B_ QuRT_ User_ Guide(17)

11 Semaphores

Threads use semaphores to synchronously access shared resources . When a semaphore is initialized , It is assigned an integer value . This value indicates the number of threads that can simultaneously access shared resources through the semaphore . The default value is 1.
When a thread executes on a semaphore down In operation , The result depends on the count value of the signal lamp .

• If the count value is non-zero , Is decremented , The thread gains access to the shared resource and continues execution .
• If the count value is zero , Will not be decremented , The thread is suspended on the signal . When the count value becomes non-zero （ Because another thread released the signal ）, It will be decremented , The suspended thread wakes up and gains access to the shared resource .
  When a thread to a semaphore When working up ,semaphore The count value of is incremented . The result depends on the number of threads waiting on the signal .
• If no thread is waiting , The current thread releases access to shared resources and continues execution .
• If one or more threads are waiting , also semaphore The value of is non-zero , The kernel wakes up the waiting thread with the highest priority , And decreasing semaphore The numerical . If the awakened thread has a higher priority than the current thread , Context switching will occur .
  add The operation is similar to up, But you can put semaphore The value of is increased to be greater than 1 The degree of . therefore ,add It is possible to wake up multiple waiting threads in one operation .
  try down Operation enables a thread to attempt to access a shared resource , If it's semaphore The count value of is 0, There is no risk of suspension .
• If the count is non-zero , Then the attempted lowering operation is the same as the conventional lowering operation .
• If the count is zero ,try down A value representing the zero count status will be returned .Semaphores It's a shared object , The following operations are supported .
• qurt_sem_add()
• qurt_sem_destroy()
• qurt_sem_down()
• qurt_sem_get_val()
• qurt_sem_init()
• qurt_sem_init_val()
• qurt_sem_try_down()
• qurt_sem_up()
• qurt_sem_down_timed()
• Data Types

11.1 qurt_sem_add()

11.1.1 Function Documentation

11.1.1.1 int qurt_sem_add ( qurt_sem_t ∗ sem, unsigned int amt )

Release access to a shared resource （ The specified number will increase the number of semaphores ）.
When a thread is in a semaphore When performing an add operation on , The specified value increases semaphore Count of . The result depends on the number of threads waiting on the signal .

• When no thread is waiting , The current thread releases access to shared resources and continues execution .
• When there is one or more threads waiting , also semaphore When the count is non-zero , The kernel will repeatedly wake up the waiting thread with the highest priority , And decreasing semaphore Count , Until there is no waiting thread or semaphore The count is zero . If any awakened thread has a higher priority than the current thread , Context switching will occur .
  Related data types
  qurt_sem_t
  Parameters
  
  Return value
  Unused integer value.
  Dependency relationship
  None.

11.2 qurt_sem_destroy()

11.2.1 Function Documentation

11.2.1.1 void qurt_sem_destroy ( qurt_sem_t ∗ sem )

Destroy designated semaphore.
Be careful ： When semaphores When it's no longer in use , It must be destroyed . Failure to do so will result in QuRT Kernel resource leak .
Semaphores Cannot be destroyed while still in use . If this happens ,QuRT The behavior of will not be defined .
Related data types
qurt_sem_t
Parameters

Return value
None.
Dependency relationship
None.

11.3 qurt_sem_down()

11.3.1 Function Documentation

11.3.1.1 int qurt_sem_down ( qurt_sem_t ∗ sem )

Request access to a shared resource . When a thread moves down a semaphore , As a result,
Depending on semaphore The count of .

• When the count value is non-zero , It is decremented , The thread gains access to the shared resource and continues execution .
• When the count is zero , It will not be decremented , Thread is suspended on semaphore . When the count value becomes non-zero （ Because another thread released the signal ）, It is decremented , The suspended thread wakes up and gains access to the shared resource .
  Related data types
  qurt_sem_t
  Parameters
  
  Return value
  Unused integer value.

Dependency relationship
None.

11.4 qurt_sem_get_val()

11.4.1 Function Documentation

11.4.1.1 static unsigned short qurt_sem_get_val ( qurt_sem_t ∗ sem )

obtain semaphore The count of .
Returns the specified semaphore The current count of .

Related data types
qurt_sem_t
Parameters

Return value
Integer semaphore count value
Dependency relationship
None.

11.5 qurt_sem_init()

11.5.1 Function Documentation

11.5.1.1 void qurt_sem_init ( qurt_sem_t ∗ sem )

Initialize a semaphore object . The default initial value is 1, namely semaphore Count value .
Parameters

Return value
None.
Dependency relationship
None.