Is it legal to overrun one element of a struct to view another?

Question

Given the following contrived example code:

struct abc
{
    int x[5];
    int y[5];
};

void main()
{
    struct abc test;
    test.y[0] = 10;
    printf("%n", test.x[5]);
}

The output of the program is 10.

While not the best programming practice, this does work. However, is this an artifact of the compiler and platform, or is this legal code? (i.e. defined by the C standard?)

Even if the result is not guaranteed to be 10, is there ever an instance where this would be "illegal" (i.e. writing to memory I do not "own")?

Answer 1

No, it's not legal nor guaranteed to work. The compiler could add padding into the struct, to aid in alignment, depending on the architecture, etc.

Edit: To sum up some of the stuff in these comments and clarify...

I do believe you "own" the memory there, since as edA-qa mort-ora-y points out, memcpy() of a struct needs/is expected to work. Where this is specifically guaranteed though, I'm not sure.

That being said, undefined behavior is something to avoid at all costs. What a program with undefined behavior does could change between two separate runs of the same code five seconds apart. It could cause subtle memory corruption in your program, a segfault, or run just fine, but there's no reason to ever use code that relies on undefined behavior.

Answer 2

This is undefined behaviour - you are (un)lucky in this case. Further more (aside from the mentioned padding issue), there is the maintainability issue - it's incredibly fragile - what if someone sticks something else in between. I'm sure it's a contrived example, but the recommendation is - don't do it.