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CMSIS-RTOS2
Version 2.0.0
Real-Time Operating System: API and RTX Reference Implementation
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Access shared resources simultaneously from different threads. More...
Data Structures | |
struct | osSemaphoreAttr_t |
Attributes structure for semaphore. More... | |
Typedefs | |
typedef void * | osSemaphoreId_t |
Functions | |
osSemaphoreId_t | osSemaphoreNew (uint32_t max_count, uint32_t initial_count, const osSemaphoreAttr_t *attr) |
Create and Initialize a Semaphore object. More... | |
const char * | osSemaphoreGetName (osSemaphoreId_t semaphore_id) |
Get name of a Semaphore object. More... | |
osStatus_t | osSemaphoreAcquire (osSemaphoreId_t semaphore_id, uint32_t timeout) |
Acquire a Semaphore token or timeout if no tokens are available. More... | |
osStatus_t | osSemaphoreRelease (osSemaphoreId_t semaphore_id) |
Release a Semaphore token that was acquired by osSemaphoreAcquire. More... | |
uint32_t | osSemaphoreGetCount (osSemaphoreId_t semaphore_id) |
Get current Semaphore token count. More... | |
osStatus_t | osSemaphoreDelete (osSemaphoreId_t semaphore_id) |
Delete a Semaphore object. More... | |
Semaphores are used to manage and protect access to shared resources. Semaphores are very similar to 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 to access a pool of shared resources. Using semaphores, access to a group of identical peripherals can be managed (for example multiple DMA channels).
A semaphore object should be initialized to the maximum number of available tokens. This number of available resources is specified as parameter of the osSemaphoreNew function. Each time a semaphore token is obtained with osSemaphoreAcquire, the semaphore count is decremented. When the semaphore count is 0, no semaphore token can be obtained. The thread that tries to obtain the semaphore token needs to wait until the next token is free. Semaphores are released with osSemaphoreRelease incrementing the semaphore count.
Follow these steps to create and use a semaphore:
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. Ths following example shows the code for one of the threads that might access the resource:
struct osSemaphoreAttr_t |
Semaphore ID identifies the semaphore.
osSemaphoreId_t osSemaphoreNew | ( | uint32_t | max_count, |
uint32_t | initial_count, | ||
const osSemaphoreAttr_t * | attr | ||
) |
[in] | max_count | maximum number of available tokens. |
[in] | initial_count | initial number of available tokens. |
[in] | attr | semaphore attributes; NULL: default values. |
Create and initialize a Semaphore object that is used to manage access to shared resources. The parameter count specifies the number of available resources. The max_count value 1 creates a binary semaphore.
Code Example
const char * osSemaphoreGetName | ( | osSemaphoreId_t | semaphore_id | ) |
[in] | semaphore_id | semaphore ID obtained by osSemaphoreNew. |
osStatus_t osSemaphoreAcquire | ( | osSemaphoreId_t | semaphore_id, |
uint32_t | timeout | ||
) |
[in] | semaphore_id | semaphore ID obtained by osSemaphoreNew. |
[in] | timeout | Timeout Value or 0 in case of no time-out. |
Wait until a Semaphore token becomes available and acquires it for the thread if available. When no Semaphore token is available, the function waits for the time specified with the parameter timeout.
The argument timeout specifies how long the system waits for a Semaphore token to become available. While the system waits the thread that is calling this function is put into the state BLOCKED. The millisec timeout can have the following values:
The return value indicates the number of available tokens (the semaphore count value). If 0 is returned, then no semaphore was available.
osStatus_t osSemaphoreRelease | ( | osSemaphoreId_t | semaphore_id | ) |
[in] | semaphore_id | semaphore ID obtained by osSemaphoreNew. |
Release a Semaphore token. This increments the count of available semaphore tokens.
osStatus_t return values:
uint32_t osSemaphoreGetCount | ( | osSemaphoreId_t | semaphore_id | ) |
[in] | semaphore_id | semaphore ID obtained by osSemaphoreNew. |
Returns the count of available semaphores of the semaphore object specified by semaphore_id.
osStatus_t osSemaphoreDelete | ( | osSemaphoreId_t | semaphore_id | ) |
[in] | semaphore_id | semaphore ID obtained by osSemaphoreNew. |
Delete a Semaphore object. The function releases internal memory obtained for Semaphore handling. After this call the semaphore_id is no longer valid and cannot be used. The Semaphore may be created again using the function osSemaphoreNew.
osStatus_t return values: