stk::sync::Semaphore Class Reference

Counting semaphore primitive for resource management and signaling. More...

#include <stk_sync_semaphore.h>

Inheritance diagram for stk::sync::Semaphore:

Collaboration diagram for stk::sync::Semaphore:

Public Member Functions
	Semaphore (uint16_t initial_count=0U, uint16_t max_count=COUNT_MAX)
	Constructor.
	~Semaphore ()
	Destructor.
bool	Wait (Timeout timeout=WAIT_INFINITE)
	Wait for a signal (decrement counter).
bool	TryWait ()
	Poll the semaphore without blocking (decrement counter if available).
void	Signal ()
	Post a signal (increment counter).
uint16_t	GetCount () const
	Get current counter value.
void	SetTraceName (const char *name)
	Set name.
const char *	GetTraceName () const
	Get name.

Static Public Attributes
static const size_t	COUNT_MAX = 0xFFFEU
	Max count supported.

Private Types
typedef DLHeadType	ListHeadType
	List head type for ISyncObject elements.
typedef DLEntryType	ListEntryType
	List entry type of ISyncObject elements.
typedef DListEntry< ISyncObject, _ClosedLoop >	DLEntryType
	Convenience alias for this entry type. Used to avoid repeating the full template spelling.
typedef DListHead< ISyncObject, _ClosedLoop >	DLHeadType
	Convenience alias for the corresponding list head type.

Private Member Functions
	STK_NONCOPYABLE_CLASS (Semaphore)
virtual void	AddWaitObject (IWaitObject *wobj)
	Called by kernel when a new task starts waiting on this event.
virtual void	RemoveWaitObject (IWaitObject *wobj)
	Called by kernel when a waiting task is being removed (timeout expired, wait aborted, task terminated etc.).
virtual bool	Tick (Timeout elapsed_ticks)
	Called by kernel on every system tick to handle timeout logic of waiting tasks.
void	WakeOne ()
	Wake the first task in the wait list (FIFO order).
void	WakeAll ()
	Wake all tasks currently in the wait list.
DLHeadType *	GetHead () const
	Get the list head this entry currently belongs to.
DLEntryType *	GetNext () const
	Get the next entry in the list.
DLEntryType *	GetPrev () const
	Get the previous entry in the list.
bool	IsLinked () const
	Check whether this entry is currently a member of any list.
	operator ISyncObject * ()
	Implicit conversion to a mutable pointer to the host object (T).
	operator const ISyncObject * () const
	Implicit conversion to a const pointer to the host object (T).
void	Link (DLHeadType *head, DLEntryType *next, DLEntryType *prev)
	Wire this entry into a list between prev and next.
void	Unlink ()
	Remove this entry from its current list.

Private Attributes
uint16_t	m_count
	Internal resource counter.
uint16_t	m_count_max
	Counter max limit.
IWaitObject::ListHeadType	m_wait_list
	tasks blocked on this object
DLHeadType *	m_head
	Owning list head, or NULL when the entry is not linked.
DLEntryType *	m_next
	Next entry in the list, or NULL (open list boundary) / first entry (closed loop).
DLEntryType *	m_prev
	Previous entry in the list, or NULL (open list boundary) / last entry (closed loop).

Detailed Description

Counting semaphore primitive for resource management and signaling.

Counting semaphore maintains an internal counter to manage access to a limited number of resources. Unlike a Condition Variable, a Semaphore is stateful: if Signal() is called when no tasks are waiting, the signal is "remembered" by incrementing the internal counter.

Note

This implementation uses a Direct Handover policy: when a task is waiting, Signal() gives the resource "token" directly to the first task in the wait list without incrementing the counter. The waking task is then guaranteed ownership of that token upon returning from Wait().

// Example: Resource throttling
// Initialize with 3 permits (e.g., max 3 concurrent tasks accessing the same resource)
stk::sync::Semaphore g_Limiter(3);
 
void Worker() {
    // attempt to acquire a permit with a 1000 tick timeout
    if (g_Limiter.Wait(1000)) {
 
        // ... access limited resource ...
 
        // release the permit back to the semaphore
        g_Limiter.Signal();
    }
}

See also

ISyncObject, IWaitObject, IKernelService::Wait

Note

Only available when kernel is compiled with KERNEL_SYNC mode enabled.

Definition at line 55 of file stk_sync_semaphore.h.

