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• Jan 31st, 2011

 

From the C language standard, section 6.3.2.3, paragraph 3:
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An integer constant expression with the value 0, or such an expression cast to 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.
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55) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.17.
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IOW, a null pointer constant such as the NULL macro will always evaluate to 0; a null pointer value may or may not evaluate to 0.  It's up to the compiler to map the two, so that expressions like "if (!ptr)" and "if (ptr == NULL)" work.