kernel: msgq: adding support for k_msgq_put_urgent and sample code

This commit introduces the k_msgq_put_urgent API for sending
messages to a queue in a LIFO scheme. It also adds a simple
sample code that demonstrates how it works.

Signed-off-by: Alexander pinheiro paschoaletto <axelpinheiro@gmail.com>

Author: Alexander pinheiro paschoaletto

include/zephyr/kernel.h

@@ -4785,7 +4785,7 @@ __syscall int k_msgq_alloc_init(struct k_msgq *msgq, size_t msg_size,
 int k_msgq_cleanup(struct k_msgq *msgq);
 
 /**
- * @brief Send a message to a message queue.
+ * @brief Send a message to the end of a message queue.
  *
  * This routine sends a message to message queue @a q.
  *
@@ -4806,6 +4806,30 @@ int k_msgq_cleanup(struct k_msgq *msgq);
  */
 __syscall int k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout);
 
+/**
+ * @brief Send a message to the beginning of a message queue.
+ *
+ * This routine sends a message to the beginning (head) of message queue @a q.
+ * Messages sent with this method will be retrieved before any pre-existing
+ * messages in the queue.
+ *
+ * @note The message content is copied from @a data into @a msgq and the @a data
+ * pointer is not retained, so the message content will not be modified
+ * by this function.
+ *
+ * @funcprops \isr_ok
+ *
+ * @param msgq Address of the message queue.
+ * @param data Pointer to the message.
+ * @param timeout Waiting period to add the message, or one of the special
+ *                values K_NO_WAIT and K_FOREVER.
+ *
+ * @retval 0 Message sent.
+ * @retval -ENOMSG Returned without waiting or queue purged.
+ * @retval -EAGAIN Waiting period timed out.
+ */
+__syscall int k_msgq_put_urgent(struct k_msgq *msgq, const void *data, k_timeout_t timeout);
+
 /**
  * @brief Receive a message from a message queue.
  *

kernel/msg_q.c

@@ -187,6 +187,100 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 	return result;
 }
 
+int z_impl_k_msgq_put_urgent(struct k_msgq *msgq, const void *data, k_timeout_t timeout)
+{
+	__ASSERT(!arch_is_in_isr() || K_TIMEOUT_EQ(timeout, K_NO_WAIT), "");
+
+	struct k_thread *pending_thread;
+	k_spinlock_key_t key;
+	int result;
+
+	key = k_spin_lock(&msgq->lock);
+
+	SYS_PORT_TRACING_OBJ_FUNC_ENTER(k_msgq, put, msgq, timeout);
+
+	if (msgq->used_msgs < msgq->max_msgs) {
+		/* message queue isn't full */
+		pending_thread = z_unpend_first_thread(&msgq->wait_q);
+		if (unlikely(pending_thread != NULL)) {
+			SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, 0);
+
+			/* give message to waiting thread */
+			(void)memcpy(pending_thread->base.swap_data, data,
+			       msgq->msg_size);
+			/* wake up waiting thread */
+			arch_thread_return_value_set(pending_thread, 0);
+			z_ready_thread(pending_thread);
+			z_reschedule(&msgq->lock, key);
+			return 0;
+		} else {
+			/*
+			 * urgent put = write message to the head of the queue.
+			 * this is achieved in practice by simply:
+			 *
+			 * 1. decrementing the read pointer. This effectively
+			 * opens space for the incoming message in the head of
+			 * the queue.
+			 *
+			 * 2. temporarily matching the read pointer with the
+			 * write pointer. This way the message will be written
+			 * in the recently opened space.
+			 *
+			 * 3. reverting the write pointer after the write to its
+			 * original value. The read pointer, by its turn, should
+			 * be always at the head, so it doesn't need reverting
+			 * because we just wrote to the head.
+			 */
+			char *original_write_ptr = msgq->write_ptr;
+
+			/* decrement the read pointer */
+			msgq->read_ptr -= msgq->msg_size;
+			if(msgq->read_ptr < msgq->buffer_start){
+				msgq->read_ptr = msgq->buffer_end - msgq->msg_size;
+			}
+			
+			/* match read and write pointers */
+			msgq->write_ptr = msgq->read_ptr;
+
+			/* put message in queue (like usual) */
+			__ASSERT_NO_MSG(msgq->write_ptr >= msgq->buffer_start &&
+					msgq->write_ptr < msgq->buffer_end);
+			(void)memcpy(msgq->write_ptr, (char *)data, msgq->msg_size);
+			msgq->write_ptr += msgq->msg_size;
+			if (msgq->write_ptr == msgq->buffer_end) {
+				msgq->write_ptr = msgq->buffer_start;
+			}
+			msgq->used_msgs++;
+
+			/* revert write pointer to the original value */
+			msgq->write_ptr = original_write_ptr;
+
+#ifdef CONFIG_POLL
+			handle_poll_events(msgq, K_POLL_STATE_MSGQ_DATA_AVAILABLE);
+#endif /* CONFIG_POLL */
+		}
+		result = 0;
+	} else if (K_TIMEOUT_EQ(timeout, K_NO_WAIT)) {
+		/* don't wait for message space to become available */
+		result = -ENOMSG;
+	} else {
+		SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_msgq, put, msgq, timeout);
+
+		/* wait for put message success, failure, or timeout */
+		_current->base.swap_data = (void *) data;
+
+		result = z_pend_curr(&msgq->lock, key, &msgq->wait_q, timeout);
+		SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, result);
+		return result;
+	}
+
+	SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, result);
+
+	z_reschedule(&msgq->lock, key);
+
+	return result;
+}
+
 #ifdef CONFIG_USERSPACE
 static inline int z_vrfy_k_msgq_put(struct k_msgq *msgq, const void *data,
 				    k_timeout_t timeout)
@@ -196,6 +290,15 @@ static inline int z_vrfy_k_msgq_put(struct k_msgq *msgq, const void *data,
 
