C Aptitude : NULL POINTERS

Some Questions Regarding Null Pointers

1. What is this infamous null pointer, anyway?

Ans: The language definition states that for each pointer type, there is a special value -- the "null pointer" -- which is distinguishable from all other pointer values and which is not the address of any object. That is the address-of operator & will never yield a null pointer, nor will a successful call to malloc. (malloc returns a null pointer when it fails, and this is a typical use of null pointers: as a "special" pointer value with some other meaning, usually "not allocated" or "not pointing anywhere yet.")

A null pointer is conceptually different from an uninitialized pointer. A null pointer is known not to point to any object; an uninitialized pointer might point anywhere. See also questions 49, 55, and 85.

As mentioned in the definition above, there is a null pointer for each pointer type, and the internal values of null pointers for different types may be different. Although programmers need not know the internal values, the compiler must always be informed which type of null pointer is required, so it can make the distinction if necessary (see below).

References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 5.3 p. 91; ANSI Sec. 3.2.2.3 p. 38.

2. How do I "get" a null pointer in my programs?

Ans: According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal:

char *p = 0;

if(p != 0)

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. For instance, the Unix system call "execl" takes a variable-length, null-pointer-terminated list of character pointer arguments. To generate a null pointer in a function call context, an explicit cast is typically required:

execl("/bin/sh", "sh", "-c", "ls", (char *)0);

If the (char *) cast were omitted, the compiler would not know to pass a null pointer, and would pass an integer 0 instead. (Note that many Unix manuals get this example wrong.)

When function prototypes are in scope, argument passing becomes an "assignment context," and most casts may safely be omitted, since the prototype tells the compiler that a pointer is required, and of which type, enabling it to correctly cast unadorned 0's. Function prototypes cannot provide the types for variable arguments in variable-length argument lists, however, so explicit casts are still required for those arguments. It is safest always to cast null pointer function arguments, to guard against varargs functions or those without prototypes, to allow interim use of non-ANSI compilers, and to demonstrate that you know what you are doing.

Summary:

Unadorned 0 okay: Explicit cast required:

initialization function call,

no prototype in scope

assignment

variable argument in

comparison varargs function call

function call,

prototype in scope,

fixed argument

References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II Sec. A7.10 p. 207, Sec. A7.17 p. 209; H&S Sec. 4.6.3 p. 72; ANSI Sec. 3.2.2.3.

3. What is NULL and how is it #defined?

Ans: As a matter of style, many people prefer not to have unadorned 0's scattered throughout their programs. For this reason, the preprocessor macro NULL is #defined (by or ), with value 0 (or (void *)0, about which more later). A programmer who wishes to make explicit the distinction between 0 the integer and 0 the null pointer can then use NULL whenever a null pointer is required. This is a stylistic convention only; the preprocessor turns NULL back to 0 which is then recognized by the compiler (in pointer contexts) as before. In particular, a cast may still be necessary before NULL (as before 0) in a function call argument.

(The table under question 2 above applies for NULL as well as 0.)

NULL should _only_ be used for pointers; see question 8.

References: K&R I Sec. 5.4 pp. 97-8; K&R II Sec. 5.4 p. 102; H&S Sec. 13.1 p. 283; ANSI Sec. 4.1.5 p. 99, Sec. 3.2.2.3 p. 38, Rationale Sec. 4.1.5 p. 74.

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

Ans: Programmers should never need to know the internal representation(s) of null pointers, because they are normally taken care of by the compiler. If a machine uses a nonzero bit pattern for null pointers, it is the compiler's responsibility to generate it when the programmer requests, by writing "0" or "NULL," a null pointer. Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as valid as on any other, because the compiler must (and can) still generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts.

5. If NULL were defined as follows:

#define NULL (char *)0

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

Ans: Not in general. The problem is that there are machines, which use different internal representations for pointers to different types of data. The suggested #definition would make uncast NULL arguments to functions expecting pointers to characters to work correctly, but pointer arguments to other types would still be problematical, and legal constructions such as

FILE *fp = NULL;

could fail.

Nevertheless, ANSI C allows the alternate

#define NULL (void *)0

definition for NULL. Besides helping incorrect programs to work (but only on machines with homogeneous pointers, thus questionably valid assistance) this definition may catch programs which use NULL incorrectly (e.g. when the ASCII NUL character was really intended).

6. I use the preprocessor macro

#define Nullptr(type) (type *)0

to help me build null pointers of the correct type.

Ans: This trick, though popular in some circles, does not buy much. It is not needed in assignments and comparisons; see question 2. It does not even save keystrokes. Its use suggests to the reader that the author is shaky on the subject of null pointers, and requires the reader to check the #definition of the macro, its invocations, and _all_ other pointer usages much more carefully.

7. Is the abbreviated pointer comparison "if(p)" to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?

Ans: When C requires the boolean value of an expression (in the if, while, for, and do statements, and with the &&, ||,!, and ?: operators), a false value is produced when the expression compares equal to zero, and a true value otherwise. That is, whenever one writes

if(expr)

where "expr" is any expression at all, the compiler essentially acts as if it had been written as

if(expr != 0)

Substituting the trivial pointer expression "p" for "expr," we have

if(p) is equivalent to if(p != 0)

and this is a comparison context, so the compiler can tell that the (implicit) 0 is a null pointer, and use the correct value. There is no trickery involved here; compilers do work this way, and generate identical code for both statements. The internal representation of a pointer does _not_ matter.

