CMSIS-RTOS2: Event Flags

| CMSIS-RTOS2 Version 2.1.3 Real-Time Operating System: API and RTX Reference Implementation

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Event Flags
CMSIS-RTOS API v2

Synchronize threads using event flags. More...

## Data Structures
struct | osEventFlagsAttr_t
| Attributes structure for event flags. More...

## Typedefs
typedef void * | osEventFlagsId_t

## Functions
osEventFlagsId_t | osEventFlagsNew (const osEventFlagsAttr_t *attr)
| Create and Initialize an Event Flags object. More...
uint32_t | osEventFlagsSet (osEventFlagsId_t ef_id, uint32_t flags)
| Set the specified Event Flags. More...
uint32_t | osEventFlagsClear (osEventFlagsId_t ef_id, uint32_t flags)
| Clear the specified Event Flags. More...
uint32_t | osEventFlagsGet (osEventFlagsId_t ef_id)
| Get the current Event Flags. More...
uint32_t | osEventFlagsWait (osEventFlagsId_t ef_id, uint32_t flags, uint32_t options, uint32_t timeout)
| Wait for one or more Event Flags to become signaled. More...
osStatus_t | osEventFlagsDelete (osEventFlagsId_t ef_id)
| Delete an Event Flags object. More...
const char * | osEventFlagsGetName (osEventFlagsId_t ef_id)
| Get name of an Event Flags object. More...

## Description

The event flags management functions in CMSIS-RTOS allow you to control or wait for event flags. Each signal has up to 31 event flags.

A thread

- can wait for event flags to be set (using osEventFlagsWait). Using this function, it enters the BLOCKED state.
- may set one or more flags in any other given thread (using osEventFlagsSet).
- may clear its own signals or the signals of other threads (using osEventFlagsClear).

When a thread wakes up and resumes execution, its signal flags are automatically cleared (unless event flags option osFlagsNoClear is specified).

Note
  The functions osEventFlagsSet, osEventFlagsClear, osEventFlagsGet, and osEventFlagsWait can be called from Interrupt Service Routines.
  Refer to Event Flags Configuration for RTX5 configuration options.

## Working with Events

Here is a simple example that shows how two thread can communicate with each others using event flags:

Simple event communication

The following steps are required to use event flags:

1. In the thread that is supposed to send a event with id sig1_id, call the set function:
   osDelay(1000U); // wait for 1 second
   osEventFlagsSet(sig1_id, 0x0001U); // set the flag 0x0001U for event sig1_id
2. In another thread (or threads) that are supposed to wait for the event, call the wait function:
   osEventFlagsWait(sig1_id, 0x0001U, NULL, osWaitForever); // wait forever for any flag

The following complete example code can be directly used with the "CMSIS-RTOS2 main template" and is also provided as a stand-alone template for RTX5:

Code Example

#include "cmsis_os2.h" // CMSIS RTOS header file
/*----------------------------------------------------------------------------
* Event Flags creation & usage
*---------------------------------------------------------------------------*/
#define FLAGS_MSK1 0x00000001U
osEventFlagsId_t evt_id; // event flags id
osThreadId_t tid_Thread_EventSender; // thread id 1
osThreadId_t tid_Thread_EventReceiver; // thread id 2
void Thread_EventSender (void *argument); // thread function 1
void Thread_EventReceiver (void *argument); // thread function 2
int Init_Events (void) {
evt_id = osEventFlagsNew(NULL);
if (evt_id == NULL) {
; // Event Flags object not created, handle failure
}
tid_Thread_EventSender = osThreadNew(Thread_EventSender, NULL, NULL);
if (tid_Thread_EventSender == NULL) {
return(-1);
}
tid_Thread_EventReceiver = osThreadNew(Thread_EventReceiver, NULL, NULL);
if (tid_Thread_EventReceiver == NULL) {
return(-1);
}
return(0);
}
void Thread_EventSender (void *argument) {
while (1) {
osEventFlagsSet(evt_id, FLAGS_MSK1);
osThreadYield(); // suspend thread
}
}
void Thread_EventReceiver (void *argument) {
uint32_t flags;
while (1) {
flags = osEventFlagsWait(evt_id, FLAGS_MSK1, osFlagsWaitAny, osWaitForever);
//handle event
}
}

## Data Structure Documentation

struct osEventFlagsAttr_t

Attributes to configure an event flag set.

