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Dangling pointer

Dangling pointer

Dangling pointer definition

A dangling pointer is a pointer (an object that stores a memory address) to a memory location that has been deallocated or freed, resulting in an invalid or unpredictable state. Because the memory region that the pointer points to is no longer valid, accessing the pointer can cause the program to behave unpredictably.

See also: memory allocation

Causes of dangling pointers

  • Incorrect deallocation: A programmer frees memory but forgets to update or invalidate the corresponding pointers.
  • Premature deallocation: A deallocated pointer early in the program’s execution causes other parts of the code that hold references to that memory location and produce dangling pointers.
  • Returning local variables: When a function refers the pointer to a local variable (a variable created inside the function’s scope), the pointer becomes dangling once the function completes and the variable is deleted.

Dangling pointer issues

  • Accessing invalid memory: Reading or writing to invalid memory locations may result in program crashes or data corruption.
  • Unexpected behavior: Dangling pointers can lead to unpredictable program behavior, making it difficult to debug the code.
  • Security vulnerabilities: Some security exploits (such as use-after-free vulnerabilities) can abuse dangling pointers to execute arbitrary code or gain unauthorized access.

Preventing dangling pointers

  • Employ proper memory management — make sure that memory is deallocated only when it is no longer needed and update all pointers accordingly.
  • Nullify pointers after deallocating memory by assigning NULL (or a similar invalid value) to the corresponding pointers.
  • Use automatic memory management (such as garbage collection) to handle memory deallocation and reduce the risk of dangling pointers.

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