CMSIS-RTOS: Thread Management

| CMSIS-RTOS Version 1.03 Real-Time Operating System: API and RTX Reference Implementation.

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Thread Management
CMSIS-RTOS API

Define, create, and control thread functions. More...

## Macros
#define | osThreadDef(name, priority, instances, stacksz)
| Create a Thread Definition with function, priority, and stack requirements. More...
#define | osThread(name) &os_thread_def_##name
| Access a Thread definition. More...

## Enumerations
enum | osPriority { osPriorityIdle = -3, osPriorityLow = -2, osPriorityBelowNormal = -1, osPriorityNormal = 0, osPriorityAboveNormal = +1, osPriorityHigh = +2, osPriorityRealtime = +3, osPriorityError = 0x84 }
| Priority used for thread control. More...

## Functions
osThreadId | osThreadCreate (const osThreadDef_t *thread_def, void *argument)
| Create a thread and add it to Active Threads and set it to state READY. More...
osThreadId | osThreadGetId (void)
| Return the thread ID of the current running thread. More...
osStatus | osThreadTerminate (osThreadId thread_id)
| Terminate execution of a thread and remove it from Active Threads. More...
osStatus | osThreadSetPriority (osThreadId thread_id, osPriority priority)
| Change priority of an active thread. More...
osPriority | osThreadGetPriority (osThreadId thread_id)
| Get current priority of an active thread. More...
osStatus | osThreadYield (void)
| Pass control to next thread that is in state READY. More...

## Description

The Thread Management function group allows defining, creating, and controlling thread functions in the system. The function main is a special thread function that is started at system initialization and has the initial priority osPriorityNormal.

Threads can be in the following states:

- RUNNING: The thread that is currently running is in the RUNNING state. Only one thread at a time can be in this state.
- READY: Threads which are ready to run are in the READY state. Once the RUNNING thread has terminated or is WAITING, the next READY thread with the highest priority becomes the RUNNING thread.
- WAITING: Threads that are waiting for an event to occur are in the WAITING state.
- INACTIVE: Threads that are not created or terminated are in the INACTIVE state. These threads typically consume no system resources.

Thread State and State Transitions

A CMSIS-RTOS assumes that threads are scheduled as shown in the figure Thread State and State Transitions. The thread states change as follows:

- A thread is created using the function osThreadCreate. This puts the thread into the READY or RUNNING state (depending on the thread priority).
- CMSIS-RTOS is pre-emptive. The active thread with the highest priority becomes the RUNNING thread provided it does not wait for any event. The initial priority of a thread is defined with the osThreadDef but may be changed during execution using the function osThreadSetPriority.
- The RUNNING thread transfers into the WAITING state when it is waiting for an event.
- Active threads can be terminated any time using the function osThreadTerminate. Threads can terminate also by just returning from the thread function. Threads that are terminated are in the INACTIVE state and typically do not consume any dynamic memory resources.

## Macro Definition Documentation

#define osThread | ( |  | name | ) | &os_thread_def_##name

Access to the thread definition for the function osThreadCreate.

Parameters

  name | name of the thread definition object.

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

#define osThreadDef | ( |  | name,
|  |  | priority,
|  |  | instances,
|  |  | stacksz
| ) |  |

Define the attributes of a thread functions that can be created by the function osThreadCreate using osThread. The argument instances defines the number of times that osThreadCreate can be called for the same osThreadDef.

Code Example

#include "cmsis_os.h"
void Thread (void const *arg); // function prototype for a Thread
osThreadDef (Thread, osPriorityNormal, 3, 0); // define Thread and specify to allow three instances
void ThreadCreate_example (void) {
osThreadId id1, id2, id3;
id1 = osThreadCreate (osThread (Thread), NULL); // create the thread with id1
id2 = osThreadCreate (osThread (Thread), NULL); // create the thread with id2
id3 = osThreadCreate (osThread (Thread), NULL); // create the thread with id3
if (id1 == NULL) { // handle thread creation for id1
// Failed to create the thread with id1
}
if (id2 == NULL) { // handle thread creation for id2
// Failed to create the thread with id2
}
if (id3 == NULL) { // handle thread creation for id3
// Failed to create the thread with id3
}
:
osThreadTerminate (id1); // stop the thread with id1
osThreadTerminate (id2); // stop the thread with id2
osThreadTerminate (id3); // stop the thread with id3
}

Parameters

  name | name of the thread function.
  priority | initial priority of the thread function.
  instances | number of possible thread instances.
  stacksz | stack size (in bytes) requirements for the thread function.

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

## Enumeration Type Documentation

enum osPriority

Note
  MUST REMAIN UNCHANGED: osPriority shall be consistent in every CMSIS-RTOS.
  Cannot be called from Interrupt Service Routines.

The osPriority value specifies the priority for a thread. The default thread priority should be osPriorityNormal. If a Thread is active that has a higher priority than the currently executing thread, then a thread switch occurs immediately to execute the new task.

To prevent from a priority inversion, a CMSIS-RTOS compliant OS may optionally implement a priority inheritance method. A priority inversion occurs when a high priority thread is waiting for a resource or event that is controlled by a thread with a lower priority.

Enumerator
osPriorityIdle | priority: idle (lowest)
osPriorityLow | priority: low
osPriorityBelowNormal | priority: below normal
osPriorityNormal | priority: normal (default)
osPriorityAboveNormal | priority: above normal
osPriorityHigh | priority: high
osPriorityRealtime | priority: realtime (highest)
osPriorityError | system cannot determine priority or thread has illegal priority

## Function Documentation

osThreadId osThreadCreate | ( | const osThreadDef_t * | thread_def,
|  | void * | argument
| ) |  |

Parameters

  [in] | thread_def | thread definition referenced with osThread.
  [in] | argument | pointer that is passed to the thread function as start argument.

Returns
  thread ID for reference by other functions or NULL in case of error.

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

Start a thread function by adding it to the Active Threads list and set it to state READY. The thread function receives the argument pointer as function argument when the function is started. When the priority of the created thread function is higher than the current RUNNING thread, the created thread function starts instantly and becomes the new RUNNING thread.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os.h"
void Thread_1 (void const *arg); // function prototype for Thread_1
osThreadDef (Thread_1, osPriorityNormal, 1, 0); // define Thread_1
void ThreadCreate_example (void) {
osThreadId id;
id = osThreadCreate (osThread (Thread_1), NULL); // create the thread
if (id == NULL) { // handle thread creation
// Failed to create a thread
}
:
osThreadTerminate (id); // stop the thread
}

osThreadId osThreadGetId | ( | void |  | ) |

Returns
  thread ID for reference by other functions or NULL in case of error.

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

Get the thread ID of the current running thread.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

void ThreadGetId_example (void) {
osThreadId id; // id for the currently running thread
id = osThreadGetId ();
if (id == NULL) {
// Failed to get the id; not in a thread
}
}

osPriority osThreadGetPriority | ( | osThreadId | thread_id | ) |

Parameters

  [in] | thread_id | thread ID obtained by osThreadCreate or osThreadGetId.

Returns
  current priority value of the thread function.

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

Get the priority of an active thread. In case of a failure the value osPriorityError is returned.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os.h"
void Thread_1 (void const *arg) { // Thread function
osThreadId id; // id for the currently running thread
osPriority priority; // thread priority
id = osThreadGetId (); // Obtain ID of current running thread
if (id != NULL) {
priority = osThreadGetPriority (id);
}
else {
// Failed to get the id
}
}

osStatus osThreadSetPriority | ( | osThreadId | thread_id,
|  | osPriority | priority
| ) |  |

Parameters

  [in] | thread_id | thread ID obtained by osThreadCreate or osThreadGetId.
  [in] | priority | new priority value for the thread function.

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

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

Change the priority of an active thread.

Status and Error Codes

- osOK: the priority of the specified thread has been successfully changed.
- osErrorParameter: thread_id is incorrect.
- osErrorValue: incorrect priority value.
- osErrorResource: thread_id refers to a thread that is not an active thread.
- osErrorISR: osThreadSetPriority cannot be called from interrupt service routines.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os.h"
void Thread_1 (void const *arg) { // Thread function
osThreadId id; // id for the currently running thread
osPriority pr; // thread priority
osStatus status; // status of the executed function
:
id = osThreadGetId (); // Obtain ID of current running thread
if (id != NULL) {
status = osThreadSetPriority (id, osPriorityBelowNormal);
if (status == osOK) {
// Thread priority changed to BelowNormal
}
else {
// Failed to set the priority
}
}
else {
// Failed to get the id
}
:
}

osStatus osThreadTerminate | ( | osThreadId | thread_id | ) |

Parameters

  [in] | thread_id | thread ID obtained by osThreadCreate or osThreadGetId.

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

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

Remove the thread function from the active thread list. If the thread is currently RUNNING the execution will stop.

Note
  In case that osThreadTerminate terminates the currently running task, the function never returns and other threads that are in the READY state are started.

Status and Error Codes

- osOK: the specified thread has been successfully terminated.
- osErrorParameter: thread_id is incorrect.
- osErrorResource: thread_id refers to a thread that is not an active thread.
- osErrorISR: osThreadTerminate cannot be called from interrupt service routines.

Note
  Cannot be called from Interrupt Service Routines.
  Avoid calling the function with a thread_id that does not exist or has been terminated already.

Code Example

#include "cmsis_os.h"
void Thread_1 (void const *arg); // function prototype for Thread_1
osThreadDef (Thread_1, osPriorityNormal, 1, 0); // define Thread_1
void ThreadTerminate_example (void) {
osStatus status;
osThreadId id;
id = osThreadCreate (osThread (Thread_1), NULL); // create the thread
:
status = osThreadTerminate (id); // stop the thread
if (status == osOK) {
// Thread was terminated successfully
}
else {
// Failed to terminate a thread
}
}

osStatus osThreadYield | ( | void |  | ) |

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

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

Pass control to the next thread that is in state READY. If there is no other thread in the state READY, the current thread continues execution and no thread switching occurs.

Status and Error Codes

- osOK: the function has been correctly executed.
- osErrorISR: osThreadYield cannot be called from interrupt service routines.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os.h"
void Thread_1 (void const *arg) { // Thread function
osStatus status; // status of the executed function
:
while (1) {
status = osThreadYield(); //
if (status != osOK) {
// thread switch not occurred, not in a thread function
}
}
}