the listener code described above.

if (!zbus_chan_claim(&acc_chan, K_MSEC(200))) {
        int *message_counting = (int *) zbus_chan_user_data(&acc_chan);
        *message_counting += 1;
        zbus_chan_finish(&acc_chan);
}

The following code has the exact behavior of the code in Publishing to a channel.

if (!zbus_chan_claim(&acc_chan, K_MSEC(200))) {
        struct acc_msg *acc1 = (struct acc_msg *) zbus_chan_msg(&acc_chan);
        acc1.x = 1;
        acc1.y = 1;
        acc1.z = 1;
        zbus_chan_finish(&acc_chan);
        zbus_chan_notify(&acc_chan, K_SECONDS(1));
}

The following code has the exact behavior of the code in Reading from a channel.

if (!zbus_chan_claim(&acc_chan, K_MSEC(200))) {
        const struct acc_msg *acc1 = (const struct acc_msg *) zbus_chan_const_msg(&acc_chan);
        // access the acc_msg fields directly.
        zbus_chan_finish(&acc_chan);
}

Runtime observer registration

It is possible to add observers to channels in runtime. This feature uses the heap to allocate the nodes dynamically. The heap size limits the number of dynamic observers Zbus can create. Therefore, set the CONFIG_ZBUS_RUNTIME_OBSERVERS to enable the feature. It is possible to adjust the heap size by changing the configuration CONFIG_HEAP_MEM_POOL_SIZE. The following example illustrates the runtime registration usage.

ZBUS_LISTENER_DEFINE(my_listener, callback);
// ...
void thread_entry(void) {
        // ...
        /* Adding the observer to channel chan1 */
        zbus_chan_add_obs(&chan1, &my_listener);
        /* Removing the observer from channel chan1 */
        zbus_chan_rm_obs(&chan1, &my_listener);

Samples 

For a complete overview of zbus usage, take a look at the samples. There are the following samples available:

Hello world sample illustrates the code used above in action;
Workqueue sample shows how to define and use different kinds of observers. Note there is an example of using a work queue instead of executing the listener as an execution option;
Dynamic channel sample demonstrates how to use dynamically allocated exchanging data in zbus;
UART bridge sample shows an example of sending the operation of the channel to a host via serial;
Remote mock sample illustrates how to implement an external mock (on the host) to send and receive messages to and from the bus.
Runtime observer registration sample illustrates a way of using the runtime observer registration feature;
Benchmark sample implements a benchmark with different combinations of inputs.

Suggested Uses 

Use zbus to transfer data (messages) between threads in one-to-one, one-to-many, and many-to-many synchronously or asynchronously. Choosing the proper observer type is crucial. Use subscribers for scenarios that can tolerate message losses and duplications; when they cannot, use listeners. In addition to the listener, another asynchronous message processing mechanism (like message queues) may be necessary to retain the pending message until it gets processed.

Note

Zbus can be used to transfer streams from the producer to the consumer. However, this can increase zbus’ communication latency. So maybe consider a Pipe a good alternative for this communication topology.

Configuration Options 

For enabling zbus, it is necessary to enable the CONFIG_ZBUS option.

Related configuration options:

CONFIG_ZBUS_CHANNEL_NAME enables the name of channels to be available inside the channels metadata. The log uses this information to show the channels’ names;
CONFIG_ZBUS_OBSERVER_NAME enables the name of observers to be available inside the channels metadata;
CONFIG_ZBUS_RUNTIME_OBSERVERS enables the runtime observer registration.
CONFIG_ZBUS_CHANNELS_SYS_INIT_PRIORITY determine the SYS_INIT priority used by Zbus to organize the channels observations by channel.

API Reference 

group zbus_apis

Zbus API.

Defines

ZBUS_CHAN_ADD_OBS_WITH_MASK(_chan, _obs, _masked, _prio)

Add a static channel observervation.

This macro initializes a channel observation by receiving the channel and the observer.

