i2c smbus filter function corrupting variables

问题

I have a simple function

#define AMB_FILTER 0.7f

int32_t fValue; (this is declared in the class header)

int32_t Ambient::filter(uint32_t raw)
{
    // If we have no preliminary fValue we don't need to calculate a filter
    if(fValue == -1)
    {
        fValue = raw;
        return fValue;
    }

    float y, yy;

    y = (1.0f - AMB_FILTER) * (float) raw;
    yy = AMB_FILTER * (float) fValue;

    fValue =  (int32_t) (y + yy);

    printf("filter raw %d y %f yy %f fValue %d\n",raw, y, yy, fValue);

    return fValue; 
}

It takes in a value that was read from smbus and returns a value that was filtered with the last value it received. Here is its output from the printf statement

filter raw 454 y 136.200012 yy 317.799988 fValue 454
filter raw 454 y 136.200012 yy 317.799988 fValue 454
filter raw 454 y 136.200012 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy 317.799988 fValue 454
filter raw 454 y 136.200012 yy 317.799988 fValue 454
filter raw 455 y 136.500000 yy -731751040.000000 fValue -731750912
filter raw 455 y 136.500000 yy -512225632.000000 fValue -512225504
filter raw 455 y 136.500000 yy -358557856.000000 fValue -358557728
filter raw 455 y 136.500000 yy -250990400.000000 fValue -250990256
filter raw 454 y 136.200012 yy -175693184.000000 fValue -175693040

So what is happening? How is it that it still received the same input but all of a sudden went crazy? I don't set the fValue anywhere but this function.

I made these variables (y and yy) very internalized to the function because I was worried they were somehow being modified or collided with something else. But now that they are completely local I have no idea what is happening. I am using C++ classes so this should all be in its own space anyhow.

EDIT: In fact if I let it run a little longer the variables I keep for the different i2c Chip addresses also become corrupted to -1075766188. What the hell?

回答1:

[t1] filter raw 454 y 136.200012 yy 317.799988 fValue 454
[t2] filter raw 455 y 136.500000 yy -731751040.000000 fValue -731750912

Sometime between t1 and t2 the value of fValue was corrupted. The corruption of yy is a consequence of fValue's corruption. Look elsewhere in your program for the culprit.

If you can't use valgrind, then sprinkle sanity checks on the valueof fvalue throughout your program.
Print the value of this in your routine. See if it becomes unusual.
Print the value of other member variables from Ambient. See if they become unusual.
Try commenting out huge tracts of code. Start with 1/2 of your program. If the bug is still there, comment out 1/2 of the remainder. Continue until you find a single line that controls the corruption.

As to how it was corrupted, there are many possibilities. Perhaps you dereference a wild pointer somewhere. Perhaps you write beyond the end of an array. Perhaps you are operating on a previously destroyed Ambient.

来源：https://stackoverflow.com/questions/13310496/i2c-smbus-filter-function-corrupting-variables

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