[Solved] Variable has two values simultaneously?

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Changing a const-value yields undefined behaviour, and the “mysterious” output is just such undefined behaviour.
It is actually needless to investigate on why a behaviour in the world of undefined behaviour is as it is then. But in your case it is probably not using a as you declare it as const, so the compiler “knows” the value and may decide not to access the variable.
Just for showing something curious, try:

int main()
{
    volatile int const a = 1;
    int* pa = (int*) &a;
    *pa = 2;
    printf("%p %d %p %d", (void*) &a, a, (void*) pa, *pa);
    return 0;
}

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solved Variable has two values simultaneously?