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id="toc-C++-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Other_languages" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Other_languages"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Other languages</span> </div> </a> <ul id="toc-Other_languages-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Null_dereferencing" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Null_dereferencing"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Null dereferencing</span> </div> </a> <ul id="toc-Null_dereferencing-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mitigation" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Mitigation"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Mitigation</span> </div> </a> <ul id="toc-Mitigation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Alternatives_to_null_pointers" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Alternatives_to_null_pointers"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Alternatives to null pointers</span> </div> </a> <ul id="toc-Alternatives_to_null_pointers-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-History" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#History"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>History</span> </div> </a> <ul id="toc-History-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span 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mw-first-heading"><span class="mw-page-title-main">Null pointer</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 14 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-14" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">14 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-azb mw-list-item"><a href="https://azb.wikipedia.org/wiki/%D9%86%D8%A7%D9%84_%DA%AF%D8%A4%D8%B3%D8%AA%D9%87%E2%80%8C%D8%B1%DB%8C%D8%AC%DB%8C_(%D8%A8%DB%8C%D9%84%DA%AF%DB%8C%E2%80%8C%D8%B3%D8%A7%DB%8C%D8%A7%D8%B1_%D8%A8%DB%8C%D9%84%DB%8C%D9%85%DB%8C)" title="نال گؤسته‌ریجی (بیلگی‌سایار بیلیمی) – South Azerbaijani" lang="azb" hreflang="azb" data-title="نال گؤسته‌ریجی (بیلگی‌سایار بیلیمی)" data-language-autonym="تۆرکجه" data-language-local-name="South Azerbaijani" class="interlanguage-link-target"><span>تۆرکجه</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Punter_nul" title="Punter nul – Catalan" lang="ca" hreflang="ca" data-title="Punter nul" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Nulov%C3%BD_ukazatel" title="Nulový ukazatel – Czech" lang="cs" hreflang="cs" data-title="Nulový ukazatel" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de badge-Q70894304 mw-list-item" title=""><a href="https://de.wikipedia.org/wiki/Nullzeiger" title="Nullzeiger – German" lang="de" hreflang="de" data-title="Nullzeiger" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%A7%D8%B4%D8%A7%D8%B1%D9%87%E2%80%8C%DA%AF%D8%B1_%D9%87%DB%8C%DA%86%E2%80%8C%D9%85%D9%82%D8%AF%D8%A7%D8%B1" title="اشاره‌گر هیچ‌مقدار – Persian" lang="fa" hreflang="fa" data-title="اشاره‌گر هیچ‌مقدار" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EB%84%90_%ED%8F%AC%EC%9D%B8%ED%84%B0" title="널 포인터 – Korean" lang="ko" hreflang="ko" data-title="널 포인터" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%9E%D7%A6%D7%91%D7%99%D7%A2_%D7%94%D7%90%D7%A4%D7%A1" title="מצביע האפס – Hebrew" lang="he" hreflang="he" data-title="מצביע האפס" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%83%8C%E3%83%AB%E3%83%9D%E3%82%A4%E3%83%B3%E3%82%BF" title="ヌルポインタ – Japanese" lang="ja" hreflang="ja" data-title="ヌルポインタ" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Pointer_null" title="Pointer null – Romanian" lang="ro" hreflang="ro" data-title="Pointer null" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B8%95%E0%B8%B1%E0%B8%A7%E0%B8%8A%E0%B8%B5%E0%B9%89%E0%B8%97%E0%B8%B5%E0%B9%88%E0%B9%80%E0%B8%9B%E0%B9%87%E0%B8%99%E0%B9%82%E0%B8%A1%E0%B8%86%E0%B8%B0" title="ตัวชี้ที่เป็นโมฆะ – Thai" lang="th" hreflang="th" data-title="ตัวชี้ที่เป็นโมฆะ" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Bo%C5%9F_i%C5%9Faret%C3%A7isi" title="Boş işaretçisi – Turkish" lang="tr" hreflang="tr" data-title="Boş işaretçisi" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9D%D1%83%D0%BB%D1%8C%D0%BE%D0%B2%D0%B8%D0%B9_%D0%B2%D0%BA%D0%B0%D0%B7%D1%96%D0%B2%D0%BD%D0%B8%D0%BA" title="Нульовий вказівник – 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href="/wiki/Pointer_(computer_programming)" title="Pointer (computer programming)">pointer</a> or <a href="/wiki/Reference_(computer_science)" title="Reference (computer science)">reference</a> does not refer to a valid <a href="/wiki/Object_(computer_science)" title="Object (computer science)">object</a>. Programs routinely use null pointers to represent conditions such as the end of a <a href="/wiki/List_(computing)" class="mw-redirect" title="List (computing)">list</a> of unknown length or the failure to perform some action; this use of null pointers can be compared to <a href="/wiki/Nullable_type" title="Nullable type">nullable types</a> and to the <i>Nothing</i> value in an <a href="/wiki/Option_type" title="Option type">option type</a>. </p><p>A null pointer should not be confused with an <a href="/wiki/Uninitialized_variable" title="Uninitialized variable">uninitialized pointer</a>: 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 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 <a href="/wiki/Undefined_behavior" title="Undefined behavior">undefined behaviour</a>. 