Member Typedef Documentation

DLEntryType

typedef DListEntry<ISyncObject, _ClosedLoop> stk::util::DListEntry< ISyncObject, _ClosedLoop >::DLEntryType

inherited

Convenience alias for this entry type. Used to avoid repeating the full template spelling.

Definition at line 70 of file stk_linked_list.h.

DLHeadType

typedef DListHead<ISyncObject, _ClosedLoop> stk::util::DListEntry< ISyncObject, _ClosedLoop >::DLHeadType

inherited

Convenience alias for the corresponding list head type.

Definition at line 75 of file stk_linked_list.h.

ListEntryType

typedef DLEntryType stk::ISyncObject::ListEntryType

inherited

List entry type of ISyncObject elements.

Definition at line 371 of file stk_common.h.

ListHeadType

typedef DLHeadType stk::ISyncObject::ListHeadType

inherited

List head type for ISyncObject elements.

Definition at line 366 of file stk_common.h.

Constructor & Destructor Documentation

Semaphore()

stk::sync::Semaphore::Semaphore ( uint16_t initial_count = 0U, uint16_t max_count = COUNT_MAX )

inlineexplicit

Constructor.

Parameters

[in]

initial_count

Starting value of the semaphore.

Definition at line 65 of file stk_sync_semaphore.h.

        : m_count(initial_count), m_count_max(max_count)
    {
        STK_ASSERT(initial_count < max_count); // API contract: initial count must not exceed maximum
    }

References COUNT_MAX, m_count, m_count_max, and STK_ASSERT.

Referenced by STK_NONCOPYABLE_CLASS().

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~Semaphore()

stk::sync::Semaphore::~Semaphore ( )

inline

Destructor.

Note

If tasks are still waiting at destruction time it is considered a logical error (dangling waiters). An assertion is triggered in debug builds.

MISRA deviation: [STK-DEV-005] Rule 10-3-2.

Definition at line 77 of file stk_sync_semaphore.h.

    {
        STK_ASSERT(m_wait_list.IsEmpty()); // API contract: must not be destroyed with waiting tasks
    }

References stk::ISyncObject::m_wait_list, and STK_ASSERT.

Member Function Documentation

AddWaitObject()

virtual void stk::ISyncObject::AddWaitObject ( IWaitObject * wobj )

inlinevirtualinherited

Called by kernel when a new task starts waiting on this event.

Parameters

[in]

wobj

Wait object representing blocked task.

Definition at line 376 of file stk_common.h.

    {
        STK_ASSERT(wobj->GetHead() == nullptr);
        m_wait_list.LinkBack(wobj);
    }

References stk::util::DListEntry< T, _ClosedLoop >::GetHead(), m_wait_list, and STK_ASSERT.

Referenced by stk::Kernel< TMode, TSize, TStrategy, TPlatform >::KernelTask::WaitObject::SetupWait().

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GetCount()

uint16_t stk::sync::Semaphore::GetCount ( ) const

inline

Get current counter value.

Returns

Advisory snapshot of the counter. May be stale by the time the caller acts on it. Atomic on 32-bit aligned targets.

Note

ISR-safe on targets where a 32-bit aligned read is a single instruction (all supported STK architectures). Not safe for read-modify-write use.

Definition at line 111 of file stk_sync_semaphore.h.

{ return m_count; }

References m_count.

Referenced by osSemaphoreGetCount().

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GetHead()

DLHeadType * stk::util::DListEntry< ISyncObject, _ClosedLoop >::GetHead ( ) const

inlineinherited

Get the list head this entry currently belongs to.

Returns

Pointer to the owning DListHead, or NULL if the entry is not linked.

Definition at line 80 of file stk_linked_list.h.

{ return m_head; }

GetNext()

DLEntryType * stk::util::DListEntry< ISyncObject, _ClosedLoop >::GetNext ( ) const

inlineinherited

Get the next entry in the list.

Returns

Pointer to the next DListEntry, or NULL if this is the last entry (open list) or the first entry (closed loop, where next wraps to first).

Note

In a closed loop (_ClosedLoop == true) this pointer is never NULL when the entry is linked.

Definition at line 88 of file stk_linked_list.h.

{ return m_next; }

GetPrev()

DLEntryType * stk::util::DListEntry< ISyncObject, _ClosedLoop >::GetPrev ( ) const

inlineinherited

Get the previous entry in the list.

Returns

Pointer to the previous DListEntry, or NULL if this is the first entry (open list) or the last entry (closed loop, where prev wraps to last).

Note

In a closed loop (_ClosedLoop == true) this pointer is never NULL when the entry is linked.

