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// ==== Semaphore Management ====
/**
\addtogroup CMSIS_RTOS_SemaphoreMgmt Semaphores
\ingroup CMSIS_RTOS
\brief Access shared resources simultaneously from different threads.
\details
Semaphores are used to manage and protect access to shared resources. Semaphores are very similar to
\ref CMSIS_RTOS_MutexMgmt "Mutexes". Whereas a Mutex permits just one thread to access a shared resource at a
time, a semaphore can be used to permit a fixed number of threads/ISRs to access a pool of shared resources. Using
semaphores, access to a group of identical peripherals can be managed (for example multiple DMA channels).
\image html "Semaphore.png" "CMSIS-RTOS Semaphore"
A semaphore object should be initialized to the maximum number of available tokens. This number of available resources is
specified as parameter of the \ref osSemaphoreNew function. Each time a semaphore token is obtained with \ref osSemaphoreAcquire
(in \em available state), the semaphore count is decremented. When the semaphore count is 0 (i.e. \em depleted state), no
more semaphore tokens can be obtained. The thread/ISR that tries to obtain the semaphore token needs to wait until the next
token is free. Semaphores are released with \ref osSemaphoreRelease incrementing the semaphore count.
\image html "semaphore_states.png" "CMSIS-RTOS Semaphore States"
\note The functions \ref osSemaphoreAcquire, \ref osSemaphoreGetCount, and \ref osSemaphoreRelease can be called from
\ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
Semaphore Use Cases
-------------------
Due to their flexibility, semaphores cover a wide range of synchronizing applications. At the same time, they are perhaps the
most challenging RTOS object to understand. The following explains a use case for semaphores, taken from the book
The Little Book Of Semaphores by Allen B. Downey which
is available for free download.
Non-binary Semaphore (Multiplex)
A multiplex limits the number of threads that can access a critical section of code. For example, this could be a function
accessing DMA resources which can only support a limited number of calls.
To allow multiple threads to run the function, initialize a semaphore to the maximum number of threads that can be allowed.
The number of tokens in the semaphore represents the number of additional threads that may enter. If this number is zero,
then the next thread trying to access the function will have to wait until one of the other threads exits and releases its
token. When all threads have exited the token number is back to n. The following example shows the code for one of the
threads that might access the resource:
\code
osSemaphoreId_t multiplex_id;
void thread_n (void) {
multiplex_id = osSemaphoreNew(3U, 3U, NULL);
while(1) {
osSemaphoreAcquire(multiplex_id, osWaitForever);
// do something
osSemaphoreRelease(multiplex_id);
}
}
\endcode
Producer/Consumer Semaphore
The producer-consumer problem can be solved using two semaphores.
A first semaphore (\token{empty_id}) counts down the available (empty) buffers, i.e.
the producer thread can wait for available buffer slots by acquiring from this one.
A second semaphore (\token{filled_id}) counts up the used (filled) buffers, i.e.
the consumer thread can wait for available data by acquiring from this one.
It is crucial for the correct behaviour that the threads acquire and release on both
semaphores in the given sequence. According to this example one can have multiple
producer and/or consumer threads running concurrently.
\code
#define BUFFER_SIZE 10U
osSemaphoreId_t empty_id = osSemaphoreNew(BUFFER_SIZE, BUFFER_SIZE, NULL);
osSemaphoreId_t filled_id = osSemaphoreNew(BUFFER_SIZE, 0U, NULL);
void producer_thread (void) {
while(1) {
osSemaphoreAcquire(empty_id, osWaitForever);
// produce data
osSemaphoreRelease(filled_id);
}
}
void consumer_thread (void) {
while(1){
osSemaphoreAcquire(filled_id, osWaitForever);
// consume data
osSemaphoreRelease(empty_id);
}
}
\endcode
@{
*/
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/**
\typedef osSemaphoreId_t
\details
Returned by:
- \ref osSemaphoreNew
*/
/**
\struct osSemaphoreAttr_t
\details
Specifies the following attributes for the \ref osSemaphoreNew function.
*/
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/**
\fn osSemaphoreId_t osSemaphoreNew (uint32_t max_count, uint32_t initial_count, const osSemaphoreAttr_t *attr)
\details
The function \b osSemaphoreNew creates and initializes a semaphore object that is used to manage access to shared resources
and returns the pointer to the semaphore object identifier or \token{NULL} in case of an error. It can be safely called
before the RTOS is started (call to \ref osKernelStart), but not before it is initialized (call to \ref osKernelInitialize).
The parameter \em max_count specifies the maximum number of available tokens. A \em max_count value of 1 creates a binary
semaphore.
The parameter \em initial_count sets the initial number of available tokens.
The parameter \em attr specifies additional semaphore attributes. Default attributes will be used if set to \token{NULL}.
