Null pointer

In computing, a null pointer has a value reserved for indicating that the pointer 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, depending on the language and implementation, an uninitialized pointer may not have any 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.

Null pointers have different semantics than null values. A null pointer in most programming languages means "no value", while a null value in a relational database means "unknown value". This leads to important differences in practice: most programming languages will treat two null pointers as equal, but a relational database engine does not regard two null values as equal (since they represent unknown values, it is unknown whether they are equal).

C

In C, two null pointers of any type are guaranteed to compare equal. The macro NULL is defined as an implementation-defined null pointer constant, which in C99 can be portably expressed as the integer value 0 converted implicitly or explicitly to the type void*.

Dereferencing the NULL pointer typically results in an attempted read or write from memory that is not mapped - triggering a segmentation fault or access violation. This may represent itself to the developer as a program crash, or be transformed into an exception that can be caught. There are, however, certainly circumstances where this is not the case. For example, in x86-real mode, the address 0000:0000 is readable and usually writable, hence dereferencing the null pointer is a perfectly valid but typically unwanted action that may lead to undefined but non-crashing behaviour in the application. Note also that there are occasions when dereferencing the NULL is intentional and well defined; for example BIOS code written in C for 16-bit real-mode x86 devices may write the IDT at physical address 0 of the machine by dereferencing a NULL pointer for writing. It is also possible for the compiler to optimize away the `NULL` pointer dereference, avoiding a segmentation fault but causing other undesired behavior.

C++

In C++, while the NULL macro was inherited from C, the integer literal for zero has been traditionally preferred to represent a null pointer constant. However, C++11 has introduced an explicit nullptr constant to be used instead.

Other Languages

In some programming language environments (at least one proprietary Lisp implementation, for example), the value used as the null pointer (called nil in Lisp) may actually be a pointer to a block of internal data useful to the implementation (but not explicitly reachable from user programs), thus allowing the same register to be used as a useful constant and a quick way of accessing implementation internals. This is known as the nil vector.

In languages with a tagged architecture, a possibly null pointer can be replaced with a tagged union which enforces explicit handling of the exceptional case; in fact, a possibly null pointer can be seen as a tagged pointer with a computed tag.

Dereferencing

Because a null pointer does not point to a meaningful object, an attempt to dereference a null pointer usually (but not always) causes a run-time error or immediate program crash.

• In C, the behavior of dereferencing a null pointer is undefined. Many implementations cause such code to result in the program being halted with a segmentation fault, because the null pointer representation is chosen to be an address that is never allocated by the system for storing objects. However, this behavior is not universal.
• In Java, access to a null reference triggers a NullPointerException (NPE), which can be caught by error handling code, but the preferred practice is to ensure that such exceptions never occur.
• In .NET, access to null reference triggers a NullReferenceException to be thrown. Although catching these is generally considered bad practice, this exception type can be caught and handled by the program.
• In Objective-C, messages may be sent to a nil object (which is a null pointer) without causing the program to be interrupted; the message is simply ignored, and the return value (if any) is nil or 0, depending on the type.

History

In 2009 C.A.R. Hoare stated that he invented the null reference in 1965 as part of the Algol W language, although NIL had existed in Lisp since 1959. In that 2009 reference Hoare describes his invention as a "billion-dollar mistake":

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

External links

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