Question

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Why global and static variables are initialized to their default values?

What is the technical reason this happens? And is it supported by the standard across all platforms? Is it possible that certain implementations may return undefined variables if static variables aren’t explicitly initialized?

Answer 1

It is required by the standard (§6.7.8/10).

There’s no technical reason it would have to be this way, but it’s been that way for long enough that the standard committee made it a requirement.

Leaving out this requirement would make working with static variables somewhat more difficult in many (most?) cases. In particular, you often have some one-time initialization to do, and need a dependable starting state so you know whether a particular variable has been initialized yet or not. For example:

int foo() { 
    static int *ptr;

    if (NULL == ptr)
       // initialize it
}

If ptr could contain an arbitrary value at startup, you’d have to explicitly initialize it to NULL to be able to recognize whether you’d done your one-time initialization yet or not.