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 <a href="/wiki/Semantic_completeness" class="mw-redirect" title="Semantic completeness">semantically incomplete</a>, since they do not offer the possibility to express the difference between a "Not Applicable" value versus a "Not known" value or versus a "Future" value. </p><p>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 <b>null pointer error</b>. It is one of the most common types of software weaknesses,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Tony_Hoare" title="Tony Hoare">Tony Hoare</a>, who introduced the concept, has referred to it as a "billion dollar mistake".<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="C">C</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=1" title="Edit section: C"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a>, two null pointers of any type are guaranteed to compare equal.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Prior to C23, the preprocessor macro <code>NULL</code> was provided, defined as an implementation-defined null pointer constant in <code><<a href="/wiki/Stdlib.h" class="mw-redirect" title="Stdlib.h">stdlib.h</a>></code>,<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> which in <a href="/wiki/C99" title="C99">C99</a> can be portably expressed as <code>((void *)0)</code>, the integer value <code>0</code> <a href="/wiki/Type_conversion" title="Type conversion">converted</a> to the type <code>void*</code> (see <a href="/wiki/Pointer_to_void_type" class="mw-redirect" title="Pointer to void type">pointer to void type</a>).<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Since <a href="/wiki/C23_(C_standard_revision)" title="C23 (C standard revision)">C23</a>, a null pointer is represented with <code>nullptr</code> which is of type <code>nullptr_t</code> (something originally introduced to C++11), providing a type safe null pointer.<sup id="cite_ref-N3042_6-0" class="reference"><a href="#cite_note-N3042-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p>The C standard does not say that the null pointer is the same as the pointer to <a href="/wiki/Memory_address" title="Memory address">memory address</a> 0, though that may be the case in practice. <a href="/wiki/Dereferencing" class="mw-redirect" title="Dereferencing">Dereferencing</a> a null pointer is <a href="/wiki/Undefined_behavior" title="Undefined behavior">undefined behavior</a> in C,<sup id="cite_ref-ub_7-0" class="reference"><a href="#cite_note-ub-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> and a conforming implementation is allowed to assume that any pointer that is dereferenced is not null. </p><p>In practice, dereferencing a null pointer may result in an attempted read or write from <a href="/wiki/Computer_memory" title="Computer memory">memory</a> that is not mapped, triggering a <a href="/wiki/Segmentation_fault" title="Segmentation fault">segmentation fault</a> or memory access violation. This may manifest itself as a program crash, or be transformed into a software <a href="/wiki/Exception_handling" title="Exception handling">exception</a> that can be caught by program code. There are, however, certain circumstances where this is not the case. For example, in <a href="/wiki/Intel_x86" class="mw-redirect" title="Intel x86">x86</a> <a href="/wiki/Real_mode" title="Real mode">real mode</a>, the address <code>0000:0000</code> 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 <i>is</i> intentional and well-defined; for example, <a href="/wiki/BIOS" title="BIOS">BIOS</a> code written in C for 16-bit real-mode x86 devices may write the <a href="/wiki/Interrupt_descriptor_table" title="Interrupt descriptor table">interrupt descriptor table</a> (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.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="C++"><span id="C.2B.2B"></span>C++</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=2" title="Edit section: C++"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In C++, while the <code>NULL</code> macro was inherited from C, the integer literal for zero has been traditionally preferred to represent a null pointer constant.