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

Private Attributes
QPSet	waitSet
uint8_t volatile	count
uint8_t	max_count

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 208 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 oop)
[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.

Precondition qxk_sema:100

• count must not exceed max_count
• max_count must not be zero
• max_count must not exceed 0xFFU (dynamic range of uint8_t)

Note

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

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 oop)
[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.

Precondition qxk_sema:200

• must NOT be called from an ISR;
• the semaphore must be initialized
• be called from an extended thread;
• the thread must NOT be already blocked on any object.

  Precondition qxk_sema:201

• the thread must NOT be holding a scheduler lock.

Note

Multiple extended threads can wait for a given semaphore.

Definition at line 94 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 oop)

Returns

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

Precondition qxk_sema:300

• the semaphore must be initialized

Note

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

Definition at line 181 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 oop)

Returns

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

Precondition qxk_sema:400

• the semaphore must be initialized

Note

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

Definition at line 223 of file qxk_sema.c.

Member Data Documentation

waitSet

QPSet waitSet

private

Set of extended threads waiting on this semaphore

Definition at line 212 of file qxk.h.

count

uint8_t volatile count

private

Semaphore up-down counter

Definition at line 215 of file qxk.h.

max_count

uint8_t max_count

private

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

Definition at line 218 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