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    <p>The first syntax you use:</p> <pre><code> if (!someString) </code></pre> <p>exploits a sort of "ambiguity" of C deriving from the fact that the original standard of C lacked a proper boolean type. Therefore, any integer value equalling 0 was interpreted as "false", and any integer value different from "0" was taken as "true". The meaning of <code>!</code> is therefore defined based on this convention and current versions of the C standard have kept the original definition for compatibility.</p> <p>In your specific case, <code>someString</code> is a pointer, so it is first converted to an integer, then <code>! someString</code> is interpreted as a bool value of <code>true</code> when <code>someString</code> points at the location <code>0x000000</code>, otherwise it evals to "true".</p> <p>This is fine in most conditions (I would say always), but in theory, <code>NULL/nil</code> <a href="http://en.wikipedia.org/wiki/Pointer_(computer_programming)#Null_pointer" rel="nofollow noreferrer">could be different from <code>0x000000</code></a> under certain compilers, so (in very theory) it would be better to use the second syntax, which is more explicit:</p> <pre><code> if (someString == nil) </code></pre> <p>It is anyway more readable and since <code>someString</code> is not an integer (rather a pointer), IMO, better practice in general. </p> <p>EDIT: about the definition of NULL...</p> <p>Whether the C standard defines NULL to be 0 is an interesting topic for me...</p> <p>According to <a href="http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf" rel="nofollow noreferrer">C99 standard</a>, section 7.17, "Common definitions ":</p> <blockquote> <p>NULL [which] expands to an implementation-defined null pointer constant;</p> </blockquote> <p>So, NULL is defined in stddef.h to an <em>implementation-defined null pointer constant</em>... The same document on page 47 states:</p> <blockquote> <p>An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.55) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.</p> </blockquote> <p>So, the null pointer constant (which is <code>(void*)0</code>) can be <em>converted</em> to a null pointer and this is guaranteed to compare unequal to a pointer to any object or function. </p> <p>So, I think that basically it depends on whether the implementation decides that the result of converting a <em>null pointer constant</em> to a <em>null pointer</em> produces a pointer which converted back to an integer gives 0. It is not clear that a <em>null pointer</em> interpreted as an integer equals 0. </p> <p>I would say that the standard really try and enforce the null pointer being 0, but leaves the door open to <a href="https://stackoverflow.com/questions/2597142/when-was-the-null-macro-not-0">systems where the null pointer was not 0</a>.</p>
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    1. COI gave +1, but there is no ambiguity in C on the idea of a boolean value - it simply does not exist.
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    2. CONULL is defined to be `0` or `(void *)0` by the C standard. Even if the underlying bit-pattern used for the null pointer value is different, it does not change the fact that the null pointer constant in C is represented as `0` or `(void *)0`.
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    3. CO@Perception: It does in C99. Check out `_Bool`.
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