QXSemaphore Class Reference

Counting Semaphore of the QXK preemptive kernel. More...

#include <qxk.h>

Public Member Functions
void	QXSemaphore_init (QXSemaphore *const me, uint_fast8_t const count, uint_fast8_t const max_count)
	Initialize the counting semaphore.
bool	QXSemaphore_wait (QXSemaphore *const me, QTimeEvtCtr const nTicks)
	Wait (block) on the semaphore.
bool	QXSemaphore_tryWait (QXSemaphore *const me)
	Try wait on the semaphore (non-blocking).
bool	QXSemaphore_signal (QXSemaphore *const me)
	Signal (unblock) the semaphore.

Private Attributes
QPSet	waitSet
	Set of extended threads waiting on this semaphore.
uint8_t	count
	Semaphore up-down counter.
uint8_t	max_count
	Maximum value of the semaphore counter (1 for binary semaphore).

Detailed Description

Counting Semaphore of the QXK preemptive kernel.

Details
QXSemaphore is a blocking mechanism intended primarily for signaling extended threads. The semaphore is initialized with the maximum count (see QXSemaphore_init()), which allows you to create a binary semaphore (when the maximum count is 1) and counting semaphore when the maximum count is > 1.

Usage
The following example illustrates how to instantiate and use the semaphore in your application.

QXSemaphore BTN_sema; // semaphore to signal a button press//
int main() {
    . . .
    // initialize the BTN_sema semaphore as binary, signaling semaphore//
    QXSemaphore_init(&BTN_sema, // pointer to semaphore to initialize//
                     0U,  // initial semaphore count (signaling semaphore)//
                     1U); // maximum semaphore count (binary semaphore)//
    . . .
}
 
void main_threadXYZ(QXThread * const me) {
    while (1) {
        . . .
        QXSemaphore_wait(&BTN_sema,  // <--- pointer to semaphore to wait on//
                         QXTHREAD_NO_TIMEOUT); // timeout for waiting//
        . . .
    }
}
 
void GPIO_Handler(void) {
    . . .
    QXSemaphore_signal(&BTN_sema); // <--- pointer to semaphore to signal//
    . . .
}

Definition at line 142 of file qxk.h.

Member Function Documentation

QXSemaphore_init()

void QXSemaphore_init	(	QXSemaphore *const	me,
		uint_fast8_t const	count,
		uint_fast8_t const	max_count )

Initialize the counting semaphore.

Details
Initializes a semaphore with the specified count and maximum count. If the semaphore is used for resource sharing, both the initial count and maximum count should be set to the number of identical resources guarded by the semaphore. If the semaphore is used as a signaling mechanism, the initial count should set to 0 and maximum count to 1 (binary semaphore).

Parameters

[in,out]	me	current instance pointer (see SAS_QP_OOA)
[in]	count	initial value of the semaphore counter
[in]	max_count	maximum value of the semaphore counter. The purpose of the max_count is to limit the counter so that the semaphore cannot unblock more times than the maximum.

Note

QXSemaphore_init() must be called before the semaphore can be used (signaled or waited on).

Definition at line 48 of file qxk_sema.c.

QXSemaphore_wait()

bool QXSemaphore_wait	(	QXSemaphore *const	me,
		QTimeEvtCtr const	nTicks )

Wait (block) on the semaphore.

Details
When an extended thread calls QXSemaphore_wait() and the value of the semaphore counter is greater than 0, QXSemaphore_wait() decrements the semaphore counter and returns (true) to its caller. However, if the value of the semaphore counter is 0, the function places the calling thread in the waiting list for the semaphore. The thread waits until the semaphore is signaled by calling QXSemaphore_signal(), or the specified timeout expires. If the semaphore is signaled before the timeout expires, QXK resumes the highest-priority extended thread waiting for the semaphore.

Parameters

[in,out]	me	current instance pointer (see SAS_QP_OOA)
[in]	nTicks	number of clock ticks (at the associated rate) to wait for the semaphore. The value of QXTHREAD_NO_TIMEOUT indicates that no timeout will occur and the semaphore will wait indefinitely.

Returns

'true' if the semaphore has been taken and 'false' if a timeout occurred.

Note

Multiple extended threads can wait for a given semaphore.

Definition at line 70 of file qxk_sema.c.

QXSemaphore_tryWait()

bool QXSemaphore_tryWait

(

QXSemaphore *const

me

)

Try wait on the semaphore (non-blocking).

Details
This function checks if the semaphore counter is greater than 0, in which case the counter is decremented.

Parameters

[in,out]

me

current instance pointer (see SAS_QP_OOA)

Returns

'true' if the semaphore was taken and 'false' if not taken (would BLOCK).

Note

This function can be called from any context, including ISRs and basic threads (active objects).

Definition at line 152 of file qxk_sema.c.

QXSemaphore_signal()

bool QXSemaphore_signal

(

QXSemaphore *const

me

)

Signal (unblock) the semaphore.

Details
If the semaphore counter value is 0 or more, it is incremented, and this function returns to its caller. If the extended threads are waiting for the semaphore to be signaled, QXSemaphore_signal() removes the highest-priority thread waiting for the semaphore from the waiting list and makes this thread ready-to-run. The QXK scheduler is then called to determine if the awakened thread is now the highest-priority thread that is ready-to-run.

Parameters

[in,out]

me

current instance pointer (see SAS_QP_OOA)

Returns

'true' when the semaphore signaled and 'false' when the semaphore count exceeded the maximum.

Note

A semaphore can be signaled from many places, including from ISRs, basic threads (AOs), and extended threads.

Definition at line 187 of file qxk_sema.c.

Member Data Documentation

waitSet

QPSet waitSet

private

Set of extended threads waiting on this semaphore.

Definition at line 143 of file qxk.h.

count

uint8_t count

private

Semaphore up-down counter.

Definition at line 144 of file qxk.h.

max_count

uint8_t max_count

private

Maximum value of the semaphore counter (1 for binary semaphore).

Definition at line 145 of file qxk.h.

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The documentation for this class was generated from the following files:

• qxk.h
• qxk_sema.c
• qxk.dox