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<html><head><title>EXAMPLE 6: SHARED MEMORY COMMUNICATION BETWEEN RTL AND LINUX</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex05_isr.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex07_sem.htm">[next]</a><h1>Example 6: Shared Memory Communication Between RTL and Linux</h1>This example demonstrates the use of "shared memory" to communicate betweenRT Linux tasks and normal Linux processes. Shared memory is exactlythat, memory shared between two or more tasks or processes. It isanalogous to Unix "interprocess communication" (IPC) shared memory. <p>Refer to the <ahref="../ex06_shm/shm_task.c">commented real-time tasksource code</a> and <ahref="../ex06_shm/shm_app.c">commented Linux processsource code</a> for the details.<h2>Principle of Operation</h2><ul><li>Shared memory is typically used to hold a data structure whosemembers are read and written by two different processes.<li>Unlike FIFOs, shared memory is not queued. Any data written toshared memory overwrites the previous data.<li>Shared memory can also be accessed piecewise. Individual membersof a large shared memory structure can be read and writtenindependently.<li>Because it is a shared resource, care must be taken to ensure dataconsistency. In RT Linux, a real-time task can interrupt a Linux processes, butnot vice-versa. There are two cases for a reader-writer pair:<ul><li>the RT task is the reader, and it interrupts the Linux processduring its write. Here, the RT task will see an inconsistent datastructure, with fresh data at the beginning and stale data at the end.<li>The RT task is the writer, and it interrupts the Linux processduring its read. Here, the Linux process will see an inconsistent datastructure, with stale data at the beginning and fresh data at theend.</ul><li>In this example, a single RT Linux task writes a data array toshared memory periodically, and a single Linux process reads this dataperiodically and prints the results. Various types of data consistencytechniques are demonstrated, as discussed in the following section on<a href="mutex.htm">Data Consistency Techniques</a>.</ul><h2>Creating Shared Memory</h2><ul><li>On the real-time side, shared memory is created by calling'rtai_kmalloc()', the RTAI version of the kernel 'malloc' function, toallocate a block of shared memory. For example,<pre>struct mystruct * ptr;ptr = rtai_kmalloc(101, sizeof(struct mystruct));</pre>allocates a block of shared memory for the data structure 'mystruct',registered with key 101, and returns a pointer to the sharedmemory. A null pointer is returned in the case of an error.<ul><li>The key, 101 in the example, serves to identify this sharedmemory. Other processes who want to share this will use the same key.<li>This function is similar to the C function 'malloc()' ("memoryallocate"). You pass it the size, it returns a pointer.<li>In RTAI, the shared memory must first be created by the real-timetask before the Linux task accesses it.</ul><li>On the Linux side, shared memory is accessed by calling'rtai_malloc()'. Strictly speaking,this is not an "allocation," since the memory is allocated by thereal-time task first. This is really a "connection". For example,<pre>struct mystruct * ptr;ptr = rtai_malloc(101, sizeof(struct mystruct));</pre>returns a pointer to the shared memory allocated by the previous'rt_kmalloc()' call in the real-time process.<ul><li>Note the shared key, 101, used by both sides. You can have anynumber of shared memory segments, each with its own unique id that isshared among the participants. <li>The actual pointer value will in general be different for thetasks that share memory.</ul><li>Once created, the shared memory pointer can be referenced justlike any pointer. The results are immediately available to others whoshare the memory.</ul><h2>Deleting Shared Memory</h2><ul><li>On the real-time side, shared memory is deleted by calling'rtai_kfree()', the RTAI version of the kernel 'free()' function,using the same key as was used to create the shared memory:<pre>rtai_kfree(101);</pre><li>Likewise, on the Linux side, call 'rtai_free()' with both the keyand the pointer:<pre>rtai_free(101, ptr);</pre><li>These should be done in the reverse order as the creation: theLinux process should free first, then the RT task.</ul><h2>Running the Demo</h2>To run the demo, change to the 'ex06_shm' 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 "Shared Memory" button.You'll see diagnostics messages printing out.<p><a href="../ex06_shm/shm_task.c">See the Real-Time Task Code</a><p><a href="../ex06_shm/shm_app.c">See the Linux Application Code</a><hr><a href="./ex07_sem.htm">Next: Example 7, RTL Semaphores</a><p><a href="./ex05_isr.htm">Back: Example 5, Interrupt Service Routine</a></body></html>⌨️ 快捷键说明
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