Parameters:

_chan – Channel instance.
_obs – Observer instance.
_masked – Observation state.
_prio – Observer notification sequence priority.

ZBUS_CHAN_ADD_OBS(_chan, _obs, _prio)

Add a static channel observervation.

This macro initializes a channel observation by receiving the channel and the observer.

Parameters:

_chan – Channel instance.
_obs – Observer instance.
_prio – Observer notification sequence priority.

ZBUS_OBS_DECLARE(...)

This macro list the observers to be used in a file.

Internally, it declares the observers with the extern statement. Note it is only necessary when the observers are declared outside the file.

ZBUS_CHAN_DECLARE(...)

This macro list the channels to be used in a file.

Internally, it declares the channels with the extern statement. Note it is only necessary when the channels are declared outside the file.

ZBUS_OBSERVERS_EMPTY: This macro indicates the channel has no observers.

ZBUS_OBSERVERS(...)

This macro indicates the channel has listed observers.

Note the sequence of observer notification will follow the same as listed.

ZBUS_CHAN_DEFINE(_name, _type, _validator, _user_data, _observers, _init_val)

Zbus channel definition.

This macro defines a channel.

See also

struct zbus_channel

Parameters:

_name – The channel’s name.
_type – The Message type. It must be a struct or union.
_validator – The validator function.
_user_data – A pointer to the user data.
_observers – The observers list. The sequence indicates the priority of the observer. The first the highest priority.
_init_val – The message initialization.

ZBUS_MSG_INIT(_val, ...)

Initialize a message.

This macro initializes a message by passing the values to initialize the message struct or union.

Parameters:

_val – [in] Variadic with the initial values. ZBUS_INIT(0) means {0}, as ZBUS_INIT(.a=10, .b=30) means {.a=10, .b=30}.

ZBUS_SUBSCRIBER_DEFINE_WITH_ENABLE(_name, _queue_size, _enable)

Define and initialize a subscriber.

This macro defines an observer of subscriber type. It defines a message queue where the subscriber will receive the notification asynchronously, and initialize the struct zbus_observer defining the subscriber.

Parameters:

_name – [in] The subscriber’s name.
_queue_size – [in] The notification queue’s size.
_enable – [in] The subscriber initial enable state.

ZBUS_SUBSCRIBER_DEFINE(_name, _queue_size)

Define and initialize a subscriber.

This macro defines an observer of subscriber type. It defines a message queue where the subscriber will receive the notification asynchronously, and initialize the struct zbus_observer defining the subscriber. The subscribers are defined in the enabled state with this macro.

Parameters:

_name – [in] The subscriber’s name.
_queue_size – [in] The notification queue’s size.

ZBUS_LISTENER_DEFINE_WITH_ENABLE(_name, _cb, _enable)

Define and initialize a listener.

This macro defines an observer of listener type. This macro establishes the callback where the listener will be notified synchronously, and initialize the struct zbus_observer defining the listener.

Parameters:

_name – [in] The listener’s name.
_cb – [in] The callback function.
_enable – [in] The listener initial enable state.

ZBUS_LISTENER_DEFINE(_name, _cb)

Define and initialize a listener.

This macro defines an observer of listener type. This macro establishes the callback where the listener will be notified synchronously and initialize the struct zbus_observer defining the listener. The listeners are defined in the enabled state with this macro.

Parameters:

_name – [in] The listener’s name.
_cb – [in] The callback function.

Enums

enum zbus_observer_type

Type used to represent an observer type.

A observer can be a listener or a subscriber.

Values:

enumerator ZBUS_OBSERVER_LISTENER_TYPE

enumerator ZBUS_OBSERVER_SUBSCRIBER_TYPE

Functions

int zbus_chan_pub(const struct zbus_channel *chan, const void *msg, k_timeout_t timeout)

Publish to a channel.

This routine publishes a message to a channel.

