Control access to shared resources.
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The Semaphore Management function group is used to manage and protect access to shared resources. For example, with a Semaphore the access to a group of identical peripherals can be managed. The number of available resources is specified as parameter of the osSemaphoreCreate function.
Each time a Semaphore token is obtained with osSemaphoreWait the semaphore count is decremented. When the semaphore count is 0, no Semaphore token can be obtained. Semaphores are released with osSemaphoreRelease; this function increments the semaphore count.
CMSIS-RTOS Semaphore
#define osFeature_Semaphore 65535 |
CMSIS-RTOS RTX supports an index count up to 65535 for a semaphore .
#define osSemaphore |
( |
|
name | ) |
&os_semaphore_def_##name |
Access to semaphore object for the functions osSemaphoreCreate.
- Parameters
-
name | name of the semaphore object. |
#define osSemaphoreDef |
( |
|
name | ) |
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Define a semaphore object that is referenced by osSemaphore.
- Parameters
-
name | name of the semaphore object. |
- Parameters
-
[in] | semaphore_def | semaphore definition referenced with osSemaphore. |
[in] | count | number of available resources. |
- Returns
- semaphore ID for reference by other functions or NULL in case of error.
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 count value 1 creates a binary semaphore.
- Note
- The value count is only used to set the initial token count. The initial token count is not stored in memory (to save RAM as the RTX implementation tries to use as little resource as possible). Therefore no error is generated when osSemaphoreRelease is used to release more tokens than specified with the initial token count.
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- Cannot be called from Interrupt Service Routines.
Example
void thread1 (void const *argument) {
int32_t value;
while (1) {
if (val > 0) {
:
}
}
}
void thread2 (void const *argument) {
while (1) {
:
}
}
void StartApplication (void) {
:
}
- Parameters
-
- Returns
- status code that indicates the execution status of the function.
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 After this call the semaphore_id is no longer valid and cannot be used.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 osSemaphoreCreate.
Status and Error Codes
- osOK: the semaphore object has been deleted.
- osErrorISR: osSemaphoreDelete cannot be called from interrupt service routines.
- osErrorResource: the semaphore object could not be deleted.
- osErrorParameter: the parameter semaphore_id is incorrect.
- Note
- Cannot be called from Interrupt Service Routines.
- Parameters
-
- Returns
- status code that indicates the execution status of the function.
Release a Semaphore token. This increments the count of available semaphore tokens.
- Note
- Interrupt Service Routines can call this function.
Status and Error Codes
- osOK: the semaphore has been released.
- osErrorResource: all tokens have already been released.
- osErrorParameter: the parameter semaphore_id is incorrect.
int32_t osSemaphoreWait |
( |
osSemaphoreId |
semaphore_id, |
|
|
uint32_t |
millisec |
|
) |
| |
- Parameters
-
- Returns
- number of available tokens, or -1 in case of incorrect parameters.
Wait until a Semaphore token becomes available. When no Semaphore token is available, the function waits for the time specified with the parameter millisec.
The argument millisec 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 WAITING. The millisec timeout can have the following values:
- when millisec is 0, the function returns instantly.
- when millisec is set to osWaitForever the function will wait for an infinite time until the Semaphore token becomes available.
- all other values specify a time in millisecond for a timeout.
The return value indicates the number of available tokens (the semaphore count value). If 0 is returned, then no semaphore was available.
- Note
- Cannot be called from Interrupt Service Routines.