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> However, <a href="/wiki/C%2B%2B11" title="C++11">C++11</a> introduced the explicit null pointer constant <code>nullptr</code> and type <code>nullptr_t</code> to be used instead, providing a type safe null pointer. <code>nullptr</code> and type <code>nullptr_t</code> were later introduced to C in C23. </p> <div class="mw-heading mw-heading2"><h2 id="Other_languages">Other languages</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=3" title="Edit section: Other languages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In some programming language environments (at least one proprietary Lisp implementation, for example),<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2011)">citation needed</span></a></i>]</sup> the value used as the null pointer (called <code>nil</code> in <a href="/wiki/Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a>) 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 <code>nil</code> vector. </p><p>In languages with a <a href="/wiki/Tagged_architecture" title="Tagged architecture">tagged architecture</a>, a possibly null pointer can be replaced with a <a href="/wiki/Tagged_union" title="Tagged union">tagged union</a> which enforces explicit handling of the exceptional case; in fact, a possibly null pointer can be seen as a <a href="/wiki/Tagged_pointer" title="Tagged pointer">tagged pointer</a> with a computed tag. </p><p>Programming languages use different literals for the <i>null pointer</i>. In <a href="/wiki/Python_(programming_language)" title="Python (programming language)">Python</a>, for example, a null value is called <code>None</code>. In <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a> and <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, the literal <code>null</code> is provided as a literal for reference types. In <a href="/wiki/Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a> and <a href="/wiki/Swift_(programming_language)" title="Swift (programming language)">Swift</a>, a null pointer is called <code>nil</code>. In <a href="/wiki/Eiffel_(programming_language)" title="Eiffel (programming language)">Eiffel</a>, it is called a <code>void</code> reference. </p> <div class="mw-heading mw-heading2"><h2 id="Null_dereferencing">Null dereferencing</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=4" title="Edit section: Null dereferencing"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Because a null pointer does not point to a meaningful object, an attempt to <a href="/wiki/Dereference" class="mw-redirect" title="Dereference">dereference</a> (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. <a href="/wiki/MITRE" class="mw-redirect" title="MITRE">MITRE</a> lists the null pointer error as one of the most commonly exploited software weaknesses.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> </p> <ul><li>In <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a>, dereferencing a null pointer is <a href="/wiki/Undefined_behavior" title="Undefined behavior">undefined behavior</a>.<sup id="cite_ref-ub_7-1" class="reference"><a href="#cite_note-ub-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Many implementations cause such code to result in the program being halted with an <a href="/wiki/Access_violation" class="mw-redirect" title="Access violation">access violation</a>, 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.</li> <li>In <a href="/wiki/Delphi_(programming_language)" class="mw-redirect" title="Delphi (programming language)">Delphi</a> and many other Pascal implementations, the constant <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code> 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 <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">EAccessViolation</code> exception instance if the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">System.SysUtils</code> unit is linked in the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">uses</code> clause.</li> <li>In <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a>, access to a null reference (<code>null</code>) causes a <code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/lang/NullPointerException.html">NullPointerException</a></code> (NPE), which can be caught by error handling code, but the preferred practice is to ensure that such exceptions never occur.</li> <li>In <a href="/wiki/Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a>, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code> is a <a href="/wiki/First_class_object" class="mw-redirect" title="First class object">first class object</a>. By convention, <code>(first nil)</code> is <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code>, as is <code>(rest nil)</code>. So dereferencing <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code> in these contexts will not cause an error, but poorly written code can get into an infinite loop.