Parameters:

chan – The channel’s reference.
msg – Reference to the message where the publish function copies the channel’s message data from.
timeout – Waiting period to publish the channel, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Channel published.
-ENOMSG – The message is invalid based on the validator function or some of the observers could not receive the notification.
-EBUSY – The channel is busy.
-EAGAIN – Waiting period timed out.
-EFAULT – A parameter is incorrect, the notification could not be sent to one or more observer, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

int zbus_chan_read(const struct zbus_channel *chan, void *msg, k_timeout_t timeout)

Read a channel.

This routine reads a message from a channel.

Parameters:

chan – [in] The channel’s reference.
msg – [out] Reference to the message where the read function copies the channel’s message data to.
timeout – [in] Waiting period to read the channel, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Channel read.
-EBUSY – The channel is busy.
-EAGAIN – Waiting period timed out.
-EFAULT – A parameter is incorrect, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

int zbus_chan_claim(const struct zbus_channel *chan, k_timeout_t timeout)

Claim a channel.

This routine claims a channel. During the claiming period the channel is blocked for publishing, reading, notifying or claiming again. Finishing is the only available action.

Warning

After calling this routine, the channel cannot be used by other thread until the zbus_chan_finish routine is performed.

Warning

This routine should only be called once before a zbus_chan_finish.

Parameters:

chan – [in] The channel’s reference.
timeout – [in] Waiting period to claim the channel, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Channel claimed.
-EBUSY – The channel is busy.
-EAGAIN – Waiting period timed out.
-EFAULT – A parameter is incorrect, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

int zbus_chan_finish(const struct zbus_channel *chan)

Finish a channel claim.

This routine finishes a channel claim. After calling this routine with success, the channel will be able to be used by other thread.

Warning

This routine must only be used after a zbus_chan_claim.

Parameters:

chan – The channel’s reference.

Return values:

0 – Channel finished.
-EPERM – The channel was claimed by other thread.
-EINVAL – The channel’s mutex is not locked.
-EFAULT – A parameter is incorrect, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

int zbus_chan_notify(const struct zbus_channel *chan, k_timeout_t timeout)

Force a channel notification.

This routine forces the event dispatcher to notify the channel’s observers even if the message has no changes. Note this function could be useful after claiming/finishing actions.

Parameters:

chan – The channel’s reference.
timeout – Waiting period to notify the channel, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Channel notified.
-EPERM – The current thread does not own the channel.
-EBUSY – The channel’s mutex returned without waiting.
-EAGAIN – Timeout to acquiring the channel’s mutex.
-EFAULT – A parameter is incorrect, the notification could not be sent to one or more observer, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

static inline const char *zbus_chan_name(const struct zbus_channel *chan)

Get the channel’s name.

This routine returns the channel’s name reference.

Parameters:

chan – The channel’s reference.

Returns:

Channel’s name reference.

static inline void *zbus_chan_msg(const struct zbus_channel *chan)

Get the reference for a channel message directly.

This routine returns the reference of a channel message.

Warning

This function must only be used directly for acquired (locked by mutex) channels. This can be done inside a listener for the receiving channel or after claim a channel.

Parameters:

chan – The channel’s reference.

Returns:

Channel’s message reference.

static inline const void *zbus_chan_const_msg(const struct zbus_channel *chan)

Get a constant reference for a channel message directly.

This routine returns a constant reference of a channel message. This should be used inside listeners to access the message directly. In this way zbus prevents the listener of changing the notifying channel’s message during the notification process.

Warning

This function must only be used directly for acquired (locked by mutex) channels. This can be done inside a listener for the receiving channel or after claim a channel.

Parameters:

chan – The channel’s constant reference.

Returns:

A constant channel’s message reference.

static inline uint16_t zbus_chan_msg_size(const struct zbus_channel *chan)

Get the channel’s message size.

This routine returns the channel’s message size.

Parameters:

chan – The channel’s reference.