Definition at line 96 of file stk_linked_list.h.

{ return m_prev; }

GetTraceName()

const char * stk::ITraceable::GetTraceName ( ) const

inlineinherited

Get name.

Returns

Name string, or NULL if not set or if STK_SYNC_DEBUG_NAMES is 0.

Definition at line 336 of file stk_common.h.

    {
    #if STK_SYNC_DEBUG_NAMES
        return m_trace_name;
    #else
        return nullptr;
    #endif
    }

IsLinked()

bool stk::util::DListEntry< ISyncObject, _ClosedLoop >::IsLinked ( ) const

inlineinherited

Check whether this entry is currently a member of any list.

Returns

true if linked (m_head != NULL); false otherwise.

Definition at line 101 of file stk_linked_list.h.

{ return (GetHead() != nullptr); }

Link()

void stk::util::DListEntry< ISyncObject, _ClosedLoop >::Link ( DLHeadType * head, DLEntryType * next, DLEntryType * prev )

inlineprivateinherited

Wire this entry into a list between prev and next.

Parameters

[in]	head	The owning DListHead. Stored as a back-pointer for IsLinked() and ownership checks.
[in]	next	The entry that will follow this one, or NULL if this becomes the last entry.
[in]	prev	The entry that will precede this one, or NULL if this becomes the first entry.

Note

Called exclusively by DListHead::Link(). Assumes the entry is not currently linked. Updates the neighbours' forward/back pointers to splice this entry in.

Definition at line 137 of file stk_linked_list.h.

    {
        m_head = head;
        m_next = next;
        m_prev = prev;
 
        if (m_prev != nullptr)
            m_prev->m_next = this;
 
        if (m_next != nullptr)
            m_next->m_prev = this;
    }

operator const ISyncObject *()

stk::util::DListEntry< ISyncObject, _ClosedLoop >::operator const ISyncObject * ( ) const

inlineinherited

Implicit conversion to a const pointer to the host object (T).

Note

Safe because T must derive from DListEntry<T, _ClosedLoop>. Eliminates the need for explicit static_cast at call sites.

MISRA deviation: [STK-DEV-004] Rule 5-2-x.

Definition at line 115 of file stk_linked_list.h.

{ return static_cast<const T *>(this); }

operator ISyncObject *()

stk::util::DListEntry< ISyncObject, _ClosedLoop >::operator ISyncObject* ( )

inlineinherited

Implicit conversion to a mutable pointer to the host object (T).

Note

Safe because T must derive from DListEntry<T, _ClosedLoop>. Eliminates the need for explicit static_cast at call sites.

MISRA deviation: [STK-DEV-004] Rule 5-2-x.

Definition at line 108 of file stk_linked_list.h.

{ return static_cast<T *>(this); }

RemoveWaitObject()

virtual void stk::ISyncObject::RemoveWaitObject ( IWaitObject * wobj )

inlinevirtualinherited

Called by kernel when a waiting task is being removed (timeout expired, wait aborted, task terminated etc.).

Parameters

[in]

wobj

Wait object to remove from the wait list.

Reimplemented in stk::sync::Event.

Definition at line 385 of file stk_common.h.

    {
        STK_ASSERT(wobj->GetHead() == &m_wait_list);
        m_wait_list.Unlink(wobj);
    }

References stk::util::DListEntry< T, _ClosedLoop >::GetHead(), m_wait_list, and STK_ASSERT.

Referenced by stk::sync::Event::RemoveWaitObject().

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SetTraceName()

void stk::ITraceable::SetTraceName ( const char * name )

inlineinherited

Set name.

Parameters

[in]

name

Null-terminated string or NULL.

Note

If STK_SYNC_DEBUG_NAMES is 0 then calling this function has no effect.

Definition at line 324 of file stk_common.h.

    {
    #if STK_SYNC_DEBUG_NAMES
        m_trace_name = name;
    #else
        STK_UNUSED(name);
    #endif
    }

References STK_UNUSED.

Signal()

void stk::sync::Semaphore::Signal ( )

inline

Post a signal (increment counter).

Note

Gives "token" directly to the waking task. The count is not incremented, and the waking task does not decrement it.

ISR-safe.

Definition at line 152 of file stk_sync_semaphore.h.