</li> <li>In <a href="/wiki/.Net_(programming_language)" class="mw-redirect" title=".Net (programming language)">.NET</a> and <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, access to null reference (<code>null</code>) causes a <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">NullReferenceException</code> to be thrown. Although catching these is generally considered bad practice, this exception type can be caught and handled by the program.</li> <li>In <a href="/wiki/Objective-C" title="Objective-C">Objective-C</a>, messages may be sent to a <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code> 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 <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">nil</code> or <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">0</code>, depending on the type.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup></li> <li>Before the introduction of <a href="/wiki/Supervisor_Mode_Access_Prevention" title="Supervisor Mode Access Prevention">Supervisor Mode Access Prevention</a> (SMAP), a null pointer dereference bug could be exploited by mapping <a href="/wiki/Zero_page" title="Zero page">page zero</a> into the attacker's <a href="/wiki/Address_space" title="Address space">address space</a> and hence causing the null pointer to point to that region. This could lead to <a href="/wiki/Arbitrary_code_execution" title="Arbitrary code execution">code execution</a> in some cases.<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup></li></ul> <div class="mw-heading mw-heading2"><h2 id="Mitigation">Mitigation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=5" title="Edit section: Mitigation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>While we could have languages with no nulls, most do have the possibility of nulls so there are techniques to avoid or aid debugging null pointer dereferences.<sup id="cite_ref-BondNethercote2007_13-0" class="reference"><a href="#cite_note-BondNethercote2007-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> Bond et al.<sup id="cite_ref-BondNethercote2007_13-1" class="reference"><a href="#cite_note-BondNethercote2007-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> suggest modifying the Java Virtual Machine (JVM) to keep track of null propagation. </p><p>We have three levels of handling null references, in order of effectiveness: </p> <ol><li>languages with no null;</li> <li>languages that can statically analyse code to avoid the possibility of null dereference at run time;</li> <li>if null dereference can occur at runtime, tools that aid debugging.</li></ol> <p>Pure <a href="/wiki/Functional_languages" class="mw-redirect" title="Functional languages">functional languages</a> 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 <a href="/wiki/Interpreter_(computing)" title="Interpreter (computing)">interpreted</a> or virtual-machine languages generally does not suffer the problem of null pointer dereferencing.<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Accuracy_dispute#Disputed_statement" title="Wikipedia:Accuracy dispute"><span title="The material near this tag is possibly inaccurate or nonfactual. (September 2024)">dubious</span></a> – <a href="/wiki/Talk:Null_pointer#VM_claim" title="Talk:Null pointer">discuss</a></i>]</sup> </p><p>Where a language does provide or utilise pointers which could become void, it is possible to avoid runtime null dereferences by providing <a href="/wiki/Void_safety" title="Void safety">compilation-time checking</a> via <a href="/wiki/Static_analysis" title="Static analysis">static analysis</a> or other techniques, with syntactic assistance from language features such as those seen in the <a href="/wiki/Eiffel_programming_language" class="mw-redirect" title="Eiffel programming language">Eiffel programming language</a> with Void safety<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> to avoid null derefences, <a href="/wiki/D_programming_language" class="mw-redirect" title="D programming language">D</a>,<sup id="cite_ref-SafeD_15-0" class="reference"><a href="#cite_note-SafeD-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Rust_programming_language" class="mw-redirect" title="Rust programming language">Rust</a>.