The Boolean negation operator, !, can be described as follows:

!expr is essentially equivalent to expr?0:1

It is left as an exercise for the reader to show that

if(!p) is equivalent to if(p == 0)

"Abbreviations" such as if(p), though perfectly legal, are considered by some to be bad style.

See also question 71.

References: K&R II Sec. A7.4.7 p. 204; H&S Sec. 5.3 p. 91; ANSI Secs. 3.3.3.3, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, and 3.6.5 .

8. If "NULL" and "0" are equivalent, which should I use?

Ans: Many programmers believe that "NULL" should be used in all pointer contexts, as a reminder that the value is to be thought of as a pointer. Others feel that the confusion surrounding "NULL" and "0" is only compounded by hiding "0" behind a #definition, and prefer to use unadorned "0" instead. There is no one right answer.

C programmers must understand that "NULL" and "0" are interchangeable and that an uncast "0" is perfectly acceptable in initialization, assignment, and comparison contexts. Any usage of "NULL" (as opposed to "0") should be considered a gentle reminder that a pointer is involved; programmers should not depend on it (either for their own understanding or the compiler's) for

distinguishing pointer 0's from integer 0's.

NULL should _not_ be used when another kind of 0 is required, even though it might work, because doing so sends the wrong stylistic message. (ANSI allows the #definition of NULL to be (void *)0, which will not work in non-pointer contexts.) In particular, do not use NULL when the ASCII null character (NUL) is desired.

Provide your own definition

#define NUL '\0'

if you must.

Reference: K&R II Sec. 5.4 p. 102.

9. 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 null pointers?

Ans: No. Although symbolic constants are often used in place of numbers because the numbers might change, this is _not_ the reason that NULL is used in place of 0. Once again, the language guarantees that source-code 0's (in pointer contexts) generate null pointers.

NULL is used only as a stylistic convention.

10. I'm confused. NULL is guaranteed to be 0, but the null pointer is not?

Ans: When the term "null" or "NULL" is casually used, one of several things may be meant:

1. The conceptual null pointer, the abstract language concept defined in question 1. It is implemented with...

2. The internal (or run-time) representation of a null pointer, which may or may not be all-bits-0 and which may be different for different pointer types. The actual values should be of concern only to compiler writers. Authors of C programs never see them, since they use...

3. The source code syntax for null pointers, which is the single character "0". It is often hidden behind...

4. The NULL macro, which is #defined to be "0" or "(void *)0". Finally, as a red herring, we have...

5. The ASCII null character (NUL), which does have all bits zero, but has no relation to the null pointer except in name.

This article always uses the phrase "null pointer" (in lower case) for sense 1, the character "0" for sense 3, and the capitalized word "NULL" for sense 4.

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

Ans: C programmers traditionally like to know more than they need to about the underlying machine implementation. The fact that null pointers are represented both in source code, and internally to most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) suggests that the value might change later, or on some weird machine. The construct "if(p == 0)" is easily misread as calling for conversion of p to an integral type, rather than 0 to a pointer type, before the comparison. Finally, the distinction between the several uses of the term "null" (listed above) is often overlooked.

One good way to wade out of the confusion is to imagine that C had a keyword (perhaps "nil", like Pascal) with which null pointers were requested. The compiler could either turn "nil" into the correct type of null pointer, when it could determine the type from the source code (as it does with 0's in reality), or complain when it could not. Now, in fact, in C the keyword for a null pointer is not "nil" but "0", which works almost as well, except that an uncast "0" in a non-pointer context generates an integer zero instead of an error message, and if that uncast 0 was supposed to be a null pointer, the code may not work.

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

Ans: Follow these two simple rules:

1. When you want to refer to a null pointer in source code, use "0" or "NULL".

2. If the usage of "0" or "NULL" is an argument in a function call, cast it to the pointer type expected by the function being called.

The rest of the discussion has to do with other people's misunderstandings, or with the internal representation of null pointers, which you shouldn't need to know. Understand questions 1, 2, and 3 and consider 8 and 11, and you'll do fine.

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

Ans: If for no other reason, doing so would be ill-advised because it would unnecessarily constrain implementations which would otherwise naturally represent null pointers by special, nonzero bit patterns, particularly when those values would trigger automatic hardware traps for invalid accesses.

Besides, what would this requirement really accomplish? Proper understanding of null pointers does not require knowledge of the internal representation, whether zero or nonzero. Assuming that null pointers are internally zero does not make any code easier to write (except for a certain ill-advised usage of calloc; see question 55). Known-zero internal pointers would not obviate casts in function calls, because the _size_ of the pointer might still be different from that of an int. (If "nil" were used to request null pointers rather than "0," as mentioned in question 11, the urge to assume an internal zero representation would not even arise.)

14. Seriously, have any actual machines really used nonzero null pointers?

Ans: "Certain Prime computers use a value different from all-bits-0 to encode the null pointer. Also, some large Honeywell-Bull machines use the bit pattern 06000 to encode the null pointer."

-- Portable C, by H. Rabinowitz and Chaim Schaap, Prentice-Hall, 1990, page 147.

The "certain Prime computers" were the segmented 50 series, which used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to all the extant poorly-written C code which made incorrect assumptions.

The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair (basically a nonexistent handle) as a C null pointer.