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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.〔ISO/IEC 9899, clause 6.3.2.3, paragraph 4.〕 The macro NULL is defined as an implementation-defined null pointer constant,〔ISO/IEC 9899, clause 7.17, paragraph 3: ''NULL... which expands to 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.〔ISO/IEC 9899, clause 6.3.2.3, paragraph 3.〕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 ). 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「Null pointer」の詳細全文を読む スポンサード リンク
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