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xEventGroupSync() function
Atomically set bits within an event group, then wait for a combination of bits to be set within the same event group. This functionality is typically used to synchronise multiple tasks, where each task has to wait for the other tasks to reach a synchronisation point before proceeding. This function cannot be used from an interrupt. The function will return before its block time expires if the bits specified by the uxBitsToWait parameter are set, or become set within that time. In this case all the bits specified by uxBitsToWait will be automatically cleared before the function returns. Example usage:
// Bits used by the three tasks.
#define TASK_0_BIT ( 1 << 0 )
#define TASK_1_BIT ( 1 << 1 )
#define TASK_2_BIT ( 1 << 2 )
#define ALL_SYNC_BITS ( TASK_0_BIT | TASK_1_BIT | TASK_2_BIT )
// Use an event group to synchronise three tasks. It is assumed this event
// group has already been created elsewhere.
EventGroupHandle_t xEventBits;
void vTask0( void *pvParameters )
{
EventBits_t uxReturn;
TickType_t xTicksToWait = 100 / portTICK_PERIOD_MS;
for( ;; )
{
// Perform task functionality here.
// Set bit 0 in the event flag to note this task has reached the
// sync point. The other two tasks will set the other two bits defined
// by ALL_SYNC_BITS. All three tasks have reached the synchronisation
// point when all the ALL_SYNC_BITS are set. Wait a maximum of 100ms
// for this to happen.
uxReturn = xEventGroupSync( xEventBits, TASK_0_BIT, ALL_SYNC_BITS, xTicksToWait );
if( ( uxReturn & ALL_SYNC_BITS ) == ALL_SYNC_BITS )
{
// All three tasks reached the synchronisation point before the call
// to xEventGroupSync() timed out.
}
}
}
void vTask1( void *pvParameters )
{
for( ;; )
{
// Perform task functionality here.
// Set bit 1 in the event flag to note this task has reached the
// synchronisation point. The other two tasks will set the other two
// bits defined by ALL_SYNC_BITS. All three tasks have reached the
// synchronisation point when all the ALL_SYNC_BITS are set. Wait
// indefinitely for this to happen.
xEventGroupSync( xEventBits, TASK_1_BIT, ALL_SYNC_BITS, portMAX_DELAY );
// xEventGroupSync() was called with an indefinite block time, so
// this task will only reach here if the syncrhonisation was made by all
// three tasks, so there is no need to test the return value.
}
}
void vTask2( void *pvParameters )
{
for( ;; )
{
// Perform task functionality here.
// Set bit 2 in the event flag to note this task has reached the
// synchronisation point. The other two tasks will set the other two
// bits defined by ALL_SYNC_BITS. All three tasks have reached the
// synchronisation point when all the ALL_SYNC_BITS are set. Wait
// indefinitely for this to happen.
xEventGroupSync( xEventBits, TASK_2_BIT, ALL_SYNC_BITS, portMAX_DELAY );
// xEventGroupSync() was called with an indefinite block time, so
// this task will only reach here if the syncrhonisation was made by all
// three tasks, so there is no need to test the return value.
}
}
Syntax
EventBits_t xEventGroupSync( EventGroupHandle_t xEventGroup,
const EventBits_t uxBitsToSet,
const EventBits_t uxBitsToWaitFor,
TickType_t xTicksToWait );
Return value
The value of the event group at the time either the bits being waited for became set, or the block time expired. Test the return value to know which bits were set. If xEventGroupSync() returned because its timeout expired then not all the bits being waited for will be set. If xEventGroupSync() returned because all the bits it was waiting for were set then the returned value is the event group value before any bits were automatically cleared.
Related Functions
Found 12 other functions taking a ![]()
EventGroupDef_t
argument:
Function
Description
Set bits within an event group. This function cannot be called from an interrupt. xEventGroupSetBitsFromISR() is a version that can be called from an interrupt. Setting bits in an event group will automatically unblock tasks that are blocked waiting for the bits. Example usage:
#define BIT_0 ( 1 << 0 )
#define BIT_4 ( 1 << 4 )
void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;
// Set bit 0 and bit 4 in xEventGroup.
uxBits = xEventGroupSetBits(
xEventGroup, // The event group being updated.
BIT_0 | BIT_4 );// The bits being set.
if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
{
// Both bit 0 and bit 4 remained set when the function returned.
}
else if( ( uxBits & BIT_0 ) != 0 )
{
// Bit 0 remained set when the function returned, but bit 4 was
// cleared. It might be that bit 4 was cleared automatically as a
// task that was waiting for bit 4 was removed from the Blocked
// state.
}
else if( ( uxBits & BIT_4 ) != 0 )
{
// Bit 4 remained set when the function returned, but bit 0 was
// cleared. It might be that bit 0 was cleared automatically as a
// task that was waiting for bit 0 was removed from the Blocked
// state.
}
else
{
// Neither bit 0 nor bit 4 remained set. It might be that a task
// was waiting for both of the bits to be set, and the bits were
// cleared as the task left the Blocked state.
