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<html><head><title>EXAMPLE 7: RTL SEMAPHORES</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex06_shm.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex08_rcservo.htm">[next]</a><h1>Example 7: RTL Semaphores</h1><p>This example demonstrates two-process mutual exclusion of real-timetasks using semaphores.<ul><li><i>Semaphores</i> are data structures with a count and two associatedoperations, <i>give</i> and <i>take</i>.<li>A "give" operation increments the count and returns immediately.<li>A "take" operation decrements the count and returns immediately,unless the count is already zero. In this case the operation blocksuntil another process gives the semaphore.<li>A <i>binary semaphore</i> is a semaphore with a maximum count of1. Binary semaphores are useful to enforce mutual exclusion: only oneprocess can have the semaphore at any one time. Other takers willblock until the holder returns it. Shared data will remain consistent,since there is no possibility of being interrupted during access.<li>Various semantics determine which of the possibly many blockedprocesses attempting to take a semaphore will awaken first when it isgiven, for example first-in first-out (queued) or priority-based.<li>While binary semaphores would appear to be anatural choice to enforce mutual exclusion for shared memory, they areproblematic here because they could cause the real-time task to becomeblocked based on non-real-time actions. Because of this, there isno support for semaphores between Linux processes and real-time tasks,only Linux-Linux or RT-RT. In this example we show RT-RT semaphores.</ul><p>Refer to the <ahref="../ex07_sem/sem_task.c">commented source code</a> of the example for the details.<h2>Principle of Operation</h2><ul><li>Two tasks are created that share a data array. One task is alow-priority slow task that writes the data array with a heartbeat,and the other is a high-priority fast tasks that reads it. This willtend to cause data inconsistency.<li>A semaphore is used to mediate access to the data array. Thesemaphore must first be initialized by calling 'sem_init()', e.g.,<pre>SEM sem; /* here's the semaphore data structure */rt_sem_init(&sem, 1); /* 1 means it's initially available */</pre>If the semaphore were initialized to 0, it would be initially"taken".<li>To take the semaphore, call 'rt_sem_wait()', e.g.,<pre>rt_sem_wait(&sem); /* blocked waiting... *//* we returned, and now have the semaphore */</pre>This will return immediately if you have the semaphore, or block andreturn when another task gives the semaphore.<li>To give the semaphore, call 'rt_sem_signal()', e.g.,<pre>rt_sem_signal(&sem); /* will always return immediately *//* someone blocked on 'sem' may have just run */</pre><li>The reader checks the data after it successfully gets thesemaphore. If the data array is not all set to the same value, a baddata count is incremented.<li>The example runs twice, once with semaphores disabled and thenwith semaphores enabled. Statistics are printed showing the number ofreads, writes and bad data. Without semaphores, you should see lots ofbad data. With semaphores, you should see none.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex07_sem' subdirectory of thetop-level tutorial directory, and run the 'run' script by typing<pre>./run</pre>Alternatively, change to the top-level tutorial directory and run the'runall' script there by typing<pre>./runall</pre>and selecting the "Semaphores" button.<p>You'll see a window pop up with a log of diagnostics messagesprinting out. Look for the the results of the test, with and withoutsemaphores, e.g.,<pre>Sep 1 11:53:41 letti kernel: no sem: read/write/bad counts = 50753/538/47218Sep 1 11:53:46 letti kernel: with sem: read/write/bad counts = 50665/537/0</pre>The 'bad counts' signify times where the shared data was detected tobe inconsistent. This should be 0 with the semaphore, and is likely tobe nonzero without the semaphore.<p><a href="../ex07_sem/sem_task.c">See the Code</a><hr><a href="./ex08_rcservo.htm">Next: Example 8, RC Airplane Servomotor Control</a><p><a href="./ex06_shm.htm">Back: Example 6, Shared Memory Communication</a></body></html>⌨️ 快捷键说明
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