Why Are Global and Static Variables in C/C Default-Initialized Instead of Containing Arbitrary Values?

Why Default Initialization for Global and Static Variables?

In C/C , global and static variables are initialized to their default values upon declaration. Why not leave them with arbitrary "garbage" values instead? Several crucial reasons warrant this default initialization:

1. Security:
Uninitialized memory can contain sensitive information from other processes or the kernel. Default initialization ensures these variables are cleared, preventing data leakage.

2. Efficiency:
Before usage, global and static variables must be initialized with useful values. Initializing them to their default values (often zero) is more efficient than assigning specific values throughout the code. Zeroing operations can be optimized with unrolled loops, and even performed during system idle time by the OS.

3. Reproducibility:
Leaving variables uninitialized would result in non-repeatable program behavior, making debugging challenging. Default initialization ensures consistent behavior and facilitates identification of errors.

4. Cleanliness:
Many programming languages, including C/C , strive for code clarity and simplicity. Default initialization eliminates the need for explicit initializers, making code more concise and readable.

A Note on Automatic Variables:
In contrast to global and static variables, automatic (function-local) variables are not always initialized to default values. Instead, they hold the remnants of previously assigned values. This approach is taken primarily to avoid runtime performance overheads associated with initializing such variables on every function call.

However, automatic variables stored on the stack's initial page do start as zeroed values. This page remains clear of previous function-call remnants, while subsequent pages may contain uninitialized data.

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