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p><p>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. </p><p>The last resort of level 3 is when a null reference occurs at runtime, debugging aids can help. </p> <div class="mw-heading mw-heading2"><h2 id="Alternatives_to_null_pointers">Alternatives to null pointers</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=6" title="Edit section: Alternatives to null pointers"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>As a rule of thumb, for each type of struct or class, define some objects representing some state of the business logic replacing the undefined behaviour on null. For example, "future" to indicate a field inside a structure that will not be available right now (but for which we know in advance that in the future it will be defined), "not applicable" to indicate a field in a non-normalized structure, "error", "timeout" to indicate that the field could not be initialized (probably stopping normal execution of the full program, thread, request or command). </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=7" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In 2009, <a href="/wiki/Tony_Hoare" title="Tony Hoare">Tony Hoare</a> stated<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> that he invented the null reference in 1965 as part of the <a href="/wiki/ALGOL_W" title="ALGOL W">ALGOL W</a> language. In that 2009 reference Hoare describes his invention as a "billion-dollar mistake": </p> <style data-mw-deduplicate="TemplateStyles:r1244412712">.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 32px}.mw-parser-output .templatequotecite{line-height:1.5em;text-align:left;margin-top:0}@media(min-width:500px){.mw-parser-output .templatequotecite{padding-left:1.6em}}</style><blockquote class="templatequote"><p>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.</p></blockquote> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=8" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Memory_debugger" title="Memory debugger">Memory debugger</a></li> <li><a href="/wiki/Zero_page" title="Zero page">Zero page</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=9" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://cwe.mitre.org/data/definitions/476.html">"CWE-476: NULL Pointer Dereference"</a>. <i>MITRE</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=MITRE&rft.atitle=CWE-476%3A+NULL+Pointer+Dereference&rft_id=https%3A%2F%2Fcwe.mitre.org%2Fdata%2Fdefinitions%2F476.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare/">"Null References: The Billion Dollar Mistake"</a>. <i>InfoQ</i><span class="reference-accessdate">. Retrieved <span class="nowrap">5 September</span> 2024</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=InfoQ&rft.atitle=Null+References%3A+The+Billion+Dollar+Mistake&rft_id=https%3A%2F%2Fwww.infoq.com%2Fpresentations%2FNull-References-The-Billion-Dollar-Mistake-Tony-Hoare%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a href="#c-std">ISO/IEC 9899</a>, clause 6.3.2.3, paragraph 4.</span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><a href="#c-std">ISO/IEC 9899</a>, clause 7.17, paragraph 3: <i>NULL... which expands to an implementation-defined null pointer constant...</i></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><a href="#c-std">ISO/IEC 9899</a>, clause 6.3.2.3, paragraph 3.</span> </li> <li id="cite_note-N3042-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-N3042_6-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://open-std.org/JTC1/SC22/WG14/www/docs/n3042.htm">"WR14-N3042: Introduce the nullptr constant"</a>. <i>open-std.org</i>. 2022-07-22. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20221224043228/https://open-std.org/JTC1/SC22/WG14/www/docs/n3042.htm">Archived</a> from the original on December 24, 2022.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=open-std.org&rft.atitle=WR14-N3042%3A+Introduce+the+nullptr+constant&rft.date=2022-07-22&rft_id=https%3A%2F%2Fopen-std.org%2FJTC1%2FSC22%2FWG14%2Fwww%2Fdocs%2Fn3042.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-ub-7"><span class="mw-cite-backlink">^ <a href="#cite_ref-ub_7-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-ub_7-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#c-std">ISO/IEC 9899</a>, clause 6.5.3.2, paragraph 4, esp. footnote 87.</span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFLattner2011" class="citation web cs1">Lattner, Chris (2011-05-13). <a rel="nofollow" class="external text" href="https://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html">"What Every C Programmer Should Know About Undefined Behavior #1/3"</a>. <i>blog.llvm.org</i>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20230614124121/https://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html">Archived</a> from the original on 2023-06-14<span class="reference-accessdate">. Retrieved <span class="nowrap">2023-06-14</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=blog.llvm.org&rft.atitle=What+Every+C+Programmer+Should+Know+About+Undefined+Behavior+%231%2F3&rft.date=2011-05-13&rft.aulast=Lattner&rft.aufirst=Chris&rft_id=https%3A%2F%2Fblog.llvm.org%2F2011%2F05%2Fwhat-every-c-programmer-should-know.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFStroustrup2001" class="citation book cs1"><a href="/wiki/Bjarne_Stroustrup" title="Bjarne Stroustrup">Stroustrup, Bjarne</a> (March 2001). "Chapter 5: <br />The <code>const</code> qualifier (§5.4) prevents accidental redefinition of <code>NULL</code> and ensures that <code>NULL</code> can be used where a constant is required.". <i><a href="/wiki/The_C%2B%2B_Programming_Language" title="The C++ Programming Language">The C++ Programming Language</a></i> (14th printing of 3rd ed.). United States and Canada: Addison–Wesley. p. <a rel="nofollow" class="external text" href="https://archive.org/details/cprogramminglang00stro_0/page/88">88</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-201-88954-4" title="Special:BookSources/0-201-88954-4"><bdi>0-201-88954-4</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Chapter+5%3A+%3Cbr+%2F%3EThe+%3Ccode%3Econst%3C%2Fcode%3E+qualifier+%28%C2%A75.4%29+prevents+accidental+redefinition+of+%3Ccode%3ENULL%3C%2Fcode%3E+and+ensures+that+%3Ccode%3ENULL%3C%2Fcode%3E+can+be+used+where+a+constant+is+required.&rft.btitle=The+C%2B%2B+Programming+Language&rft.place=United+States+and+Canada&rft.pages=88&rft.edition=14th+printing+of+3rd&rft.pub=Addison%E2%80%93Wesley&rft.date=2001-03&rft.isbn=0-201-88954-4&rft.aulast=Stroustrup&rft.aufirst=Bjarne&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://cwe.mitre.org/data/definitions/476.html">"CWE-476: NULL Pointer Dereference"</a>. <i>MITRE</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=MITRE&rft.atitle=CWE-476%3A+NULL+Pointer+Dereference&rft_id=https%3A%2F%2Fcwe.mitre.org%2Fdata%2Fdefinitions%2F476.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><i>The Objective-C 2.0 Programming Language</i>, <a rel="nofollow" class="external text" href="https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW7">section "Sending Messages to nil"</a>.</span> </li> <li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://project-zero.issues.chromium.org/issues/42452311">"OS X exploitable kernel NULL pointer dereference in AppleGraphicsDeviceControl"</a></span> </li> <li id="cite_note-BondNethercote2007-13"><span class="mw-cite-backlink">^ <a href="#cite_ref-BondNethercote2007_13-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-BondNethercote2007_13-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBondNethercoteKentGuyer2007" class="citation book cs1">Bond, Michael D.; Nethercote, Nicholas; Kent, Stephen W.; Guyer, Samuel Z.; McKinley, Kathryn S. (2007). "Tracking bad apples". <i>Proceedings of the 22nd annual ACM SIGPLAN conference on Object oriented programming systems and applications - OOPSLA '07</i>. p. 405. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F1297027.1297057">10.1145/1297027.1297057</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/9781595937865" title="Special:BookSources/9781595937865"><bdi>9781595937865</bdi></a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:2832749">2832749</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Tracking+bad+apples&rft.btitle=Proceedings+of+the+22nd+annual+ACM+SIGPLAN+conference+on+Object+oriented+programming+systems+and+applications+-+OOPSLA+%2707&rft.pages=405&rft.date=2007&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A2832749%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1145%2F1297027.1297057&rft.isbn=9781595937865&rft.aulast=Bond&rft.aufirst=Michael+D.&rft.au=Nethercote%2C+Nicholas&rft.au=Kent%2C+Stephen+W.&rft.au=Guyer%2C+Samuel+Z.&rft.au=McKinley%2C+Kathryn+S.&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.eiffel.org/doc/eiffel/Void-safety-_Background%2C_definition%2C_and_tools">"Void-safety: Background, definition, and tools"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">2021-11-24</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Void-safety%3A+Background%2C+definition%2C+and+tools&rft_id=https%3A%2F%2Fwww.eiffel.org%2Fdoc%2Feiffel%2FVoid-safety-_Background%252C_definition%252C_and_tools&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-SafeD-15"><span class="mw-cite-backlink"><b><a href="#cite_ref-SafeD_15-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBartosz_Milewski" class="citation web cs1">Bartosz Milewski. <a rel="nofollow" class="external text" href="http://dlang.org/safed.html">"SafeD – D Programming Language"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">17 July</span> 2014</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=SafeD+%E2%80%93+D+Programming+Language&rft.au=Bartosz+Milewski&rft_id=http%3A%2F%2Fdlang.org%2Fsafed.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-16"><span class="mw-cite-backlink"><b><a href="#cite_ref-16">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://hacks.mozilla.org/2019/01/fearless-security-memory-safety/">"Fearless Security: Memory Safety"</a>. