Accessing array out of bounds in C/C++

An array in C is a fixed-size sequential collection of elements of the same data type where all elements are stored in contiguous memory. When an array is accessed out of bounds, undefined behavior occurs in C, unlike higher-level languages that throw exceptions.

Syntax

type arrayName[size];
// Valid indices: 0 to (size-1)
// Out of bounds: index < 0 or index >= size

Accessing Out of Bounds Memory

Accessing out-of-bounds memory means trying to access an array index outside its valid range (index < 0 or index >= array size). This returns garbage values from adjacent memory locations.

Example: Reading Beyond Array Bounds

In this example, an array of 5 elements is initialized and we attempt to access 7 elements −

#include <stdio.h>

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    printf("The elements of array: ");
    for (int i = 0; i < 7; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    return 0;
}

The elements of array: 1 2 3 4 5 32765 0

The first 5 elements are valid, while indices 5 and 6 contain garbage values from uninitialized memory.

Out of Bounds Array Assignment

Writing to memory outside an array's bounds can corrupt adjacent data structures or cause segmentation faults, depending on the memory location accessed.

Example: Writing Beyond Array Bounds

Here we attempt to assign a value at index 10 when the array size is only 5 −

#include <stdio.h>

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    arr[10] = 57;  // Writing beyond bounds
    printf("Value at arr[10]: %d\n", arr[10]);
    return 0;
}

Segmentation fault (core dumped)

Prevention Methods

Always validate array indices before access to prevent undefined behavior −

#include <stdio.h>

#define ARRAY_SIZE 5

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    int index = 7;
    
    if (index >= 0 && index < ARRAY_SIZE) {
        printf("arr[%d] = %d\n", index, arr[index]);
    } else {
        printf("Error: Index %d is out of bounds (0-%d)\n", index, ARRAY_SIZE-1);
    }
    return 0;
}

Error: Index 7 is out of bounds (0-4)

Key Points

• C does not perform automatic bounds checking on arrays
• Reading out of bounds returns garbage values
• Writing out of bounds can cause segmentation faults or memory corruption
• Always validate array indices before access

Conclusion

Array bounds checking is the programmer's responsibility in C. Out-of-bounds access leads to undefined behavior, which can manifest as garbage values or program crashes. Always validate indices to ensure safe array access.