C language Null pointers questions asked in technical interviews of reputed companies - 2008
1. What is this infamous null pointer, anyway?
A: 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 "guaranteed to compare unequal to a pointer to any object or function." That is, the address-of operator & will never yield a null pointer, nor will a successful call to malloc(). (malloc() does return 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 or function; an uninitialized pointer might point anywhere.
As mentioned 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 that it can make the distinction if necessary
2. How do I get a null pointer in my programs?
A: 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. 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. For example, the Unix system call execl takes a variable-length, null-pointer-terminated list of character pointer arguments, and is correctly called like this:
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. (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 convert an unadorned 0. 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 probably safest to properly cast all null pointer constants in function calls, to guard against varargs functions or those without prototypes.
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
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?
A: When C requires the Boolean value of an expression, a false value is inferred 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 actually a null pointer constant, and use the correct null pointer value. There is no trickery involved here; compilers do work this way, and generate identical code for both constructs. The internal representation of a null pointer does *not* matter.
The boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to (expr)?0:1 or to ((expr) == 0)
which leads to the conclusion 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
4. What is NULL and how is it #defined?
A: As a matter of style, many programmers prefer not to have unadorned 0's scattered through their programs. Therefore, the preprocessor macro NULL is #defined (by
other headers) with the value 0, possibly cast to (void *). A programmer who wishes to make explicit
the distinction between 0 the integer and 0 the null pointer constant can then use NULL whenever a null pointer is required.
Using NULL is a stylistic convention only; the preprocessor turns NULL back into 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. T
5. How should NULL be defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?
A: The same as on any other machine: as 0 (or some version of 0;).
Whenever a programmer requests a null pointer, either by writing "0" or "NULL", it is the compiler's responsibility to generate whatever bit pattern the machine uses for that null pointer. Therefore, #defining NULL as 0 on a machine for which internal null pointers are nonzero is as valid as on any other: the compiler must always be able to generate the machine's correct null pointers in response to unadorned 0's seen in pointer contexts.
6. If NULL were defined as follows:
#define NULL ((char *)0)
wouldn't that make function calls which pass an un cast NULL work?
A: Not in general. The complication 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 work correctly, but pointer arguments of other types would still be problematical, and legal constructions such as
FILE *fp = NULL;
could fail.
Nevertheless, ANSI C allows the alternate definition
#define NULL ((void *)0)
for NULL. Besides potentially 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 NULL character was really intended;).
7.If NULL and 0 are equivalent as null pointer constants, which should I use?
A: 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 macro, and prefer to use unadorned 0 instead. There is no one right answer. C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. 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. (Furthermore, ANSI allows the definition of
NULL to be ((void *)0), which will not work at all 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.
8. 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?
A: No. (Using NULL may be preferable, but not for this reason.) 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.
9. I use the preprocessor macro
#define Nullptr(type) (type *)0
to help me build null pointers of the correct type.
A: This trick, though popular and superficially attractive, does
not buy much. It is not needed in assignments or comparisons;
(It does not even save keystrokes.)
10. This is strange. NULL is guaranteed to be 0, but the null pointer is not?
A: When the term "null" or "NULL" is casually used, one of several
things may be meant:
1. The conceptual null pointer
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 null pointer constant, which is a constant integer 0 It is often hidden behind...
4. The NULL macro, which is #defined to be 0 Finally, as red herrings, we have...
5. The ASCII null character (NUL), which does have all bits zero, but has no necessary relation to the null pointer except in name; and...
6. The "null string," which is another name for the empty string (""). Using the term "null string" can be confusing in C, because an empty string involves a null ('\0') character, but *not* a null pointer, which brings us full circle...
11.Why is there so much confusion surrounding null pointers? Why do these questions come up so often?
A: C programmers traditionally like to know more than they might 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) may seem to suggest that the value could change some day, 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" is often overlooked.
One good way to wade out of the confusion is to imagine that C used a keyword (perhaps "nil", like Pascal) as a null pointer constant. The compiler could either turn "nil" into the appropriate type of null pointer when it could unambiguously determine that type from the source code, or complain when it could not. Now in fact, in C the keyword for a null pointer constant 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 constant, the code may not work.
12. I'm confused. I just can't understand all this null pointer stuff.
A: Here are two simple rules you can follow:
1. When you want a null pointer constant 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, with the internal representation of null pointers (which you shouldn't need to know), and with the complexities of function prototypes. (Taking those complexities into account, we find that rule 2 is conservative, of course; but it doesn't hurt.)
13. Given all the confusion surrounding null pointers, wouldn't it be easier simply to require them to be represented internally by zeroes?
A: 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 such a 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(); ). 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, the urge to assume an internal zero representation would not even arise.)
14 Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?
A: The Prime 50 series 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. Older, word-addressed Prime machines were also notorious for requiring
larger byte pointers (char *'s) than word pointers (int *'s).
The Eclipse MV series from Data General has three architecturally supported pointer formats (word, byte, and bit pointers), two of which are used by C compilers: byte pointers for char * and void *, and word pointers for everything else.
Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most users (in ring 11) have null pointers of 0xB00000000000. It was common on old CDC ones- complement machines to use an all-one-bits word as a special flag for all kinds of data, including invalid addresses.
The old HP 3000 series uses a different addressing scheme for byte addresses than for word addresses; like several of the machines above it therefore uses different representations for
char * and void * pointers than for other pointers.
The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair
pointer.
Depending on the "memory model" in use, 8086-family processors (PC compatibles) may use 16-bit data pointers and 32-bit function pointers, or vice versa.
Some 64-bit Cray machines represent int * in the lower 48 bits of a word; char * additionally uses the upper 16 bits to indicate a byte address within a word.
How can I track it down?
A: This message, which typically occurs with MS-DOS compilers, means that you've written, via a null (perhaps because uninitialized) pointer, to an invalid location (probably offset 0 in the default data segment).
A debugger may let you set a data watchpoint on location 0. Alternatively, you could write a bit of code to stash away a copy of 20 or so bytes from location 0, and periodically check that the memory at location 0 hasn't changed. See also question 16.8.
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