How to know when a value of a variable change?

This is a discussion on How to know when a value of a variable change? within the C Programming forums, part of the General Programming Boards category; Dear all, I have a big program. A variable change unexpectedly, so I want to find what function change this ...

  1. #1
    Registered User Hannibal2010's Avatar
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    How to know when a value of a variable change?

    Dear all,
    I have a big program. A variable change unexpectedly, so I want to find what function change this variable.
    How can I track a variable in Linux environment?
    Is there any method?
    Any help are welcome.

  2. #2
    Code Goddess Prelude's Avatar
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    You can watch it in a debugger, but the bigger problem is that access to the variable isn't strictly controlled (ie. it's probably global). This generally makes tracking changes more difficult, as you've independently discovered.
    My best code is written with the delete key.

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    Registered User Hannibal2010's Avatar
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    I mean, I want to set a trigger, a function will run when a variable changes.
    Can I do it?

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    Your best bet is to simply call a function with the new value...

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    Registered User Hannibal2010's Avatar
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    But, when do I should call this function?
    I want this function runs when the variable changes.
    Can I do it?

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    Quote Originally Posted by Hannibal2010 View Post
    I mean, I want to set a trigger, a function will run when a variable changes.
    Can I do it?
    In general, no. In a debugger, you can set something like a "watchpoint" (the name and method to set one depend on the debugger) so, when the variable changes, execution is suspended at the instruction that caused the change.

    If you want to somehow modify your code so a function will be called if a variable changes then the possibilities are remote (for example, is creating a thread which monitors the value that you know is being changed unexpectedly). The problem is that changes the layout of your program itself in memory (as both the thread function and the method of creating a thread consume some resources). A side effect can therefore be that a different variable starts being changed.

    There is a very good reason that people actively discourage hacking of code, and debugging. Yes, it is difficult to design code does not clobber variables it shouldn't. But it is actually harder, particularly in large programs, to find the culprit when some variable is unexpectedly being changed.
    Right 98% of the time, and don't care about the other 3%.

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    Quote Originally Posted by Hannibal2010 View Post
    But, when do I should call this function?
    I want this function runs when the variable changes.
    Can I do it?
    You would call the function wherever the variable is being changed... that is, instead of a = 10 ... call a function that changes a and does the other stuff you need done because it was changed.

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    Have a look at the original post, Tater. Existing program with a variable being unexpectedly changed. Wanting to somehow call a function when that unexpected change occurs.

    Hence Prelude's recommendation (and mine) to use a debugger.
    Right 98% of the time, and don't care about the other 3%.

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    Quote Originally Posted by grumpy View Post
    Have a look at the original post, Tater. Existing program with a variable being unexpectedly changed. Wanting to somehow call a function when that unexpected change occurs.

    Hence Prelude's recommendation (and mine) to use a debugger.
    Well, yes... I take your point... I had something else in mind; thinking of a variable being changed by a second thread and trying to catch it somehow...

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    Registered User manasij7479's Avatar
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    Why not do it in the way the debugger itself does it?
    ...(AFAIK) ..by running the monitored code as a inferior( child?) process to the monitoring code.
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
    1.None of the other participants are fast and steady.
    2.The fast and unsteady suddenly falls asleep while running !



  11. #11
    Code Goddess Prelude's Avatar
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    >Why not do it in the way the debugger itself does it?
    Why not just use a debugger instead of reinventing the wheel with no good reason?
    My best code is written with the delete key.

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    Registered User manasij7479's Avatar
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    Quote Originally Posted by Prelude View Post
    Why not just use a debugger instead of reinventing the wheel with no good reason?
    Because it is fun and educational ! ...and can have a slight chance of producing a better wheel.
    (And...out of curiosity...can this be utilised to implement a better even driven model compared to callback functions?)
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
    1.None of the other participants are fast and steady.
    2.The fast and unsteady suddenly falls asleep while running !



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    Quote Originally Posted by manasij7479 View Post
    Because it is fun and educational ! ...and can have a slight chance of producing a better wheel.
    Fun and educational, possibly.

    While there is always the possibility of some breakthrough what leads to a better wheel, it is more likelihood of recreating the square wheel and then expending a lot of effort trying to get it to rotate.

    Quote Originally Posted by manasij7479 View Post
    (And...out of curiosity...can this be utilised to implement a better even driven model compared to callback functions?)
    That depends on what you call "better". The implementation of debuggers amounts to one program setting up an environment that allows it to control and monitor the internal execution of another program. You could use the same techniques as an alternative to callbacks, I suppose.

    However, I have in mind images of sledgehammers and nuts.
    Right 98% of the time, and don't care about the other 3%.

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