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ure 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 an object however in general most languages do not offer such a guarantee for uninitialized pointers 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 behavior also in languages offering such support the correct use depends on the individual experience of each developer and linter tools even when used properly null pointers are semantically incomplete since they do not offer the possibility to express the difference between not applicable not currently known and not yet determined values 1 in systems 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 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 or null pointer exception it is one of the most common types of software weaknesses 2 and tony hoare who introduced the concept has referred to it as a billion dollar mistake 3 c edit in c two null pointers of any type are guaranteed to compare equal 4 the preprocessor macro null is provided defined as an implementation defined null pointer constant in stdlib h 5 which in c99 can be portably expressed with define null void 0 the integer value 0 converted to the type void see pointer to void type 6 since c23 a null pointer is represented with nullptr which is of type nullptr_t first introduced to c 11 providing a type safe null pointer 7 the c standard does not say that the null pointer is the same as the pointer to memory address 0 though that may be the case in practice dereferencing a null pointer is undefined behavior in c 8 and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null in practice dereferencing a null pointer may result in an attempted read or write from memory that is not mapped triggering a segmentation fault or memory access violation this may manifest itself as a program crash or be transformed into a software exception that can be caught by program code there are however certain circumstances where this is not the case for example in x86 real mode the address 0000 0000 is readable and also usually writable and dereferencing a pointer to that address is a perfectly valid but typically unwanted action that may lead to undefined but non crashing behavior in the application if a null pointer is represented as a pointer to that address dereferencing it will lead to that behavior there are occasions when dereferencing a pointer to address zero is intentional and well defined for example bios code written in c for 16 bit real mode x86 devices may write the interrupt descriptor table idt at physical address 0 of the machine by dereferencing a pointer with the same value as 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 9 c edit 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 10 however c 11 introduced the explicit null pointer constant nullptr and type nullptr_t to be used instead providing a type safe null pointer nullptr and type nullptr_t were later introduced to c in c23 other languages edit programming languages use different literals for the null pointer in java and c the literal null is provided as a literal for reference types in pascal and swift a null pointer is called nil in eiffel it is called a void reference in rust the absence of a value is denoted as none but a true null pointer is std ptr null null dereferencing edit because a null pointer does not point to a meaningful object an attempt to dereference i e access the data stored at that memory location a null pointer usually but not always causes a run time error or immediate program crash mitre lists the null pointer error as one of the most commonly exploited software weaknesses 11 in c dereferencing a null pointer is undefined behavior 8 many implementations cause such code to result in the program being halted with an access violation 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 it is also not guaranteed since compilers are permitted to optimize programs under the assumption that they are free of undefined behavior this behaviour is the same in c as there is no null pointer exception in the c language on platforms such as unix like systems and windows with the visual studio compiler an access violation causes a c c sigsegv signal to be issued although in c c null dereferences are not exceptions which can be caught in c try catch blocks it is possible to catch such an access violation by using std signal in c c to specify a handler to be called when that signal is issued some external c libraries such as poco c libraries include a nullpointerexception class unlike java where java lang nullpointerexception extends java lang runtimeexception poco nullpointerexception instead extends poco logicexception 12 in cyclone a failed null pointer check will throw a null_exception in d much like c a null pointer dereference results in a segmentation fault in delphi and many other pascal implementations the constant nil represents a null pointer to the first address in memory which is also used to initialize managed variables dereferencing it raises an external os exception which is mapped onto a pascal eaccessviolation exception instance if the system sysutils unit is linked in the uses clause in java access to a null reference null causes 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 and c access to null reference null causes a system 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 13 in rust dereferencing a null pointer std ptr null in an unsafe block results in undefined behaviour which usually results in a segmentation fault or corrupted memory before the introduction of supervisor mode access prevention smap a null pointer dereference bug could be exploited by mapping page zero into the attacker s address space and hence causing the null pointer to point to that region this could lead to code execution in some cases 14 mitigation edit while there could be languages with no nulls most do have the possibility of nulls so there are techniques to avoid or aid debugging null pointer dereferences 15 bond et al suggest modifying the java virtual machine jvm to keep track of null propagation 15 there are three levels of handling null references in order of effectiveness languages with no null languages that can statically analyse code to avoid the possibility of null dereference at run time if null dereference can occur at runtime tools that aid debugging pure functional languages are an example of level 1 since no direct access is provided to pointers and all code and data is immutable user code running in interpreted or virtual machine languages generally does not suffer the problem of null pointer dereferencing dubious discuss where a language does provide or utilise pointers which could become void it is possible to avoid runtime null dereferences by providing compilation time checking via static analysis or other techniques with syntactic assistance from language features such as those seen in the eiffel programming language with void safety 16 to avoid null dereferences d 17 and rust 18 nullable type systems have also been widely adopted in mainstream programming languages kotlin swift and typescript include them from the start while c and scala have been retrofitted with nullable type systems 19 in some languages analysis can be performed using external tools but these are weak compared to direct language support with compiler checks since they are limited by the language definition itself the last resort of level 3 is when a null reference occurs at runtime debugging aids can help history edit in 2009 tony hoare stated 3 20 21 that he invented the null reference in 1965 as part of the algol w language 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 see also edit memory debugger zero page notes edit toussaint etienne guagliardo paolo libkin leonid sequeda juan 2022 troubles with nulls views from the users proceedings of the vldb endowment 15 2613 2625 doi 10 14778 3551793 3551818 hdl 20 500 11820 29fe40ac 24be 4b44 a67f 5c191e97092a cwe 476 null pointer dereference mitre 1 2 tony hoare 2009 08 25 null references the billion dollar mistake infoq com iso iec 9899 clause 6 3 2 3 paragraph 4 iso iec 9899 clause 7 17 paragraph 3 null which expands to an implementation defined null pointer constant iso iec 9899 clause 6 3 2 3 paragraph 3 wr14 n3042 introduce the nullptr constant open std org 2022 07 22 archived from the original on december 24 2022 1 2 iso iec 9899 clause 6 5 3 2 paragraph 4 esp footnote 87 lattner chris 2011 05 13 what every c programmer should know about undefined behavior 1 3 blog llvm org archived from the original on 2023 06 14 retrieved 2023 06 14 stroustrup bjarne march 2001 chapter 5 the const qualifier 5 4 prevents accidental redefinition of null and ensures that null can be used where a constant is required the c programming language 14th printing of 3rd ed united states and canada addison wesley p 88 isbn 0 201 88954 4 cwe 476 null pointer dereference mitre class poco nullpointerexception docs pocoproject org retrieved 17 august 2025 the objective c 2 0 programming language section sending messages to nil os x exploitable kernel null pointer dereference in applegraphicsdevicecontrol 1 2 bond michael d nethercote nicholas kent stephen w guyer samuel z mckinley kathryn s 2007 tracking bad apples proceedings of the 22nd annual acm sigplan conference on object oriented programming systems and applications oopsla 07 p 405 doi 10 1145 1297027 1297057 isbn 9781595937865 s2cid 2832749 void safety background definition and tools retrieved 2021 11 24 bartosz milewski safed d programming language retrieved 17 july 2014 fearless security memory safety archived from the original on 8 november 2020 retrieved 4 november 2020 madsen magnus pol jaco van de 2021 relational nullable types with boolean unification proceedings of the acm on programming languages 5 1 28 doi 10 1145 3485487 tony hoare 2009 08 25 presentation null references the billion dollar mistake infoq com contieri maxi 2025 06 18 code smell 304 null pointer exception maximiliano contieri software design retrieved 2025 06 18 references edit joint technical committee iso iec jtc 1 subcommittee sc 22 working group wg 14 2007 09 08 international standard iso iec 9899 pdf committee draft cite book cs1 maint multiple names authors list link cs1 maint numeric names authors list link v t e nulls in computing null character null device null function null modem null object pattern null pointer null in sql null string null coalescing operator see also null coalescing operator nullable type undefined value v t e tony hoare software programming algol w monitors null references occam structured programming algorithms dining philosophers problem quickselect quicksort formal methods csp duration calculus hoare logic utp books structured programming 1972 communicating sequential processes 1985 unifying theories of programming 1998 prentice hall international series in computer science from 1981 workplaces moscow state university elliott brothers queen s university belfast university of oxford microsoft research cambridge collaborations colleagues richard bird jonathan bowen michael butler ole johan dahl edsger dijkstra leslie fox mike gordon he jifeng eric hehner andrey kolmogorov gary t leavens john lucas david may robin milner jayadev misra carroll morgan peter o hearn ernst rüdiger olderog jill pym anders p ravn bill roscoe natarajan shankar mike spivey joe stoy bernard sufrin niklaus wirth zhou chaochen students cliff jones bill roscoe augusto sampaio groups ifip working group 2 1 programming research group procos awards distinguished fellow of the british computer society 1978 turing award 1980 harry h goode memorial award 1981 fellow of the royal society 1982 iet faraday medal 1985 computer pioneer award 1990 knight bachelor 2000 kyoto prize 2000 fellow of the royal academy of engineering 2005 fellow of the computer history museum 2006 friedrich l bauer prize 2007 ieee john von neumann medal 2011 fellow of the acm 2020 royal medal 2023 category wikiquote retrieved from https en wikipedia org w index php title null_pointer oldid 1359146938 category pointers computer programming hidden categories articles with short description short description is different from wikidata all accuracy disputes articles with disputed statements from september 2024 cs1 maint multiple names authors list cs1 maint numeric names authors list this page was last edited on 13 june 2026 at 11 48 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie 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