Refer to Memory Management for details about usage of

- osEventFlagsAttr_t::cb_mem
- osEventFlagsAttr_t::cb_size

Data Fields
const char * | name | name of the event flags Pointer to a constant string with a human readable name (displayed during debugging) of the event flag object. Default: NULL no name specified.
uint32_t | attr_bits | attribute bits Reserved for future use (must be set to '0' for future compatibility).
void * | cb_mem | memory for control block Pointer to a memory for the event flag control block object. Refer to Static Object Memory for more information. Default: NULL to use Automatic Dynamic Allocation for the event flag control block.
uint32_t | cb_size | size of provided memory for control block The size (in bytes) of memory block passed with cb_mem. For RTX, the minimum value is defined with osRtxEventFlagsCbSize (higher values are permitted). Default: 0 as the default is no memory provided with cb_mem.

## Typedef Documentation

osEventFlagsId_t

Event Flags ID identifies the event flags.

Returned by:

- osEventFlagsNew

## Function Documentation

osEventFlagsId_t osEventFlagsNew | ( | const osEventFlagsAttr_t * | attr | ) |

Parameters

  [in] | attr | event flags attributes; NULL: default values.

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

The function osEventFlagsNew creates a new event flags object that is used to send events across threads and returns the pointer to the event flags object identifier or NULL in case of an error. It can be safely called before the RTOS is started (call to osKernelStart), but not before it is initialized (call to osKernelInitialize).

The parameter attr sets the event flags attributes (refer to osEventFlagsAttr_t). Default attributes will be used if set to NULL, i.e. kernel memory allocation is used for the event control block.

Note
  Cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os2.h" // CMSIS RTOS header file
osEventFlagsId_t evt_id; // message queue id
int Init_Events (void) {
evt_id = osEventFlagsNew(NULL);
if (evt_id == NULL) {
; // Event Flags object not created, handle failure
return(-1);
}
return(0);
}

uint32_t osEventFlagsSet | ( | osEventFlagsId_t | ef_id,
|  | uint32_t | flags
| ) |  |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.
  [in] | flags | specifies the flags that shall be set.

Returns
  event flags after setting or error code if highest bit set.

The function osEventFlagsSet sets the event flags specified by the parameter flags in an event flags object specified by parameter ef_id. All threads waiting for the flag set will be notified to resume from BLOCKED state. The function returns the event flags after setting or an error code (highest bit is set, refer to Flags Functions Error Codes).

Possible Flags Functions Error Codes return values:

- osFlagsErrorUnknown: unspecified error.
- osFlagsErrorParameter: parameter ef_id does not identify a valid event flags object or flags has highest bit set.
- osFlagsErrorResource: the event flags object is in an invalid state.

Note
  This function may be called from Interrupt Service Routines.

Code Example

#include "cmsis_os2.h" // CMSIS RTOS header file
osEventFlagsId_t evt_id; // event flasg id
void Thread_EventSender (void *argument) {
while (1) {
osEventFlagsSet(evt_id, 0x00000001U);
osThreadYield(); // suspend thread
}
}

uint32_t osEventFlagsClear | ( | osEventFlagsId_t | ef_id,
|  | uint32_t | flags
| ) |  |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.
  [in] | flags | specifies the flags that shall be cleared.

Returns
  event flags before clearing or error code if highest bit set.

The function osEventFlagsClear clears the event flags specified by the parameter flags in an event flags object specified by parameter ef_id. The function returns the event flags before clearing or an error code (highest bit is set, refer to Flags Functions Error Codes).

Possible Flags Functions Error Codes return values:

- osFlagsErrorUnknown: unspecified error.
- osFlagsErrorParameter: parameter ef_id does not identify a valid event flags object or flags has highest bit set.
- osFlagsErrorResource: the event flags object is in an invalid state.

