---
title: Overflow and underflow of buffers
framework: xcode
role: article
role_heading: Article
path: xcode/overflow-and-underflow-of-buffers
---

# Overflow and underflow of buffers

Detects when you access memory outside of a buffer’s boundaries.

## Overview

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: 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.

## See Also

### Address Sanitizer

- [Use of deallocated memory](xcode/use-of-deallocated-memory.md)
- [Deallocation of deallocated memory](xcode/deallocation-of-deallocated-memory.md)
- [Deallocation of nonallocated memory](xcode/deallocation-of-nonallocated-memory.md)
- [Use of stack memory after function return](xcode/use-of-stack-memory-after-function-return.md)
- [Use of out-of-scope stack memory](xcode/use-of-out-of-scope-stack-memory.md)
- [Overflow of C++ containers](xcode/overflow-of-c-containers.md)