C Language Accessing memory beyond allocated chunk


Example

A a pointer to a piece of memory containing n elements may only be dereferenced if it is in the range memory and memory + (n - 1). Dereferencing a pointer outside of that range results in undefined behavior. As an example, consider the following code:

int array[3];
int *beyond_array = array + 3;
*beyond_array = 0; /* Accesses memory that has not been allocated. */

The third line accesses the 4th element in an array that is only 3 elements long, leading to undefined behavior. Similarly, the behavior of the second line in the following code fragment is also not well defined:

int array[3];
array[3] = 0;

Note that pointing past the last element of an array is not undefined behavior (beyond_array = array + 3 is well defined here), but dereferencing it is (*beyond_array is undefined behavior). This rule also holds for dynamically allocated memory (such as buffers created through malloc).