Why Don't Array Out-of-Bounds Accesses Always Cause Segmentation Faults?

Understanding Segmentation Faults: Why Array Overflows Don't Always Crash

When accessing an array out of bounds, one might intuitively expect a segmentation fault. However, in the provided code:

int *a = new int[2];

// Accessing array elements beyond the allocated size
a[0] = 0;
a[1] = 1;
a[2] = 2;
a[3] = 3;
a[100] = 4;

no error or segmentation fault occurs during compilation or runtime. This surprising behavior stems from the nature of undefined behavior.

Undefined behavior is a behavior explicitly left unspecified by the programming language definition. In this case, accessing array elements outside the allocated bounds results in unpredictable consequences. In some cases, it may lead to a segmentation fault, crashing the program. However, in other scenarios, it may not trigger an error, as seen in this example.

The absence of a segmentation fault in this code is primarily due to the following factors:

• Memory Allocation: When you allocate an array using new, the system allocates a contiguous block of memory for the specified number of elements. In this case, the block is only large enough for two integers.
• Array Overflows: When you access elements beyond the allocated size, you're venturing into undefined territory. The compiler has no idea how to handle such accesses.
• Possible Consequences: In this particular instance, the program doesn't crash because the memory allocated after the array is uninitialized and writable. This allows the program to write data without encountering any immediate errors.
• Danger of Undefined Behavior: However, it's essential to note that undefined behavior can have potentially hazardous consequences. It can lead to subtle bugs, unpredictable program behavior, and even security vulnerabilities.

Therefore, it's crucial to avoid relying on undefined behavior in your code and always adhere to the allocated bounds of arrays to prevent unexpected results.

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