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copied!<p>In the C99 standard, §7.17.3 states that <code>NULL</code> “expands to an implementation defined <em>null pointer constant</em>”. Meanwhile §6.3.2.3.3 defines <em>null pointer constant</em> as “an integer constant expression with the value 0, or such an expression cast to type <code>void *</code>”. As there is no other definition for a null pointer constant, a conforming definition of <code>NULL</code> must expand to an integer constant expression with the value zero (or this cast to <code>void *</code>).</p> <p>Further quoting from the <a href="http://c-faq.com/null/machnon0.html" rel="nofollow noreferrer">C FAQ question 5.5</a> (emphasis added):</p> <blockquote> Section 4.1.5 of the C Standard states that NULL “expands to an implementation-defined null pointer constant,” which means that the implementation gets to choose which form of 0 to use and whether to use a `void *` cast; see questions 5.6 and 5.7. “Implementation-defined” here <strong>does not mean that NULL might be #defined to match some implementation-specific nonzero internal null pointer value</strong>. </blockquote> <p>It makes perfect sense; since the standard requires a zero integer constant in pointer contexts to <em>compile</em> into a null pointer (regardless of whether or not the machine's internal representation of that has a value of zero), the case where <code>NULL</code> is defined as zero must be handled anyhow. The programmer is not required to type <code>NULL</code> to obtain null pointers; it's just a stylistic convention (and may help catch errors e.g. when a <code>NULL</code> defined as <code>(void *)0</code> is used in a non-pointer context).</p> <p>Edit: One source of confusion here seems to be the concise language used by the standard, i.e. it does not explicitly say that there is no <em>other</em> value that might be considered a null pointer constant. However, when the standard says “…is called a null pointer constant”, it means that <em>exactly</em> the given definitions are called null pointer constants. It does not need to explicitly follow every definition by stating what is non-conforming when (by definition) the standard defines what <em>is</em> conforming.</p>

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