Книга: Real-Time Concepts for Embedded Systems
6.3.2 Acquiring and Releasing Semaphores
6.3.2 Acquiring and Releasing Semaphores
Table 6.2 identifies the operations used to acquire or release semaphores.
Table 6.2: Semaphore acquire and release operations.
| Operation | Description |
|---|---|
| Acquire | Acquire a semaphore token |
| Release | Release a semaphore token |
The operations for acquiring and releasing a semaphore might have different names, depending on the kernel: for example, take and give, sm_p and sm_v,pend and post, and lock and unlock. Regardless of the name, they all effectively acquire and release semaphores.
Tasks typically make a request to acquire a semaphore in one of the following ways:
· Wait forever - task remains blocked until it is able to acquire a semaphore.
· Wait with a timeout - task remains blocked until it is able to acquire a semaphore or until a set interval of time, called the timeout interval, passes. At this point, the task is removed from the semaphore’s task-waiting list and put in either the ready state or the running state.
· Do not wait - task makes a request to acquire a semaphore token, but, if one is not available, the task does not block.
Note that ISRs can also release binary and counting semaphores. Note that most kernels do not support ISRs locking and unlocking mutexes, as it is not meaningful to do so from an ISR. It is also not meaningful to acquire either binary or counting semaphores inside an ISR.
Any task can release a binary or counting semaphore; however, a mutex can only be released (unlocked) by the task that first acquired (locked) it. Note that incorrectly releasing a binary or counting semaphore can result in losing mutually exclusive access to a shared resource or in an I/O device malfunction.
For example, a task can gain access to a shared data structure by acquiring an associated semaphore. If a second task accidentally releases that semaphore, this step can potentially free a third task waiting for that same semaphore, allowing that third task to also gain access to the same data structure. Having multiple tasks trying to modify the same data structure at the same time results in corrupted data.
Оглавление книги
- Разработка приложений баз данных InterBase на Borland Delphi
- Open Source Insight and Discussion
- Introduction to Microprocessors and Microcontrollers
- Chapter 6. Traversing of tables and chains
- Chapter 8. Saving and restoring large rule-sets
- Chapter 11. Iptables targets and jumps
- Chapter 5 Installing and Configuring VirtualCenter 2.0
- Chapter 16. Commercial products based on Linux, iptables and netfilter
- Appendix A. Detailed explanations of special commands
- Appendix B. Common problems and questions
- Appendix E. Other resources and links
- IP filtering terms and expressions