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The tools that solve the preceding problem are special variables called semaphores, or mutexes (singular mutex, from mutual exclusion). You must use the shared resources only inside so-called critical sections parts of code that must be included between an Acquire(mutex_name) statement and a Release(mutex_name) statement: Acquire (mtx); /* critical section, where shared resources are used */ Release (mtx); Acquire simply waits for the mutex_name mutex variable to be released, and this waiting avoids conflicts with other tasks that require the shared resource. Release, as the term suggests, releases the mutex, thus allowing other tasks blocked at the Acquire statement line to enter the critical section. You can recast Acquire as follows: void Acquire (bool &mtx) { while (mtx) Wait(10); mtx = true; } You could replace Release as follows: void Release (bool &mtx) { mtx = false; }
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Note The & (ampersand) before the argument name means that it is passed by reference, and thus can
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be modified by the function code. In NXC, you usually pass arguments by value: the variable x passed by value to myfun(x) is not modified after the myfun call. A temporary copy of the x variable is made instead. For details, check the NXC Programming Guide.
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Let s see this technique in practice, with the help of two simple multitasking programs. The programs should do the same thing. They have two tasks running at the same time, one playing a high tone, the other playing a lower tone, alternately: high, low, high, low, and so on. Listing 5-1 shows the first, wrong version of the program. Listing 5-1. The Wrong Version of a Simple Multitask Program to Play Sounds task task1() { while(true) {
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PlayTone(1000,300); Wait(600); } } task task2() { while(true) { PlayTone(500,300); Wait(600); } } task main() { Precedes(task1,task2); } Here you can see that the main task is used just to tell the NXT scheduler to execute the tasks named task1 and task2, after the main task itself has finished. Because the Precedes statement is the only thing the main task has to do, it ends at once, leaving space for the other two concurrent tasks. They are concurrent, in fact, because both are trying to play a tone, accessing the NXT loudspeaker as a shared resource. The program does not behave as expected, because the tasks alternate with no discipline about the loudspeaker usage. The sound pattern coming out is random: it could be low, high, high, high, low, low, high, high, low, low, low, low, high . . . , instead of the regular pattern described before, where the high and low tones alternate in an orderly fashion. Now you should realize why you must employ mutex variables. See the correct version of the preceding program in Listing 5-2. Listing 5-2. The Correct Version of a Simple Multitask Program to Play Alternating Sounds mutex sound; task task1() { while(true) { Acquire(sound); PlayTone(1000,300); Wait(600); Release(sound); } } task task2() { while(true)
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{ Acquire(sound); PlayTone(500,300); Wait(600); Release(sound); } } task main() { Precedes(task1,task2); } The program is the same, but now the critical sections those parts of the task code where the shared resource is used are enclosed by the Acquire/Release statements. The mutex variable is called sound, according to the function of the critical section it is meant to discipline. It is declared as a global variable, so that every function or task in the program can see and use it. The sound pattern coming out is the one you desired: high, low, high, low, high, low . . . The first sound to be played is always the high-pitched one, because task1 comes before task2 in the Precedes statement list. The first scheduled task is task1, playing the high tone first. If you swap the task names inside parentheses, such as Precedes(task2,task1), the first sound heard will be the low-pitched one, played by task2. Now you should have the essential background about multitasking to continue on.
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