 	return z_impl_k_msgq_put(msgq, data, timeout);
 }
+
+static inline int z_vrfy_k_msgq_put_urgent(struct k_msgq *msgq, const void *data,
+				    k_timeout_t timeout)
+{
+	K_OOPS(K_SYSCALL_OBJ(msgq, K_OBJ_MSGQ));
+	K_OOPS(K_SYSCALL_MEMORY_READ(data, msgq->msg_size));
+
+	return z_impl_k_msgq_put_urgent(msgq, data, timeout);
+}
 #include <zephyr/syscalls/k_msgq_put_mrsh.c>
 #endif /* CONFIG_USERSPACE */
 

samples/basic/msg_queue/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(msg_queue)
+
+target_sources(app PRIVATE src/main.c)

samples/basic/msg_queue/README.rst

@@ -0,0 +1,50 @@
+.. zephyr:code-sample:: msg_queue
+   :name: Message Queue
+
+   Implements a basic message queue producer/consumer thread pair.
+
+Overview
+********
+
+A simple sample to demonstrate the basic usage of Zephyr message queues.
+a producer thread sends both normal and urgent messages.
+
+Building and Running
+********************
+
+This application can be built and executed on QEMU as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/msg_queue
+   :host-os: unix
+   :board: qemu_x86
+   :goals: run
+   :compact:
+
+To build for another board, change "qemu_x86" above to that board's name.
+
+Sample Output
+=============
+
+Every normal message sending implies sending the message to the end of the
+queue, and the first message to go is the first to be delivered. Every "urgent"
+message, at its turn, implies sending the message to the beginning of the queue.
+
+In this sample, one producer thread sends 1 urgent message for each 2 regular
+ones. Note that message C is the first retrieved because it was the last one
+sent as "urgent".
+
+.. code-block:: console
+
+   [producer] sending: 0
+   [producer] sending: 1
+   [producer] sending: A (urgent)
+   [producer] sending: 2
+   [producer] sending: 3
+   [producer] sending: B (urgent)
+   [producer] sending: 4
+   [producer] sending: 5
+   [producer] sending: C (urgent)
+   [consumer] got sequence: CBA012345
+
+Exit QEMU by pressing :kbd:`CTRL+A` :kbd:`x`.

samples/basic/msg_queue/prj.conf

@@ -0,0 +1 @@
+# nothing here

samples/basic/msg_queue/sample.yaml

@@ -0,0 +1,17 @@
+sample:
+  description: A message queue usage sample
+  name: message queue sample
+common:
+  tags:
+    - message queues
+tests:
+  sample.msgqueue:
+    build_only: true
+    platform_allow:
+      - qemu_riscv32
+      - qemu_cortex_a53
+    integration_platforms:
+      - qemu_riscv32
+    tags:
+      - message
+      - queue

samples/basic/msg_queue/src/main.c

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2025 Instituto Superior de Engenharia do Porto (ISEP).
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+
+#define BUF_SIZE 10
+#define INACTIVE -1
+#define PRIORITY 5
+
+K_MSGQ_DEFINE(my_msgq, sizeof(char), BUF_SIZE, 1);
+
+void producer_function(void *rec, void *p2, void *p3){
+	
+	k_tid_t receiving_thread = (k_tid_t) rec;
+
+	char normal_data = '0';
+	char urgent_data = 'A'; 
+	int total_sent = 0;
+
+	/*
+	 * sends messages every 100 msec, in repeating
+	 * sequence normal, normal, urgent, ...
+	 */
+	while(total_sent < (BUF_SIZE - 1)){
+		for(int i = 0; i < 2; i++){
+			printk("[producer] sending: %c\n", normal_data);
+			k_msgq_put(&my_msgq, &normal_data, K_NO_WAIT);
+			k_sleep(K_MSEC(100));
+			normal_data++;
+		}
+		printk("[producer] sending: %c (urgent)\n", urgent_data);
+		k_msgq_put_urgent(&my_msgq, &urgent_data, K_NO_WAIT);
+		k_sleep(K_MSEC(100));
+		urgent_data++;
+
+		total_sent += 3;
+	}
+
+	/*
+	 * finished sending messages, now start the receiving thread.
+	 * keep in mind both threads can be running at the same time,
+	 * but in this example we wish to see the queue accumulate some
+	 * messages before the receiver thread starts reading them out.
+	 */
+	k_thread_start(receiving_thread);
+}
+
+void consumer_function(void *p1, void *p2, void *p3){
+	char received[BUF_SIZE];
+	
+	for(int i = 0; i < (BUF_SIZE - 1); i++){
+		k_msgq_get(&my_msgq, &received[i], K_NO_WAIT);
+	}
+
+	received[BUF_SIZE - 1] = '\0';
+	/* we expect to see CBA012345... */
+	printk("[consumer] got sequence: %s\n", received);
+}
+
+K_THREAD_DEFINE(consumer_thread, 2048, consumer_function, NULL, NULL, NULL, PRIORITY, 0, INACTIVE);
+K_THREAD_DEFINE(producer_thread, 2048, producer_function, ((void *) consumer_thread), NULL, NULL, PRIORITY, 0, 0);