What is an Access Violation?

What is an Access Violation?

An access violation is an error raised by the operating system when a program tries to access a region of memory in a way that is not allowed. This typically results in the program crashing or terminating with an error like:

  • Access Violation (Exception Code 0xC0000005)
  • Segmentation fault (on Unix/Linux systems)
  • General Protection Fault

Memory Protection Basics

Modern operating systems use virtual memory management to protect memory. Each process gets its own virtual address space, and pages of memory have permissions:

  • Read (R): can read from this page
  • Write (W): can write to this page
  • Execute (X): can execute code on this page

 

Common Causes of Access Violations

  1. Null Pointer Dereference
    Trying to access memory via a pointer that’s NULL or nullptr.
  2. Dangling Pointer
    Using a pointer after the memory it points to has been freed or deleted.
  3. Buffer Overflows / Underruns
    Accessing memory outside the bounds of an array or buffer.
  4. Writing to Read-only Memory
    Attempting to modify memory marked as read-only (e.g., string literals, code sections).
  5. Incorrect Pointer Arithmetic
    Miscalculations leading to accessing invalid memory locations.
  6. Stack Overflow / Corruption
    Writing beyond stack limits or corrupting the call stack.
  7. Executing Non-executable Memory
    Trying to run code in memory pages not marked executable.

How Does the OS Detect This?

  • The OS uses the Memory Management Unit (MMU) and page tables to map virtual memory to physical memory.
  • Each page has permission bits.
  • If a program tries to access memory without appropriate permission, the CPU triggers a page fault.
  • The OS examines the fault; if it’s illegal, it sends a signal or exception (e.g., SIGSEGV on Unix, or an Access Violation Exception on Windows).

Diagnosing Access Violations

  • Use a Debugger: Set breakpoints and step through code to find the exact instruction causing the violation.
  • Analyze the Call Stack: Identify which function and line triggered the error.
  • Examine Pointers: Check if pointers are valid, initialized, and pointing to allocated memory.
  • Memory Tools:
    • Valgrind (Linux/macOS)
    • AddressSanitizer (gcc/clang)
    • Dr. Memory (Windows)
      These tools help find invalid reads/writes, memory leaks, and use-after-free bugs.

How to Fix Access Violations

  • Always initialize pointers before use.
  • Free memory only once; avoid using pointers after freeing.
  • Use safe functions for copying and accessing memory (e.g.strncpy, bounds-checked containers).
  • Use language features that manage memory automatically (smart pointers in C++, managed languages like Java/C#).
  • Validate all inputs and array indices.
  • Avoid casting pointers arbitrarily.
  • For multi-threaded code, ensure memory synchronization and avoid race conditions.

Summary

  • Access violation = illegal memory access (read/write/execute not allowed).
  • Caused by invalid pointers, buffer overruns, wrong permissions.
  • Detected by OS via page faults and protection bits.
  • Debug and fix by careful pointer management and tools.
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