}
}
Clear bits within an event group. This function cannot be called from an interrupt. Example usage:
#define BIT_0 ( 1 << 0 )
#define BIT_4 ( 1 << 4 )
void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;
// Clear bit 0 and bit 4 in xEventGroup.
uxBits = xEventGroupClearBits(
xEventGroup, // The event group being updated.
BIT_0 | BIT_4 );// The bits being cleared.
if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
{
// Both bit 0 and bit 4 were set before xEventGroupClearBits() was
// called. Both will now be clear (not set).
}
else if( ( uxBits & BIT_0 ) != 0 )
{
// Bit 0 was set before xEventGroupClearBits() was called. It will
// now be clear.
}
else if( ( uxBits & BIT_4 ) != 0 )
{
// Bit 4 was set before xEventGroupClearBits() was called. It will
// now be clear.
}
else
{
// Neither bit 0 nor bit 4 were set in the first place.
}
}
[Potentially] block to wait for one or more bits to be set within a previously created event group. This function cannot be called from an interrupt. Example usage:
#define BIT_0 ( 1 << 0 )
#define BIT_4 ( 1 << 4 )
void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;
const TickType_t xTicksToWait = 100 / portTICK_PERIOD_MS;
// Wait a maximum of 100ms for either bit 0 or bit 4 to be set within
// the event group. Clear the bits before exiting.
uxBits = xEventGroupWaitBits(
xEventGroup, // The event group being tested.
BIT_0 | BIT_4, // The bits within the event group to wait for.
pdTRUE, // BIT_0 and BIT_4 should be cleared before returning.
pdFALSE, // Don't wait for both bits, either bit will do.
xTicksToWait ); // Wait a maximum of 100ms for either bit to be set.
if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
{
// xEventGroupWaitBits() returned because both bits were set.
}
else if( ( uxBits & BIT_0 ) != 0 )
{
// xEventGroupWaitBits() returned because just BIT_0 was set.
}
else if( ( uxBits & BIT_4 ) != 0 )
{
// xEventGroupWaitBits() returned because just BIT_4 was set.
}
else
{
// xEventGroupWaitBits() returned because xTicksToWait ticks passed
// without either BIT_0 or BIT_4 becoming set.
}
}
Delete an event group that was previously created by a call to xEventGroupCreate(). Tasks that are blocked on the event group will be unblocked and obtain 0 as the event group's value.
A version of xEventGroupGetBits() that can be called from an ISR.
A version of xEventGroupClearBits() that can be called from an interrupt. Setting bits in an event group is not a deterministic operation because there are an unknown number of tasks that may be waiting for the bit or bits being set. FreeRTOS does not allow nondeterministic operations to be performed while interrupts are disabled, so protects event groups that are accessed from tasks by suspending the scheduler rather than disabling interrupts. As a result event groups cannot be accessed directly from an interrupt service routine. Therefore xEventGroupClearBitsFromISR() sends a message to the timer task to have the clear operation performed in the context of the timer task. Example usage:
#define BIT_0 ( 1 << 0 )
#define BIT_4 ( 1 << 4 )
// An event group which it is assumed has already been created by a call to
// xEventGroupCreate().
EventGroupHandle_t xEventGroup;
void anInterruptHandler( void )
{
// Clear bit 0 and bit 4 in xEventGroup.
xResult = xEventGroupClearBitsFromISR(
xEventGroup, // The event group being updated.
BIT_0 | BIT_4 ); // The bits being set.
if( xResult == pdPASS )
{
// The message was posted successfully.
}
}
A version of xEventGroupSetBits() that can be called from an interrupt. Setting bits in an event group is not a deterministic operation because there are an unknown number of tasks that may be waiting for the bit or bits being set. FreeRTOS does not allow nondeterministic operations to be performed in interrupts or from critical sections. Therefore xEventGroupSetBitsFromISR() sends a message to the timer task to have the set operation performed in the context of the timer task - where a scheduler lock is used in place of a critical section. Example usage:
#define BIT_0 ( 1 << 0 )
#define BIT_4 ( 1 << 4 )
// An event group which it is assumed has already been created by a call to
// xEventGroupCreate().
EventGroupHandle_t xEventGroup;
void anInterruptHandler( void )
{
BaseType_t xHigherPriorityTaskWoken, xResult;
// xHigherPriorityTaskWoken must be initialised to pdFALSE.
xHigherPriorityTaskWoken = pdFALSE;
// Set bit 0 and bit 4 in xEventGroup.
xResult = xEventGroupSetBitsFromISR(
xEventGroup, // The event group being updated.
BIT_0 | BIT_4 // The bits being set.
&xHigherPriorityTaskWoken );
// Was the message posted successfully?
if( xResult == pdPASS )
{
// If xHigherPriorityTaskWoken is now set to pdTRUE then a context
// switch should be requested. The macro used is port specific and
// will be either portYIELD_FROM_ISR() or portEND_SWITCHING_ISR() -
// refer to the documentation page for the port being used.
portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
}
References
Code
Location
Referrer
Call Tree
xEventGroupSync() is called by 1 function and calls 9 functions:
Data Use
xEventGroupSync() reads 2 variables and writes 1 variable: