Dangling pointer

Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references are references that do not resolve to a valid destination.

Dangling pointers arise during object destruction, when an object that is pointed to by a given pointer is deleted or deallocated, without modifying the value of that said pointer, so that the pointer still points to the memory location of the deallocated memory. The system may reallocate the previously freed memory, and if the program then dereferences the (now) dangling pointer, unpredictable behavior may result, as the memory may now contain completely different data. If the program writes to memory referenced by a dangling pointer, a silent corruption of unrelated data may result, leading to subtle bugs that can be extremely difficult to find. If the memory has been reallocated to another process, then attempting to dereference the dangling pointer can cause segmentation faults (UNIX, Linux) or general protection faults (Windows). If the program has sufficient privileges to allow it to overwrite the bookkeeping data used by the kernel's memory allocator, the corruption can cause system instabilities. In object-oriented languages with garbage collection, dangling references are prevented by only destroying objects that are unreachable, meaning they do not have any incoming pointers; this is ensured either by tracing or reference counting. However, a finalizer may create new references to an object, requiring object resurrection to prevent a dangling reference.