Why are Global and Static Variables, but not Automatic Variables, Initialized to Default Values in C/C ?

Why are Global and Static Variables Initialized to Default Values in C/C ?

In C/C , global and static variables are initialized to default values instead of being left with garbage values. This practice stems from several fundamental reasons:

1. Security: Uninitialized memory can contain sensitive information from other processes or the kernel, leading to security vulnerabilities. Default initialization ensures that these variables begin with predictable values.
2. Efficiency: By initializing global and static variables to default values, the compiler can optimize the initialization process by zeroing out large blocks of memory using efficient unrolled loops. This reduces the initialization overhead compared to leaving them with garbage values.
3. Reproducibility: Garbage values can vary depending on the system, making program behavior non-repeatable and bug identification challenging. Default initialization provides consistent starting values, ensuring predictable program execution.
4. Elegance: Default initialization allows programs to start from a clean state without the need for explicit initialization statements, keeping the code concise and readable.

Conversely, automatic variables (declared within function scopes) are not explicitly initialized to default values. This is primarily because:

• Memory conservation: Auto variables are allocated on the stack, which is a precious resource. Default initializing all auto variables could waste memory on unused or discarded data.
• Runtime performance: Default initializing auto variables can incur a quadratic (or worse) runtime penalty, especially for large arrays. By leaving them uninitialized, the program can avoid unnecessary computations.

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