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<html><head><title>THE BASICS OF RT LINUX</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./tutorial.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex01_periodic.htm">[next]</a><h1>The Basics of Real-Time Linux</h1><h2>RT Linux Tasks are Kernel Modules, not Linux Programs</h2><ul><li>RT Linux tasks are "kernel modules," meaning they run as part of theprivileged Linux kernel, similar to device drivers. <li>In both the RTAIand RTL flavors of RT Linux, the principle is the same:<ul><li>a real-timescheduler replaces the original Linux scheduler,<li>intercepts the timerinterrupt and external device interrupts,<li>runs any real-time codeassociated with these, and<li>runs any normal Linux processes in thetime left over.</ul><li>Kernel modules, like device drivers, execute in a primitiveenvironment, without direct access to many user-level Linux facilitieslike terminal or file I/O.<ul><li> Any reasonably large real-time applicationwill want these features, and these applications typically are splitbetween real-time and non-real-time parts, with the non-real-timeparts using all the nice Linux features.<li> Communication between the twoparts can be accomplished numerous ways, using shared memory,first-in, first-out (FIFO) queues and other communication pathwayswe'll describe later.<li>This split complicates RT Linux programming, incomparison with other real-time operating systems in which everythingruns in real-time.<li>On the other hand, the simpler environment makes RTLinux quite fast.</ul><li>Warning! Since your real-time code is running as a kernel module,it is effectively as privileged as the kernel. Unlike with regularLinux processes, coding errors may crash the system, requiring areboot.<p><p>"With great power comes great responsibility." <i>Uncle Ben to Peter Parker, aka Spider-Man.</i></ul><h2>Kernel Modules are Dynamically Loaded</h2><ul><li>Kernel modules are dynamically loaded using the 'insmod'insert module program, and <li>unloaded (stopped), using the 'rmmod' "remove module"program.<li>These programs are only available to the root user(administrator), although there are ways to avoid giving real-timeprogrammers true root access to the system. </ul><h2>C is the Preferred Language</h2><ul><li>Linux is written in the C programming language (with some assemblylanguage), as are RTAI and RTL, so C is the preferred language forwriting RT Linux programs.<li>Other languages (e.g., C++) can be used, but there are caveats that make Cthe much-preferred language for RT Linux development.<li>The examples inthis tutorial are all written in C. <li>C programs are normally compiled into full executable programs,but kernel modules are compiled into object code, with final linkingsuppressed.<ul><li>Instead of a full-blown executable, your code will bea loadable object '.o' "dot-oh" file, <li>possiblythe result of linking together several other '.o' files if yourproject is split into numerous files for convenience or clarity. </ul><li>In C, a program's "entry point" where execution begins is a functioncalled 'main()'.<ul><li>For a kernel module, thisentry point is called 'init_module()'.<li>'insmod' looks for this symbolwhen loading the code.</ul><li>A program's "exit point" is a function called 'cleanup_module()'. Thiswill be called when 'rmmod' removes the kernel module.</ul>Here is the minimal C code that illustrates this:<pre>/* simple.c */#define __KERNEL__#define MODULE#include <linux/kernel.h>#include <linux/module.h>int init_module(void){ printk("hello, world!\n"); /* printk = kernel printf, to the console */ return 0;}void cleanup_module(void){ printk("goodbye, world!\n"); return;}/* end of simple.c */</pre><h2>The Mechanics of Compiling and Running</h2><ul><li>The mechanics of compiling C code vary depending upon which compileryou use.<li>The Free Software Foundation's Gnu C compiler 'gcc' is installed with most Linuxdistributions.<li>With 'gcc', to compile thisexample you would do:<pre>gcc -c simple.c</pre><p><li>The '-c' "dash-c" means don't compile toa full-blown executable, just leave it as the loadable object file 'simple.o'.<li>To run this, you would use the 'insmod' program:<pre>insmod simple.o</pre><p><li>To stop this, you would use the 'rmmod' program, passing the name ofthe module without the '.o' suffix:<pre>rmmod simple</pre><p><li>Note that the output of the 'printk()' calls in simple.c appears in themain console, which may not be where you ran theprogram, especially if you're running in the windowing environment, XWindows.<li>You can get to the main console window with Control-Alt-F1 onmost Linuxes, where the other function keys toggle between variousother consoles, and X Windows.</ul><hr><a href="./ex01_periodic.htm">Next: Example 1, Pure Periodic Scheduling of a Single Task</a><p><a href="./tutorial.htm">Back: Start of Tutorial</a></body></html>?? 快捷鍵說明
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