# Overflow and underflow of buffers Detects when you access memory outside of a buffer’s boundaries. ## Overview In Xcode 7 and later, you can use this check to detect when you access memory that’s outside of a buffer’s boundaries. The check reports overflow when accessed memory is beyond the end of the buffer, and underflow when the accessed memory is before the beginning of a buffer. Xcode sanitizes heap and stack buffers, as well as global variables. Buffer overflow and underflow can result in a crash or other unpredictable behavior. ### Global, heap, and stack overflows in C In the following example, the `global_array`, `heap_buffer`, and `stack_buffer` variables each have valid indexes in the range `[0, 9]`, but the accessed index is `10`, which causes an overflow: ```occ int global_array[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; void foo() { int idx = 10; global_array[idx] = 42; // Error: out of bounds access of global variable char *heap_buffer = malloc(10); heap_buffer[idx] = 'x'; // Error: out of bounds access of heap allocated variable char stack_buffer[10]; stack_buffer[idx] = 'x'; // Error: out of bounds access of stack allocated variable } ``` #### Solution Add a bounds check before attempting to access a buffer at a specific index. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)