C: uninitialized local variable takes on unexpected value

Question

I'm really confused about the result of the following code:

#include <stdio.h>
#include <stdlib.h>

int one(int a, int b) {
    int k, t;
    k = a - b;
    t = a + b + 1;
    if (k % 2 == 0) return t;
    else return 0;
}

int two(int x, int y) {
    int m;
    printf("%d\n", m);
    return m + x + y;
}

main() {
    int result = two(5, one(4, 3));
    // printf("%d\n", one(4, 3));
    printf("result is %d\n", result);
}

one(4, 3) returns 0, which is not surprising. But I don't understand why two(5, 0) returns 8. In other words, m takes on the value 3 without being initialized. How did this happen?

When you use an un-initialized variable, you are immediately transported to the realm of 'undefined behavior'. There's no telling what will happen. — uptoNoGood, Dec 15 '16 at 04:12
Why is 8 unexpected? An uninitialized variable is allowed to contain any value, or worse, crash when you try to access it. — user253751, Dec 15 '16 at 04:16
I was just wondering if `m` is 3 because 3 was passed in `one(4, 3)`. I expected `m` to be some random value. — zhuhan, Dec 15 '16 at 04:18

score 1 · Answer 1 · answered Dec 15 '16 at 04:11

1

C does not automatically initialize values to 0 when you define them. Technically, reading that data before you initialize it is undefined behavior. In practice, this normally results in a garbage value containing whatever data was stored in that location previously.

answered Dec 15 '16 at 04:11

PC Luddite

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Yes. But why is `m` 3 instead of some other random value? Is this 3 the argument that was passed in `one(4, 3)`? – zhuhan Dec 15 '16 at 04:16
2

@HanZhu Probably because it reused memory that previously stored a `3`. at some point. It might or might not have been that particular 3. – user253751 Dec 15 '16 at 04:17
Global variables will be initialized to zero – e.jahandar Dec 15 '16 at 05:34
@e.jahandar actually just any static variable, whether it's global or not. – PC Luddite Dec 15 '16 at 12:44

C: uninitialized local variable takes on unexpected value

1 Answers1