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Dangling pointer 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, and include such phenomena as link rot on the internet. Dangling pointers arise during object destruction, when an object that has an incoming reference is deleted or deallocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the deallocated memory. As the system may reallocate the previously freed memory to another process, if the original program then dereferences the (now) dangling pointer, ''unpredictable behavior may result'', as the memory may now contain completely different data. This is especially the case if the program writes data to memory pointed by a dangling pointer, a silent corruption of unrelated data may result, leading to subtle bugs that can be extremely difficult to find, or cause segmentation faults (UNIX, Linux) or general protection faults (Windows). If the overwritten data is bookkeeping data used by the system'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. Wild pointers arise when a pointer is used prior to initialization to some known state, which is possible in some programming languages. They show the same erratic behaviour as dangling pointers, though they are less likely to stay undetected. ==Cause of dangling pointers== In many languages (e.g., the C programming language) deleting an object from memory explicitly or by destroying the stack frame on return does not alter associated pointers. The pointer still points to the same location in memory even though the reference has since been deleted and may now be used for other purposes. A straightforward example is shown below: If the operating system is able to detect run-time references to null pointers, a solution to the above is to assign 0 (null) to dp immediately before the inner block is exited. Another solution would be to somehow guarantee dp is not used again without further initialization. Another frequent source of dangling pointers is a jumbled combination of malloc() and free() library calls: a pointer becomes dangling when the block of memory it points to is freed. As with the previous example one way to avoid this is to make sure to reset the pointer to null after freeing its reference—as demonstrated below. #include void func() An all too common misstep is returning addresses of a stack-allocated local variable: once a called function returns, the space for these variables gets deallocated and technically they have "garbage values". int *func(void) Attempts to read from the pointer may still return the correct value (1234) for a while after calling func, but any functions called thereafter will overwrite the stack storage allocated for num with other values and the pointer would no longer work correctly. If a pointer to num must be returned, num must have scope beyond the function—it might be declared as static. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dangling pointer」の詳細全文を読む スポンサード リンク
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