C - false value of integer var

Question

The output in the following code is really mysterious.

#include <stdio.h>

void funktion(int x) {}

void test() {
    int foo;
    printf("test = %d\n", foo);
}

int main() {
    test();
    funktion(5);
    test();

    return 0;
}

Why will the foo-var initialised with the value of the parameter of funktion()? I don't understand why foo-var is initialised by the function-parameter. I know that I can fix it with a explicit initialisation - but I want to understand the mystery. Thanks!

It is Undefined Behavior to read a value from an uninitialized variable. It can be anything. — Christian Gibbons, Mar 19 '18 at 17:40
In C it's not undefined behavior. But the value of uninitialized local variables (like `foo`) is *indeterminate*. — Some programmer dude, Mar 19 '18 at 17:41
@Someprogrammerdude Is *printing* indeterminate value defined (yeah, I have had this discussion bookmarked somewhere..)? — Eugene Sh., Mar 19 '18 at 17:42
@EugeneSh. I *think* so, but I'm not 100% certain. I think it's only UB if the indeterminate value is a *trap value* (which for integers will be never). — Some programmer dude, Mar 19 '18 at 17:43
So it's only a coincidence that the value of foo is the parameter of funktion()? I think foo use the free spacecell from the parameter and initialised with the content of this space cell? — , Mar 19 '18 at 17:45
@Waldi It's up to the compiler on how to implement it. In your case it might be this way, but it is not something you can rely on. — Eugene Sh., Mar 19 '18 at 17:47
Yes, it is possible that the memory segment that was used for the argument to funktion happened to be reused by foo. — Christian Gibbons, Mar 19 '18 at 17:48
@Someprogrammerdude I hadn't considered that distinction. Makes sense. — Christian Gibbons, Mar 19 '18 at 17:50
@Someprogrammerdude The only type guaranteed *not* to have a trap representation is ` unsigned char`. Any other type may, depending on the implementation. On systems most people are likely to encounter (i.e. x86 Linux or Windows) there typically aren't any. A signed integer could have a trap representation if, for example, it uses one's compliment representation and does not support negative 0. — dbush, Mar 19 '18 at 18:09

Jazzwave06 · Accepted Answer · 2018-03-19T17:56:40.033

I've added comments to your main() function that explains why is it so. By the way, you're in undefined behaviour land, and different platforms might yield different results.

int main() {
    // no return, no parameter, one local variable of type int.
    // This pushes an int on the stack, uninitialized (the local variable).
    test();
    // Stack pointer is back to where it was.

    // no return, one int parameter, no local variables.
    // This pushes an int with value=5 on the stack (the parameter).
    funktion(5);
    // Stack pointer is back to where it was.

    // no return, no parameter, one local variable of type int.
    // This pushes an int on the stack, uninitialized (the local variable). 
    // Previous value of that memory comes from funktion() call.
    test();
    // Stack pointer is back to where it was.

    return 0;
}

Thank you for the explanation of the mystery! – Mar 19 '18 at 17:56 — , Mar 19 '18 at 17:56

C - false value of integer var

1 Answers1