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

1. Security: leaving memory alone would leak information from other processes or the kernel.

2. Efficiency: the values are useless until initialized to something, and it's more efficient to zero them in a block with unrolled loops. The OS can even zero freelist pages when the system is otherwise idle, rather than when some client or user is waiting for the program to start.

3. Reproducibility: leaving the values alone would make program behavior non-repeatable, making bugs really hard to find.

4. Elegance: it's cleaner if programs can start from 0 without having to clutter the code with default initializers.

One might then wonder why the auto storage class does start as garbage. The answer is two-fold:

1. It doesn't, in a sense. The very first stack frame page at each level (i.e., every new page added to the stack) does receive zero values. The "garbage", or "uninitialized" values that subsequent function instances at the same stack level see are really the previous values left by other method instances of your own program and its library.

2. There might be a quadratic (or whatever) runtime performance penalty associated with initializing auto (function locals) to anything. A function might not use any or all of a large array, say, on any given call, and it could be invoked thousands or millions of times. The initialization of statics and globals, OTOH, only needs to happen once.

Because with the proper cooperation of the OS, 0 initializing statics and globals can be implemented with no runtime overhead.

Section 6.7.8 Initialization of C99 standard (n1256) answers this question:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;

— if it has arithmetic type, it is initialized to (positive or unsigned) zero;

— if it is an aggregate, every member is initialized (recursively) according to these rules;

— if it is a union, the first named member is initialized (recursively) according to these rules.