Value indicating that a referenced dataset is invalid or doesn't exist
In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown length or the failure to perform some action; this use of null pointers can be compared to nullable types and to the Nothing value in an option type.
A null pointer should not be confused with an uninitialized pointer: a null pointer is guaranteed to compare unequal to any pointer that points to a valid object. However, in general, most languages do not offer such guarantee. It might compare equal to other, valid pointers; or it might compare equal to null pointers. It might do both at different times; or the comparison might be undefined behaviour. Also, in languages offering such support, the correct use depends on the individual experience of each developer an linter tools. Even when used properly, null pointers are semantically incomplete, since they do not offer the possibility to express the difference between of a "Not Applicable" value versus a "Not known" value or versus a "Future" value.
Because a null pointer does not point to a meaningful object, an attempt to access the data stored at that (invalid) memory location may cause a run-time error or immediate program crash. This is the null pointer error. It is one of the most common types of software weaknesses,[1] and Tony Hoare, who introduced the concept, has referred to it as a "billion dollar mistake".
In computing, a nullpointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs...
representation. Null device, a virtual file that discards data written to it, on Unix systems /dev/nullNullpointer or reference (sometimes written NULL, nil,...
often occur because of errors in pointer use, particularly in C dynamic memory allocation. Dereferencing a nullpointer, which results in undefined behavior...
unusable. This means that, for example, a pointer to 0 is never a valid pointer and can be used as a special nullpointer value. Unlike the previously mentioned...
system is able to detect run-time references to nullpointers, a solution to the above is to assign 0 (null) to dp immediately before the inner block is...
abbreviated as NUL (or NULL, though in some contexts that term is used for the nullpointer). In 8-bit codes, it is known as a null byte. The original meaning...
pointers that can be NULL require the user to check the pointer before using it and require specific code to handle the case when the object pointer is...
programmers [who?] then set the pointer variable to NULL: free(a); a = NULL; This ensures that further attempts to dereference the pointer, on most systems, will...
confuse == with =. The advantage of avoiding null behavior can also be considered a disadvantage, as nullpointer errors can be hidden and only appear much...
no appropriate object available. A null reference is impossible in standard-conforming C++. A null Animal* pointer is possible, and could be useful as...
nullpointer to his design of the ALGOL W language and called it a "mistake": I call it my billion-dollar mistake. It was the invention of the null reference...
singly linked list. The first one uses the sentinel value NULL, and the second one a (pointer to the) sentinel node Sentinel, as the end-of-list indicator...
= NULL; // initialize invalid links to nil. if (head == NULL) { head = temp; // If the linked list is empty (i.e., the head node pointer is a null pointer)...
Windows actually make the zero page inaccessible to trap uses of nullpointers. Such pointer values may legitimately indicate uninitialized values or sentinel...
offsetof(st, m) \ ((size_t)&(((st *)0)->m)) This can be understood as taking a nullpointer of type structure st, and then obtaining the address of member m within...
expression for rejecting all available candidates None, the keyword for the nullpointer in Python Nones (disambiguation) Nothing (disambiguation) Zero (disambiguation)...
might return a nullpointer and it is good programming practice to check for this: int *array = malloc(10 * sizeof(int)); if (array == NULL) { fprintf(stderr...
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