{
    ScopedCriticalSection cs_;
 
    if (m_wait_list.IsEmpty())
    {
        STK_ASSERT(m_count < m_count_max); // API contract: the count must not exceed maximum
 
        // no one is waiting, save signal for later
        m_count = static_cast<uint16_t>(m_count + 1U);
        __stk_full_memfence();
    }
    else
    {
        // give signal directly to the first waiting task
        WakeOne();
    }
}

References m_count, m_count_max, stk::ISyncObject::m_wait_list, STK_ASSERT, and stk::ISyncObject::WakeOne().

Referenced by stk_sem_signal().

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STK_NONCOPYABLE_CLASS()

stk::sync::Semaphore::STK_NONCOPYABLE_CLASS ( Semaphore )

private

References Semaphore().

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Tick()

bool stk::ISyncObject::Tick ( Timeout elapsed_ticks )

inlinevirtualinherited

Called by kernel on every system tick to handle timeout logic of waiting tasks.

Implementation of ISyncObject::Tick, see ISyncObject. Placed here as it depends on hw namespace.

Parameters

[in]

elapsed_ticks

Number of ticks elapsed between this and previous calls, in case of KERNEL_TICKLESS mode this value can be >1, for non-tickless mode it is always 1.

Returns

true if this synchronization object still has waiters with a finite timeout and requires further tick calls. false if the wait list is empty or all remaining waiters have infinite timeouts, signaling to the kernel that it may stop calling Tick() for this object until a new waiter is added.

Note

When this method returns false, the kernel unlinks this object from its active sync list. It will be re-linked automatically when the next waiter is added via AddWaitObject().

Definition at line 467 of file stk_arch.h.

{
    // note: ScopedCriticalSection usage
    //
    // Single-core: no critical section needed - Tick() runs inside the
    // SysTick ISR which already executes with interrupts disabled, making
    // re-entrancy impossible on the local core.
    //
    // Multi-core: critical section is required because the tick handler on
    // each core may call Tick() concurrently for the same Semaphore instance,
    // and ISyncObject::Tick() is not re-entrant.
#if (STK_ARCH_CPU_COUNT > 1)
    hw::CriticalSection::ScopedLock cs_;
#endif
 
    IWaitObject *itr = static_cast<IWaitObject *>(m_wait_list.GetFirst());
 
    while (itr != nullptr)
    {
        IWaitObject *next = static_cast<IWaitObject *>(itr->GetNext());
 
        if (!itr->Tick(elapsed_ticks))
            itr->Wake(true);
 
        itr = next;
    }
 
    return !m_wait_list.IsEmpty();
}

References stk::util::DListEntry< T, _ClosedLoop >::GetNext(), m_wait_list, stk::IWaitObject::Tick(), and stk::IWaitObject::Wake().

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TryWait()

bool stk::sync::Semaphore::TryWait ( )

inline

Poll the semaphore without blocking (decrement counter if available).

Checks if a resource token is available. If so, decrements the counter and returns immediately without yielding the CPU or touching the kernel wait list. If the counter is zero, returns immediately with false.

Note

ISR-safe.

Returns

true if a resource token was acquired, false if count was zero.

Definition at line 96 of file stk_sync_semaphore.h.

{ return Wait(NO_WAIT); }

References stk::NO_WAIT, and Wait().

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Unlink()

void stk::util::DListEntry< ISyncObject, _ClosedLoop >::Unlink ( )

inlineprivateinherited

Remove this entry from its current list.

Note

Called exclusively by DListHead::Unlink(). Patches the neighbours' pointers to bridge over this entry, then clears m_head, m_next, and m_prev to NULL so the entry is in a clean unlinked state.

Does not update DListHead::m_count or m_first / m_last — those are the responsibility of the calling DListHead::Unlink().

Definition at line 157 of file stk_linked_list.h.

    {
        if (m_prev != nullptr)
            m_prev->m_next = m_next;
 
        if (m_next != nullptr)
            m_next->m_prev = m_prev;
 
        m_head = nullptr;
        m_next = nullptr;
        m_prev = nullptr;
    }

Wait()

bool stk::sync::Semaphore::Wait ( Timeout timeout = WAIT_INFINITE )

inline

Wait for a signal (decrement counter).

Parameters

[in]

timeout

Maximum time to wait (ticks).

Warning

ISR-safe only with timeout = NO_WAIT, ISR-unsafe otherwise.

Returns

True if acquired, false if timeout occurred.

Definition at line 124 of file stk_sync_semaphore.h.

