C Interview Questions - Part 11

5.1 What is this infamous null pointer, anyway?

5.2 How do I get a null pointer in my programs?

5.3 Is the abbreviated pointer comparison ``if(p)'' to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?

5.4 What is NULL and how is it defined?

5.5 How should NULL be defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?

5.6 If NULL were defined as follows:

#define NULL ((char *)0)

wouldn't that make function calls which pass an uncast NULL work?

5.7 My vendor provides header files that #define NULL as 0L. Why?

5.8 Is NULL valid for pointers to functions?

5.9 If NULL and 0 are equivalent as null pointer constants, which should I use?

5.10 But wouldn't it be better to use NULL (rather than 0), in case the value of NULL changes, perhaps on a machine with nonzero internal null pointers?

5.11 I once used a compiler that wouldn't work unless NULL was used.

5.12 I use the preprocessor macro

#define Nullptr(type) (type *)0

to help me build null pointers of the correct type.

5.13 This is strange. NULL is guaranteed to be 0, but the null pointer is not?

5.14 Why is there so much confusion surrounding null pointers? Why do these questions come up so often?

5.15 I'm confused. I just can't understand all this null pointer stuff.

5.16 Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes?

5.17 Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?

5.18 Is a run-time integral value of 0, cast to a pointer, guaranteed to be a null pointer?

5.19 How can I access an interrupt vector located at the machine's location 0? If I set a pointer to 0, the compiler might translate it to some nonzero internal null pointer value.

5.20 What does a run-time ``null pointer assignment'' error mean? How can I track it down?