Exchange messages between threads in a FIFO-like operation. More...

Macros
#define	osFeature_MessageQ 1
	Message Queues: 1=available, 0=not available. More...

#define	osMessageQDef(name, queue_sz, type)
	Create a Message Queue Definition. More...

#define	osMessageQ(name) &os_messageQ_def_##name
	Access a Message Queue Definition. More...

Functions
osMessageQId	osMessageCreate (const osMessageQDef_t *queue_def, osThreadId thread_id)
	Create and Initialize a Message Queue. More...

osStatus	osMessagePut (osMessageQId queue_id, uint32_t info, uint32_t millisec)
	Put a Message to a Queue. More...

osEvent	osMessageGet (osMessageQId queue_id, uint32_t millisec)
	Get a Message or Wait for a Message from a Queue. More...

Description

Message passing is another basic communication model between threads. In the message passing model, one thread sends data explicitly, while another thread receives it. The operation is more like some kind of I/O rather than a direct access to information to be shared. In CMSIS-RTOS, this mechanism is called s message queue. The data is passed from one thread to another in a FIFO-like operation. Using message queue functions, you can control, send, receive, or wait for messages. The data to be passed can be of integer or pointer type:

CMSIS-RTOS Message Queue

Compared to a Memory Pool, message queues are less efficient in general, but solve a broader range of problems. Sometimes, threads do not have a common address space or the use of shared memory raises problems, such as mutual exclusion.

Working with Message Queues

Follow these steps to create and use a message queue:

Setup the message queue:
osMessageQDef(message_q, 5, uint32_t); // Declare a message queue

osMessageQId (message_q_id); // Declare an ID for the message queue
Then, create the message queue in a thread:
message_q_id = osMessageCreate(osMessageQ(message_q), NULL);
Fill the message queue with data:
uint32_t data = 512;

osMailPut(message_q_id, data, osWaitForever);
From the receiving thread access the data using:
osEvent event = osMessageGet(message_q_id, osWaitForever);

Macro Definition Documentation

#define osFeature_MessageQ 1

A CMSIS-RTOS implementation may support message queues.

When osFeature_MailQ is 1 message queues are supported.
When osFeature_MailQ is 0 no message queues are supported.

CMSIS-RTOS RTX Setting: osFeature_MessageQ is 1

#define osMessageQ ( name ) &os_messageQ_def_##name

Access to the message queue definition for the function osMessageCreate.

Parameters

name	name of the queue

Note: CAN BE CHANGED: The parameter to osMessageQ shall be consistent but the macro body is implementation specific in every CMSIS-RTOS.

#define osMessageQDef	(	name,
		queue_sz,
		type
	)

Define the attributes of a message queue created by the function osMessageCreate using osMessageQ.

Parameters

name	name of the queue.
queue_sz	maximum number of messages in the queue.
type	data type of a single message element (for debugger).

Note: CAN BE CHANGED: The parameter to osMessageQDef shall be consistent but the macro body is implementation specific in every CMSIS-RTOS.

Function Documentation

osMessageQId osMessageCreate	(	const osMessageQDef_t *	queue_def,
		osThreadId	thread_id
	)

Parameters

[in]	queue_def	queue definition referenced with osMessageQ.
[in]	thread_id	thread ID (obtained by osThreadCreate or osThreadGetId) or NULL.

Returns: message queue ID for reference by other functions or NULL in case of error.

Note: MUST REMAIN UNCHANGED: osMessageCreate shall be consistent in every CMSIS-RTOS.

Create and initialize a message queue. The parameter thread_id registers the receiving thread for a message and is needed for the general osWait function to deliver the message.

Note: Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os.h"
 
osThreadId tid_thread1;                          // ID for thread 1
osThreadId tid_thread2;                          // for thread 2
 
typedef struct {                                 // Message object structure
  float    voltage;                              // AD result of measured voltage
  float    current;                              // AD result of measured current
  int      counter;                              // A counter value
} T_MEAS;
 
osPoolDef(mpool, 16, T_MEAS);                    // Define memory pool
osPoolId  mpool;
osMessageQDef(MsgBox, 16, &T_MEAS);              // Define message queue
osMessageQId  MsgBox;
 
void send_thread (void const *argument);         // forward reference
void recv_thread (void const *argument);         // forward reference
                                                 // Thread definitions
osThreadDef(send_thread, osPriorityNormal, 1, 0);
osThreadDef(recv_thread, osPriorityNormal, 1, 2000);
 
//
//  Thread 1: Send thread
//
void send_thread (void const *argument) {
  T_MEAS    *mptr;
 
  mptr = osPoolAlloc(mpool);                     // Allocate memory for the message
  mptr->voltage = 223.72;                        // Set the message content
  mptr->current = 17.54;
  mptr->counter = 120786;
  osMessagePut(MsgBox, (uint32_t)mptr, osWaitForever);  // Send Message
  osDelay(100);
 
  mptr = osPoolAlloc(mpool);                     // Allocate memory for the message
  mptr->voltage = 227.23;                        // Prepare a 2nd message
  mptr->current = 12.41;
  mptr->counter = 170823;
  osMessagePut(MsgBox, (uint32_t)mptr, osWaitForever);  // Send Message
  osThreadYield();                               // Cooperative multitasking
                                                 // We are done here, exit this thread
}
 
//
//  Thread 2: Receive thread
//
void recv_thread (void const *argument) {
  T_MEAS  *rptr;
  osEvent  evt;
   
  for (;;) {
    evt = osMessageGet(MsgBox, osWaitForever);  // wait for message
    if (evt.status == osEventMessage) {
      rptr = evt.value.p;
      printf ("\nVoltage: %.2f V\n", rptr->voltage);
      printf ("Current: %.2f A\n", rptr->current);
      printf ("Number of cycles: %d\n", rptr->counter);
      osPoolFree(mpool, rptr);                  // free memory allocated for message
    }
  }
}
 
void StartApplication (void) {
  mpool = osPoolCreate(osPool(mpool));                 // create memory pool
  MsgBox = osMessageCreate(osMessageQ(MsgBox), NULL);  // create msg queue
   
  tid_thread1 = osThreadCreate(osThread(send_thread), NULL);
  tid_thread2 = osThreadCreate(osThread(recv_thread), NULL);
  :
}

osEvent osMessageGet	(	osMessageQId	queue_id,
		uint32_t	millisec
	)

Parameters

[in]	queue_id	message queue ID obtained with osMessageCreate.
[in]	millisec	Timout Value or 0 in case of no time-out.

Returns: event information that includes status code.

Note: MUST REMAIN UNCHANGED: osMessageGet shall be consistent in every CMSIS-RTOS.

Suspend the execution of the current RUNNING thread until a message arrives. When a message is already in the queue, the function returns instantly with the message information.

The argument millisec specifies how long the system waits for a message to become available. While the system waits the thread that is calling this function is put into the state WAITING. The millisec timeout value can have the following values:

when millisec is 0, the function returns instantly.
when millisec is set to osWaitForever the function will wait for an infinite time until a message arrives.
all other values specify a time in millisecond for a timeout.

Note: The parameter millisec must be 0 for using this function in an ISR.; Interrupt Service Routines can call this function.

Status and Error Codes

osOK: no message is available in the queue and no timeout was specified.
osEventTimeout: no message has arrived during the given timeout period.
osEventMessage: message received, value.p contains the pointer to message.
osErrorParameter: a parameter is invalid or outside of a permitted range.

osStatus osMessagePut	(	osMessageQId	queue_id,
		uint32_t	info,
		uint32_t	millisec
	)

Parameters

[in]	queue_id	message queue ID obtained with osMessageCreate.
[in]	info	message information.
[in]	millisec	Timout Value or 0 in case of no time-out.

Returns: status code that indicates the execution status of the function.

Note: MUST REMAIN UNCHANGED: osMessagePut shall be consistent in every CMSIS-RTOS.

Put the message info in a message queue specified by queue_id.

When the message queue is full, the system retries for a specified time with millisec. While the system retries the thread that is calling this function is put into the state WAITING. The millisec timeout can have the following values:

when millisec is 0, the function returns instantly.
when millisec is set to osWaitForever the function will wait for an infinite time until a message queue slot becomes available.
all other values specify a time in millisecond for a timeout.

Note: The parameter millisec must be 0 for using this function in an ISR.; Interrupt Service Routines can call this function.

Status and Error Codes

osOK: the message is put into the queue.
osErrorResource: no memory in the queue was available.
osErrorTimeoutResource: no memory in the queue was available during the given time limit.
osErrorParameter: a parameter is invalid or outside of a permitted range.

Macros

Functions

Description

Working with Message Queues

Macro Definition Documentation

Function Documentation