Returns:

Channel’s message size.

static inline void *zbus_chan_user_data(const struct zbus_channel *chan)

Get the channel’s user data.

This routine returns the channel’s user data.

Parameters:

chan – The channel’s reference.

Returns:

Channel’s user data.

int zbus_chan_add_obs(const struct zbus_channel *chan, const struct zbus_observer *obs, k_timeout_t timeout)

Add an observer to a channel.

This routine adds an observer to the channel.

Parameters:

chan – The channel’s reference.
obs – The observer’s reference to be added.
timeout – Waiting period to add an observer, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Observer added to the channel.
-EALREADY – The observer is already present in the channel’s runtime observers list.
-ENOMEM – Returned without waiting.
-EAGAIN – Waiting period timed out.
-EINVAL – Some parameter is invalid.

int zbus_chan_rm_obs(const struct zbus_channel *chan, const struct zbus_observer *obs, k_timeout_t timeout)

Remove an observer from a channel.

This routine removes an observer to the channel.

Parameters:

chan – The channel’s reference.
obs – The observer’s reference to be removed.
timeout – Waiting period to remove an observer, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Observer removed to the channel.
-EINVAL – Invalid data supplied.
-EBUSY – Returned without waiting.
-EAGAIN – Waiting period timed out.
-ENODATA – no observer found in channel’s runtime observer list.
-ENOMEM – Returned without waiting.

static inline int zbus_obs_set_enable(struct zbus_observer *obs, bool enabled)

Change the observer state.

This routine changes the observer state. A channel when disabled will not receive notifications from the event dispatcher.

Parameters:

obs – [in] The observer’s reference.
enabled – [in] State to be. When false the observer stops to receive notifications.

Return values:

0 – Observer set enable.
-EFAULT – A parameter is incorrect, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

static inline int zbus_obs_is_enabled(struct zbus_observer *obs, bool *enable)

Get the observer state.

This routine retrieves the observer state.

Parameters:

obs – [in] The observer’s reference.
enable – [out] The boolean output’s reference.

Returns:

Observer state.

int zbus_obs_set_chan_notification_mask(const struct zbus_observer *obs, const struct zbus_channel *chan, bool masked)

Mask notifications from a channel to an observer.

The observer can mask notifications from a specific observing channel by calling this function.

Parameters:

obs – The observer’s reference to be added.
chan – The channel’s reference.
masked – The mask state. When the mask is true, the observer will not receive notifications from the channel.

Return values:

0 – Channel notifications masked to the observer.
-ESRCH – No observation found for the related pair chan/obs.
-EINVAL – Some parameter is invalid.

int zbus_obs_is_chan_notification_masked(const struct zbus_observer *obs, const struct zbus_channel *chan, bool *masked)

Get the notifications masking state from a channel to an observer.

Parameters:

obs – The observer’s reference to be added.
chan – The channel’s reference.
masked – [out] The mask state. When the mask is true, the observer will not receive notifications from the channel.

Return values:

0 – Retrieved the masked state.
-ESRCH – No observation found for the related pair chan/obs.
-EINVAL – Some parameter is invalid.

static inline const char *zbus_obs_name(const struct zbus_observer *obs)

Get the observer’s name.

This routine returns the observer’s name reference.

Parameters:

obs – The observer’s reference.

Returns:

The observer’s name reference.

int zbus_sub_wait(const struct zbus_observer *sub, const struct zbus_channel **chan, k_timeout_t timeout)

Wait for a channel notification.

This routine makes the subscriber to wait a notification. The notification comes as a channel reference.

Parameters:

sub – [in] The subscriber’s reference.
chan – [out] The notification channel’s reference.
timeout – [in] Waiting period for a notification arrival, or one of the special values K_NO_WAIT and K_FOREVER.

