Do functions listen to global variables under while loops (Stopping functions)

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rough93 2019-9-18

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评论： Walter Roberson 2019-9-20

If I have a function running within a while loop, where the condition of the loop is a live variable (ex. a pressure sensor), will the function be stopped if the while loop condition is met while the function runs?

In this application. I have a function programmed to move motors to a certain orientation, the while loop in the main code file where the function is called is sensing if pressure = 0, if the pressure changes (~=0) before the motors have reached their orientations will the function be stopped?

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Walter Roberson 2019-9-19

In MATLAB, two functions cannot execute at the same time except when you use the Parallel Computing Toolbox (in which case each of the executing functions is in a different process.)

However:

There are internal routines that somehow fill or empty I/O buffers in the background. It is not specified as to how they work or when they can interrupt. They do not operate at the MATLAB level, but they do alter some information that can be queried at the MATLAB level, such as number of bytes currently in the buffer. To prevent inconsistencies, there is probably some locking of the appropriate variables, but this is not certain. It is possible that they operate on a different thread at the C++ level that underlies MATLAB.
There are MATLAB-level callbacks that can be triggered in response to I/O, such as when end-of-line is detected on a serial port, or end of a camera frame loaded. It is not clearly documented as to when these callbacks can be triggered, but probably it is between any two physical lines of MATLAB code (not at the end of each part of a compound command such as one separated with semi-colons -- but if those compound commands happen to invoke MATLAB code, then the interruption can happen between lines of that MATLAB code.)
There are MATLAB-level callbacks that can be triggered in response to timers. It is documented that these can be triggered between any two physical lines of MATLAB code (not at the end of each part of a compound command such as one separated with semi-colons -- but if those compound commands happen to invoke MATLAB code, then the interruption can happen between lines of that MATLAB code.)
There are MATLAB-level event listeners. At the moment I do not know what the rules are for when an event listener can be triggerred.
There are MATLAB-level GUI callbacks such as KeyPressFcn or uicontrol('style', 'button') 'Callback' function. When these requested, MATLAB checks to see if a GUI callback is already executing, and if so then it checks the GUI object's Interruptible property to determine whether an interruption can occur, or if instead the callback can be queued. You should assume that callbacks will be queued unless you deliberately changed the object properties. https://www.mathworks.com/help/matlab/creating_guis/callback-sequencing-and-interruption.html . Queued callbacks are not given a chance to run until drawnow() or pause() or uiwait() or waitfor() is executed, or until figure() is asked to either create a new figure or activate an existing figure. If you have a loop of code that depends upon a variable that is changed in a callback, then you are much better to assume you need to call drawnow() to give the callback a chance to execute to change the variable.

rough93 2019-9-20

The code is talking to a arduino in order to send commands and receieve position feedback. I've implemented the Arduino hardware control toolbox in order to communicate through the arduino (which only acts as a middleman). It sounds like your suggestion would cause the arduino to stop all running code however, which I don't want to neccessarily do, just the current loop.

Walter Roberson 2019-9-20

No, the arduino is looping around in its control loop. You would have it check to see if a command has come in from the host to set the goals. If not, then it should check the pressure sensors to see whether it should stop moving towards its current goal. If the sensor is okay, then it should figure out the next motor command to send and send it; if the sensor is exceeded, then it should send any needed motor stop commands and should mark itself as idle. If it has not been given a goal or has stopped moving towards a goal because of sensor, or has reached the goal, then it would mark the current goal as idling for which it would not bother to check pressure or send a command to the motor. But no matter what, it loops back checking for commands again. There is never a point where it stops running all code (unless you send a shutdown command), but there are points where it does not need to issue motor commands this cycle.

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Answer 1

dpb 2019-9-18

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https://ww2.mathworks.cn/matlabcentral/answers/480987-do-functions-listen-to-global-variables-under-while-loops-stopping-functions#answer_392490

在 MATLAB Online 中打开

pressure = readDevicepressure
while pressure == 0
    (x, y, z) = function(move the pressure reader to x,y,z)
end   

only reads the pressure before the loop starts; there's nothing that updates it (unless your function reads it again internally).

If you want to be continuously monitoring the pressure, then do so--

pressure = readDevicepressure;
while pressure == 0
  (x, y, z) = function(move the pressure reader to x,y,z)
  pressure = readDevicepressure;
end

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rough93 2019-9-18

Truly continuously monitoring would have to be inside the function still though right? Otherwise this function will complete and then re-read the pressure. I need to find a way to give the motor a move-to command and read pressure simultaneously while the motor is in motion so it can stop if hits something.

dpb 2019-9-18

That is true, yes, this only polls the pressure sensor when the function completes.

The typical RT system control loop in polled operation ensures there's no section of code longer than some allowable time interval before inputs are polled.

Otherwise, one needs it to be interrupt-driven, not polled. In ML that would be a timer in the base product; I don't know if there are other features in some other toolboxes for such purposes or not. The data acquisition TB can control specific data acq devices but whether your device falls under it or not we don't know.

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