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| title | Semaphore API Summary |
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| Function Name | Operation |
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OSSemCreate() | Create a semaphore. | OSSemDel() | Delete a semaphore. | OSSemPend() | Wait on a semaphore. | OSSemPendAbort() | Abort the wait on a semaphore. | OSSemPost() | Signal a semaphore. | OSSemSet() | Force the semaphore count to a desired value. |
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When used for synchronization, a semaphore keeps track of how many times it was signaled using a counter. The counter can take values between 0 and 255, 65,535, or 4,294,967,295, depending on whether the semaphore mechanism is implemented using 8, 16, or 32 bits, respectively. For µC/OS-III, the maximum value of a semaphore is determined by the data type OS_SEM_CTR (see os_type.h), which is changeable, as needed (assuming access to µC/OS-III’s source code). Along with the semaphore’s value, µC/OS-III keeps track of tasks waiting for the semaphore to be signaled.
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| title | Posting (or signaling) a Semaphore |
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OS_SEM MySem;
void MyISR (void)
{
OS_ERR err;
:
OSSemPost(&MySem, (1)
OS_OPT_POST_1, (2)
&err); (3)
/* Check "err" */
:
:
} |
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(1) Your task signals (or posts to) the semaphore by calling OSSemPost(). You specify the semaphore to post by passing its address. The semaphore must have been previously created. (2) The next argument specifies how the task wants to post. There are a number of options to choose from. When you specify OS_OPT_POST_1, you are indicating that you want to post to only one task (in case there are multiple tasks waiting on the semaphore). The task that will be made ready-to-run will be the highest-priority task waiting on the semaphore. If there are multiple tasks at the same priority, only one of them will be made ready-to-run. As shown in the figure below, tasks waiting are in priority order (HPT means High Priority Task and LPT means Low Priority Task). So, it is a fast operation to extract the HPT from the list. If specifying OS_OPT_POST_ALL, all tasks waiting on the semaphore will be posted and made ready-to-run. The calling task can “add” the option OS_OPT_POST_NO_SCHED to either of the two previous options to indicate that the scheduler is not to be called at the end of OSSemPost(), possibly because additional postings will be performed, and rescheduling should only take place when finished. This means that the signal is performed, but the scheduler is not called even if a higher-priority task was waiting for the semaphore to be signaled. This allows the calling task to perform other post functions (if needed) and make all the posts take effect simultaneously. Note that OS_OPT_POST_NO_SCHED is “additive,” meaning that it can be used with either of the previous options. You can thus specify: OS_OPT_POST_1 OS_OPT_POST_ALL OS_OPT_POST_1 + OS_OPT_POST_NO_SCHED OS_OPT_POST_ALL + OS_OPT_POST_NO_SCHED | Panel |
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| title | Tasks waiting for Semaphore |
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(3) OSSemPost() returns an error code based on the outcome of the call. If the call was successful, err will contain OS_ERR_NONE. If not, the error code will indicate the reason for the error (see Appendix A, µC-OS-III API Reference for a list of possible error codes for OSSemPost()).
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