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20201108003116/https://hacks.mozilla.org/2019/01/fearless-security-memory-safety/">Archived</a> from the original on 8 November 2020<span class="reference-accessdate">. Retrieved <span class="nowrap">4 November</span> 2020</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Fearless+Security%3A+Memory+Safety&rft_id=https%3A%2F%2Fhacks.mozilla.org%2F2019%2F01%2Ffearless-security-memory-safety%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-17"><span class="mw-cite-backlink"><b><a href="#cite_ref-17">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFTony_Hoare2009" class="citation web cs1"><a href="/wiki/Tony_Hoare" title="Tony Hoare">Tony Hoare</a> (2009-08-25). <a rel="nofollow" class="external text" href="http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare">"Null References: The Billion Dollar Mistake"</a>. InfoQ.com.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Null+References%3A+The+Billion+Dollar+Mistake&rft.pub=InfoQ.com&rft.date=2009-08-25&rft.au=Tony+Hoare&rft_id=http%3A%2F%2Fwww.infoq.com%2Fpresentations%2FNull-References-The-Billion-Dollar-Mistake-Tony-Hoare&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> <li id="cite_note-18"><span class="mw-cite-backlink"><b><a href="#cite_ref-18">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFTony_Hoare2009" class="citation web cs1"><a href="/wiki/Tony_Hoare" title="Tony Hoare">Tony Hoare</a> (2009-08-25). <a rel="nofollow" class="external text" href="https://qconlondon.com/london-2009/qconlondon.com/london-2009/presentation/Null%2BReferences_%2BThe%2BBillion%2BDollar%2BMistake.html">"Presentation: "Null References: The Billion Dollar Mistake"<span class="cs1-kern-right"></span>"</a>. InfoQ.com.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Presentation%3A+%22Null+References%3A+The+Billion+Dollar+Mistake%22&rft.pub=InfoQ.com&rft.date=2009-08-25&rft.au=Tony+Hoare&rft_id=https%3A%2F%2Fqconlondon.com%2Flondon-2009%2Fqconlondon.com%2Flondon-2009%2Fpresentation%2FNull%252BReferences_%252BThe%252BBillion%252BDollar%252BMistake.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Null_pointer&action=edit&section=10" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239549316">.mw-parser-output .refbegin{margin-bottom:0.5em}.mw-parser-output .refbegin-hanging-indents>ul{margin-left:0}.mw-parser-output .refbegin-hanging-indents>ul>li{margin-left:0;padding-left:3.2em;text-indent:-3.2em}.mw-parser-output .refbegin-hanging-indents ul,.mw-parser-output .refbegin-hanging-indents ul li{list-style:none}@media(max-width:720px){.mw-parser-output .refbegin-hanging-indents>ul>li{padding-left:1.6em;text-indent:-1.6em}}.mw-parser-output .refbegin-columns{margin-top:0.3em}.mw-parser-output .refbegin-columns ul{margin-top:0}.mw-parser-output .refbegin-columns li{page-break-inside:avoid;break-inside:avoid-column}@media screen{.mw-parser-output .refbegin{font-size:90%}}</style><div class="refbegin" style=""> <ul><li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="c-std" class="citation book cs1">Joint Technical Committee ISO/IEC JTC 1, Subcommittee SC 22, Working Group WG 14 (2007-09-08). <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf"><i>International Standard ISO/IEC 9899</i></a> <span class="cs1-format">(PDF)</span> (Committee Draft).</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=International+Standard+ISO%2FIEC+9899&rft.date=2007-09-08&rft.au=Joint+Technical+Committee+ISO%2FIEC+JTC+1%2C+Subcommittee+SC+22%2C+Working+Group+WG+14&rft_id=http%3A%2F%2Fwww.open-std.org%2Fjtc1%2Fsc22%2FWG14%2Fwww%2Fdocs%2Fn1256.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ANull+pointer" class="Z3988"></span><span class="cs1-maint citation-comment"><code class="cs1-code">{{<a href="/wiki/Template:Cite_book" title="Template:Cite book">cite book</a>}}</code>: CS1 maint: multiple names: authors list (<a href="/wiki/Category:CS1_maint:_multiple_names:_authors_list" title="Category:CS1 maint: multiple names: authors list">link</a>) CS1 maint: numeric names: authors list (<a 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hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Null_character" title="Null character">Null character</a></li> <li><a href="/wiki/Null_device" title="Null device">Null device</a></li> <li><a href="/wiki/Null_function" title="Null function">Null function</a></li> <li><a href="/wiki/Null_modem" title="Null modem">Null modem</a></li> <li><a href="/wiki/Null_object_pattern" title="Null object pattern">Null object pattern</a></li> <li><a class="mw-selflink selflink">Null pointer</a></li> <li><a href="/wiki/Null_(SQL)" title="Null (SQL)">Null in SQL</a></li> <li><a href="/wiki/Empty_string" title="Empty string">Null string</a></li> <li><a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div> <ul><li><i>See also:</i> <a href="/wiki/Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li> 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