| > the null pointer is guaranteed to equal zero in integer comparisons (and therefore false in boolean expressions). Note that this does not mean that the null pointer is 0. I'll quote the relevant sentence from section 5.2 of http://www.faqs.org/faqs/C-faq/faq/: "According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time." Later in that section, there's an example that makes the difference between "null pointers are 0" and "constant 0s in a pointer context are converted into a null pointer". The intro to that example is "However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. To generate a null pointer in a function call context, an explicit cast may be required, to force the 0 to be recognized as a pointer." The example itself is: "execl("/bin/sh", "sh", "-c", "date", (char )0); If the (char ) cast on the last argument were omitted, the compiler would not know to pass a null pointer, and would pass an integer 0 instead." See also 5.3 which starts with "Is the abbreviated pointer comparison "if(p)" to test for non- null pointers valid? What if the internal representation for null pointers is nonzero?" |