{
    ScopedCriticalSection cs_;
 
    // fast path: resource is available
    if (m_count != 0U)
    {
        m_count = static_cast<uint16_t>(m_count - 1U);
        __stk_full_memfence();
        return true;
    }
 
    // try lock behavior (timeout=NO_WAIT)
    if (timeout == NO_WAIT)
        return false;
 
    STK_ASSERT(!hw::IsInsideISR()); // API contract: caller must not be in ISR if timeout!=NO_WAIT
 
    // slow path: block until Signal() or timeout
    // note: after waking, if not a timeout, we effectively own the resource that Signal() produced
    // but didn't put into m_count (see logic of if (m_wait_list.IsEmpty()) in Signal())
    return !IKernelService::GetInstance()->Wait(this, &cs_, timeout)->IsTimeout();
}

References stk::IKernelService::GetInstance(), stk::hw::IsInsideISR(), stk::IWaitObject::IsTimeout(), m_count, stk::NO_WAIT, STK_ASSERT, and stk::IKernelService::Wait().

Referenced by osSemaphoreAcquire(), stk_sem_wait(), and TryWait().

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WakeAll()

void stk::ISyncObject::WakeAll ( )

inlineprotectedinherited

Wake all tasks currently in the wait list.

Note

Each woken task is notified with timeout=false, indicating a successful signal (not a timeout expiry).

Does nothing if no tasks are currently waiting.

Definition at line 425 of file stk_common.h.

    {
        while (!m_wait_list.IsEmpty())
            static_cast<IWaitObject *>(m_wait_list.GetFirst())->Wake(false);
    }

References m_wait_list, and stk::IWaitObject::Wake().

Referenced by stk::sync::ConditionVariable::NotifyAll_CS(), stk::sync::Event::Pulse(), and stk::sync::Event::Set().

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WakeOne()

void stk::ISyncObject::WakeOne ( )

inlineprotectedinherited

Wake the first task in the wait list (FIFO order).

Note

The woken task is notified with timeout=false, indicating a successful signal (not a timeout expiry).

Does nothing if no tasks are currently waiting.

Definition at line 415 of file stk_common.h.

    {
        if (!m_wait_list.IsEmpty())
            static_cast<IWaitObject *>(m_wait_list.GetFirst())->Wake(false);
    }

References m_wait_list, and stk::IWaitObject::Wake().

Referenced by stk::sync::ConditionVariable::NotifyOne_CS(), stk::sync::Event::Pulse(), stk::sync::Event::Set(), and stk::sync::Semaphore::Signal().

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Member Data Documentation

COUNT_MAX

const size_t stk::sync::Semaphore::COUNT_MAX = 0xFFFEU

static

Max count supported.

Definition at line 60 of file stk_sync_semaphore.h.

Referenced by osSemaphoreNew(), and Semaphore().

m_count

uint16_t stk::sync::Semaphore::m_count

private

Internal resource counter.

Definition at line 116 of file stk_sync_semaphore.h.

Referenced by GetCount(), Semaphore(), Signal(), and Wait().

m_count_max

uint16_t stk::sync::Semaphore::m_count_max

private

Counter max limit.

Definition at line 117 of file stk_sync_semaphore.h.

Referenced by Semaphore(), and Signal().

m_head

DLHeadType* stk::util::DListEntry< ISyncObject, _ClosedLoop >::m_head

privateinherited

Owning list head, or NULL when the entry is not linked.

Definition at line 170 of file stk_linked_list.h.

m_next

DLEntryType* stk::util::DListEntry< ISyncObject, _ClosedLoop >::m_next

privateinherited

Next entry in the list, or NULL (open list boundary) / first entry (closed loop).

Definition at line 171 of file stk_linked_list.h.

m_prev

DLEntryType* stk::util::DListEntry< ISyncObject, _ClosedLoop >::m_prev

privateinherited

Previous entry in the list, or NULL (open list boundary) / last entry (closed loop).

Definition at line 172 of file stk_linked_list.h.

m_wait_list

IWaitObject::ListHeadType stk::ISyncObject::m_wait_list

protectedinherited

tasks blocked on this object

Definition at line 431 of file stk_common.h.

Referenced by AddWaitObject(), ISyncObject(), stk::sync::Event::Pulse(), RemoveWaitObject(), stk::sync::Semaphore::Signal(), Tick(), stk::sync::Mutex::Unlock(), WakeAll(), WakeOne(), stk::sync::ConditionVariable::~ConditionVariable(), stk::sync::Event::~Event(), stk::sync::Mutex::~Mutex(), and stk::sync::Semaphore::~Semaphore().

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

• stk_sync_semaphore.h