Note
  This function may be called from Interrupt Service Routines.

uint32_t osEventFlagsGet | ( | osEventFlagsId_t | ef_id | ) |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.

Returns
  current event flags.

The function osEventFlagsGet returns the event flags currently set in an event flags object specified by parameter ef_id or 0 in case of an error.

Note
  This function may be called from Interrupt Service Routines.

uint32_t osEventFlagsWait | ( | osEventFlagsId_t | ef_id,
|  | uint32_t | flags,
|  | uint32_t | options,
|  | uint32_t | timeout
| ) |  |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.
  [in] | flags | specifies the flags to wait for.
  [in] | options | specifies flags options (osFlagsXxxx).
  [in] | timeout | Timeout Value or 0 in case of no time-out.

Returns
  event flags before clearing or error code if highest bit set.

The function osEventFlagsWait suspends the execution of the currently RUNNING thread until any or all event flags specified by the parameter flags in the event object specified by parameter ef_id are set. When these event flags are already set, the function returns instantly. Otherwise, the thread is put into the state BLOCKED.

The options parameter specifies the wait condition:

Option |
--- | ---
osFlagsWaitAny | Wait for any flag (default).
osFlagsWaitAll | Wait for all flags.
osFlagsNoClear | Do not clear flags which have been specified to wait for.

If `osFlagsNoClear` is set in the options osEventFlagsClear can be used to clear flags manually.

The parameter timeout specifies how long the system waits for event flags. While the system waits, the thread that is calling this function is put into the BLOCKED state. The parameter timeout can have the following values:

- when timeout is 0, the function returns instantly (i.e. try semantics).
- when timeout is set to osWaitForever the function will wait for an infinite time until the event flags become available (i.e. wait semantics).
- all other values specify a time in kernel ticks for a timeout (i.e. timed-wait semantics).

The function returns the event flags before clearing or an error code (highest bit is set, refer to Flags Functions Error Codes).

Possible Flags Functions Error Codes return values:

- osFlagsErrorUnknown: unspecified error.
- osFlagsErrorTimeout: awaited flags have not been set in the given time.
- osFlagsErrorResource: awaited flags have not been set when no timeout was specified.
- osFlagsErrorParameter: parameter ef_id does not identify a valid event flags object or flags has highest bit set.

Note
  May be called from Interrupt Service Routines if the parameter timeout is set to 0.

Code Example

#include "cmsis_os2.h" // CMSIS RTOS header file
osEventFlagsId_t evt_id; // event flasg id
void Thread_EventReceiver (void *argument) {
uint32_t flags;
while (1) {
flags = osEventFlagsWait(evt_id, 0x00000001U, osFlagsWaitAny, osWaitForever);
//handle event
}
}

osStatus_t osEventFlagsDelete | ( | osEventFlagsId_t | ef_id | ) |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.

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

The function osEventFlagsDelete deletes the event flags object specified by parameter ef_id and releases the internal memory obtained for the event flags handling. After this call, the ef_id is no longer valid and cannot be used. This can cause starvation of threads that are waiting for flags of this event object. The ef_id may be created again using the function osEventFlagsNew.

Possible osStatus_t return values:

- osOK: the specified event flags object has been deleted.
- osErrorISR: osEventFlagsDelete cannot be called from interrupt service routines.
- osErrorParameter: parameter ef_id is NULL or invalid.
- osErrorResource: the event flags object is in an invalid state.

Note
  This function cannot be called from Interrupt Service Routines.

const char * osEventFlagsGetName | ( | osEventFlagsId_t | ef_id | ) |

Parameters

  [in] | ef_id | event flags ID obtained by osEventFlagsNew.

Returns
  name as null-terminated string.

The function osEventFlagsGetName returns the pointer to the name string of the event flags object identified by parameter ef_id or NULL in case of an error.

Note
  This function cannot be called from Interrupt Service Routines.

Code Example

#include "cmsis_os2.h" // CMSIS RTOS header file
osEventFlagsId_t evt_id; // event flasg id
void EvtFlagsGetName_example (void) {
char *name;
name = osEventFlagsGetName(evt_id);
if (name == NULL) {
// Failed to get the event flags object name
}
}