\note This function \b cannot be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
Code Example
\code
#include "cmsis_os2.h" // CMSIS RTOS header file
osSemaphoreId_t sid_Semaphore; // semaphore id
osThreadId_t tid_Thread_Semaphore; // thread id
void Thread_Semaphore (void *argument); // thread function
int Init_Semaphore (void) {
sid_Semaphore = osSemaphoreNew(2U, 2U, NULL);
if (sid_Semaphore == NULL) {
; // Semaphore object not created, handle failure
}
tid_Thread_Semaphore = osThreadNew(Thread_Semaphore, NULL, NULL);
if (tid_Thread_Semaphore == NULL) {
return(-1);
}
return(0);
}
void Thread_Semaphore (void *argument) {
osStatus_t val;
while (1) {
; // Insert thread code here...
val = osSemaphoreAcquire(sid_Semaphore, 10U); // wait for max. 10 ticks for semaphore token to get available
switch (val) {
case osOK:
; // Use protected code here...
osSemaphoreRelease(sid_Semaphore); // return a token back to a semaphore
break;
case osErrorResource:
break;
case osErrorParameter:
break;
default:
break;
}
osThreadYield(); // suspend thread
}
}
\endcode
*/
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/**
\fn const char *osSemaphoreGetName (osSemaphoreId_t semaphore_id)
\details
The function \b osSemaphoreGetName returns the pointer to the name string of the semaphore identified by parameter \a
semaphore_id or \token{NULL} in case of an error.
\note This function may be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
*/
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/**
\fn osStatus_t osSemaphoreAcquire (osSemaphoreId_t semaphore_id, uint32_t timeout)
\details
The blocking function \b osSemaphoreAcquire waits until a token of the semaphore object specified by parameter
\a semaphore_id becomes available. If a token is available, the function instantly returns and decrements the token count.
The parameter \a timeout specifies how long the system waits to acquire the token. While the system waits, the thread
that is calling this function is put into the \ref ThreadStates "BLOCKED" state. The parameter \ref CMSIS_RTOS_TimeOutValue
"timeout" can have the following values:
- when \a timeout is \token{0}, the function returns instantly (i.e. try semantics).
- when \a timeout is set to \b osWaitForever the function will wait for an infinite time until the semaphore becomes
available (i.e. wait semantics).
- all other values specify a time in kernel ticks for a timeout (i.e. timed-wait semantics).
Possible \ref osStatus_t return values:
- \em osOK: the token has been obtained and the token count decremented.
- \em osErrorTimeout: the token could not be obtained in the given time.
- \em osErrorResource: the token could not be obtained when no \a timeout was specified.
- \em osErrorParameter: the parameter \a semaphore_id is \token{NULL} or invalid.
- \em osErrorSafetyClass: the calling thread safety class is lower than the safety class of the specified semaphore.
\note May be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines" if the parameter \a timeout is set to
\token{0}.
Code Example
Refer to \ref osSemaphoreNew.
*/
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/**
\fn osStatus_t osSemaphoreRelease (osSemaphoreId_t semaphore_id)
\details
The function \b osSemaphoreRelease releases a token of the semaphore object specified by parameter \a semaphore_id. Tokens
can only be released up to the maximum count specified at creation time, see \ref osSemaphoreNew. Other threads that
currently wait for a token of this semaphore object will be put into the \ref ThreadStates "READY" state.
Possible \ref osStatus_t return values:
- \em osOK: the token has been released and the count incremented.
- \em osErrorResource: the token could not be released (maximum token count has been reached).
- \em osErrorParameter: the parameter \a semaphore_id is \token{NULL} or invalid.
- \em osErrorSafetyClass: the calling thread safety class is lower than the safety class of the specified semaphore.
\note This function may be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
Code Example
Refer to \ref osSemaphoreNew.
*/
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/**
\fn uint32_t osSemaphoreGetCount (osSemaphoreId_t semaphore_id)
\details
The function \b osSemaphoreGetCount returns the number of available tokens of the semaphore object specified by parameter
\a semaphore_id. In case of an error it returns \token{0}.
\note This function may be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
*/
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/**
\fn osStatus_t osSemaphoreDelete (osSemaphoreId_t semaphore_id)
\details
The function \b osSemaphoreDelete deletes a semaphore object specified by parameter \a semaphore_id. It releases internal
memory obtained for semaphore handling. After this call, the \a semaphore_id is no longer valid and cannot be used. The
semaphore may be created again using the function \ref osSemaphoreNew.
Possible \ref osStatus_t return values:
- \em osOK: the semaphore object has been deleted.
- \em osErrorParameter: the parameter \a semaphore_id is \token{NULL} or invalid.
- \em osErrorResource: the semaphore is in an invalid state.
- \em osErrorISR: \b osSemaphoreDelete cannot be called from interrupt service routines.
- \em osErrorSafetyClass: the calling thread safety class is lower than the safety class of the specified semaphore.
\note This function \b cannot be called from \ref CMSIS_RTOS_ISR_Calls "Interrupt Service Routines".
*/
/// @}
// these struct members must stay outside the group to avoid double entries in documentation
/**
\var osSemaphoreAttr_t::attr_bits
\details
Reserved for future use (must be set to '0' for future compatibility).
\var osSemaphoreAttr_t::cb_mem
\details
Pointer to a memory for the semaphore control block object. Refer to \ref CMSIS_RTOS_MemoryMgmt_Manual for more information.
Default: \token{NULL} to use \ref CMSIS_RTOS_MemoryMgmt_Automatic for the semaphore control block.
\var osSemaphoreAttr_t::cb_size
\details
The size (in bytes) of memory block passed with \ref cb_mem. Required value depends on the underlying kernel implementation.
Default: \token{0} as the default is no memory provided with \ref cb_mem.
\var osSemaphoreAttr_t::name
\details
Pointer to a constant string with a human readable name (displayed during debugging) of the semaphore object.
Default: \token{NULL} no name specified.
*/