Return values:

0 – Notification received.
-ENOMSG – Returned without waiting.
-EAGAIN – Waiting period timed out.
-EINVAL – The observer is not a subscriber.
-EFAULT – A parameter is incorrect, or the function context is invalid (inside an ISR). The function only returns this value when the CONFIG_ZBUS_ASSERT_MOCK is enabled.

bool zbus_iterate_over_channels(bool (*iterator_func)(const struct zbus_channel *chan))

Iterate over channels.

Enables the developer to iterate over the channels giving to this function an iterator_func which is called for each channel. If the iterator_func returns false all the iteration stops.

Parameters:

iterator_func – [in] The function that will be execute on each iteration.

Return values:

true – Iterator executed for all channels.
false – Iterator could not be executed. Some iterate returned false.

bool zbus_iterate_over_channels_with_user_data(bool (*iterator_func)(const struct zbus_channel *chan, void *user_data), void *user_data)

Iterate over channels with user data.

Enables the developer to iterate over the channels giving to this function an iterator_func which is called for each channel. If the iterator_func returns false all the iteration stops.

Parameters:

iterator_func – [in] The function that will be execute on each iteration.
user_data – [in] The user data that can be passed in the function.

Return values:

true – Iterator executed for all channels.
false – Iterator could not be executed. Some iterate returned false.

bool zbus_iterate_over_observers(bool (*iterator_func)(const struct zbus_observer *obs))

Iterate over observers.

Enables the developer to iterate over the observers giving to this function an iterator_func which is called for each observer. If the iterator_func returns false all the iteration stops.

Parameters:

iterator_func – [in] The function that will be execute on each iteration.

Return values:

true – Iterator executed for all channels.
false – Iterator could not be executed. Some iterate returned false.

bool zbus_iterate_over_observers_with_user_data(bool (*iterator_func)(const struct zbus_observer *obs, void *user_data), void *user_data)

Iterate over observers with user data.

Enables the developer to iterate over the observers giving to this function an iterator_func which is called for each observer. If the iterator_func returns false all the iteration stops.

Parameters:

iterator_func – [in] The function that will be execute on each iteration.
user_data – [in] The user data that can be passed in the function.

Return values:

true – Iterator executed for all channels.
false – Iterator could not be executed. Some iterate returned false.

struct zbus_channel_data

#include <zbus.h>

Type used to represent a channel mutable data.

Every channel has a zbus_channel_data structure associated.

Public Members

int16_t observers_start_idx

Static channel observer list start index.

Considering the ITERABLE SECTIONS allocation order.

int16_t observers_end_idx

Static channel observer list end index.

Considering the ITERABLE SECTIONS allocation order.

struct k_mutex mutex

Access control mutex.

Points to the mutex used to avoid race conditions for accessing the channel.

sys_slist_t observers

Channel observer list.

Represents the channel’s observers list, it can be empty or have listeners and subscribers mixed in any sequence. It can be changed in runtime.

struct zbus_channel

#include <zbus.h>

Type used to represent a channel.

Every channel has a zbus_channel structure associated used to control the channel access and usage.

Public Members

const char *const name: Channel name.

void *const message

Message reference.

Represents the message’s reference that points to the actual shared memory region.

const size_t message_size

Message size.

Represents the channel’s message size.

void *const user_data

User data available to extend zbus features.

The channel must be claimed before using this field.

bool (*const validator)(const void *msg, size_t msg_size)

Message validator.

Stores the reference to the function to check the message validity before actually performing the publishing. No invalid messages can be published. Every message is valid when this field is empty.

struct zbus_channel_data *const data: Mutable channel data struct.

struct zbus_observer

#include <zbus.h>

Type used to represent an observer.

Every observer has an representation structure containing the relevant information. An observer is a code portion interested in some channel. The observer can be notified synchronously or asynchronously and it is called listener and subscriber respectively. The observer can be enabled or disabled during runtime by change the enabled boolean field of the structure. The listeners have a callback function that is executed by the bus with the index of the changed channel as argument when the notification is sent. The subscribers have a message queue where the bus enqueues the index of the changed channel when a notification is sent.