QXSemaphore Class Reference

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

#include <qxthread.hpp>

Collaboration diagram for QXSemaphore:

Public Member Functions
void	init (std::uint_fast16_t const count, std::uint_fast16_t const max_count=0xFFFFU) noexcept
	initialize the counting semaphore More...
bool	wait (std::uint_fast16_t const nTicks=QXTHREAD_NO_TIMEOUT) noexcept
	wait (block) on the semaphore More...
bool	tryWait (void) noexcept
	try wait on the semaphore (non-blocking) More...
bool	signal (void) noexcept
	signal (unblock) the semaphore More...

Private Attributes
QPSet	m_waitSet
	set of extended threads waiting on this semaphore More...
std::uint16_t volatile	m_count
	semaphore up-down counter More...
std::uint16_t	m_max_count
	maximum value of the semaphore counter More...

Detailed Description

Counting Semaphore of the QXK preemptive kernel.

Description

QP::QXSemaphore is a blocking mechanism intended primarily for signaling extended threads. The semaphore is initialized with the maximum count (see QP::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
    BTN_sema.init(0U,  // initial semaphore count (singaling semaphore)
                  1U); // maximum semaphore count (binary semaphore)
    . . .
}
 
void main_threadXYZ(QXThread * const me) {
    while (1) {
        . . .
        BTN_sema.wait(QXTHREAD_NO_TIMEOUT); // timeout for waiting
        . . .
    }
}
 
void GPIO_Handler(void) {
    . . .
    BTN_sema.signal();
    . . .
}

Definition at line 153 of file qxthread.hpp.

Member Function Documentation

init()

void init ( std::uint_fast16_t const  count, std::uint_fast16_t const  max_count = 0xFFFFU  )

noexcept

initialize the counting semaphore

Description

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]	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).

Precondition

max_count must be greater than zero

Definition at line 77 of file qxk_sema.cpp.

wait()

bool wait ( std::uint_fast16_t const  nTicks = QXTHREAD_NO_TIMEOUT )

noexcept

wait (block) on the semaphore

Description

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]

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 signaled, and false if the timeout occured.

Note

Multiple extended threads can wait for a given semahpre.

Precondition

this function 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.

also: the thread must NOT be holding a scheduler lock.

Definition at line 109 of file qxk_sema.cpp.

tryWait()

bool tryWait ( void  )

noexcept

try wait on the semaphore (non-blocking)

Description

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

Returns

'true' if the semaphore has count available and 'false' NOT available.

Note

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

Precondition

the semaphore must be initialized

Definition at line 191 of file qxk_sema.cpp.

signal()

bool signal ( void  )

noexcept

signal (unblock) the semaphore

Description

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.

Returns

'true' when the semaphore gets 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.

Precondition

the semaphore must be initialized

Definition at line 230 of file qxk_sema.cpp.

Field Documentation

m_waitSet

QPSet m_waitSet

private

set of extended threads waiting on this semaphore

Definition at line 169 of file qxthread.hpp.

m_count

std::uint16_t volatile m_count

private

semaphore up-down counter

Definition at line 170 of file qxthread.hpp.

m_max_count

std::uint16_t m_max_count

private

maximum value of the semaphore counter

Definition at line 171 of file qxthread.hpp.

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

• qxthread.hpp
• qxk_sema.cpp