C/C++ Null Pointers

C/C++ Null PointersSection 1. Null Pointers

1.   What is this infamous null pointer, anyway?

A:   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.

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

A:   A constant 0 in a pointer context is converted into a null pointer
at compile time.  A “pointer context” is an initialization,
assignment, or comparison with one side a variable or expression of
pointer type, and (in ANSI standard C) a function argument which
has a prototype in scope declaring a certain parameter as being of
pointer type.  In other contexts (function arguments without
prototypes, or in the variable part of variadic function calls) a
constant 0 with an appropriate explicit cast is required.

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

A:   NULL is simply a preprocessor macro, #defined as 0 (or (void *)0),
which is used (as a stylistic convention, in preference to
unadorned 0’s) to generate null pointers,

4.   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 any other machine: as 0 (or (void *)0).  (The compiler
makes the translation, upon seeing a 0, not the preprocessor.)

5.   If NULL were defined as “(char *)0,” wouldn’t that make function
calls which pass an uncast NULL work?

A:   Not in general.  The problem is that there are machines which use
different internal representations for pointers to different types
of data.  A cast is still required to tell the compiler which kind
of null pointer is required, since it may be different from
(char *)0.

6.   I use the preprocessor macro “#define Nullptr(type) (type *)0” to
help me build null pointers of the correct type.

A:   This trick, though valid, does not buy much.

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?

A:   The construction “if(p)” works, regardless of the internal
representation of null pointers, because the compiler essentially
rewrites it as “if(p != 0)” and goes on to convert 0 into the
correct null pointer.

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

A:   Either; the distinction is entirely stylistic.

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

A:   No.  NULL is, and will always be, 0.

10.  I’m confused.  NULL is guaranteed to be 0, but the null pointer is

A:   A “null pointer” is a language concept whose particular internal
value does not matter.  A null pointer is requested in source code
with the character “0”.  “NULL” is a preprocessor macro, which is
always #defined as 0 (or (void *)0).

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

A:   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.

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

A:   A simple rule is, “Always use `0′ or `NULL’ for null pointers, and
always cast them when they are used as arguments in function

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

A:   What would such a requirement really accomplish?

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

A:   Machines manufactured by Prime and by Honeywell-Bull, as well as
Symbolics Lisp Machines, have done so.

Section 2. Arrays and Pointers

15.  I had the definition char x[6] in one source file, and in another I
declared extern char *x.  Why didn’t it work?

A:   The declaration extern char *x simply does not match the actual
definition.   Use extern char x[].

16.  But I heard that char x[] was identical to char *x.

A:   Not at all.  Arrays are not pointers.

17.  You mean that a reference like x[3] generates different code
depending on whether x is an array or a pointer?

A:   Precisely.

18.  So what is meant by the “equivalence of pointers and arrays” in C?

A:   An lvalue of type array-of-T which appears in an expression decays
into a pointer to its first element; the type of the resultant
pointer is pointer-to-T.

19.  Why are array and pointer declarations interchangeable as function
formal parameters?

A:   Since functions can never receive arrays as parameters, any
parameter declarations which “look like” arrays are treated by the
compiler as if they were pointers.

20.  Someone explained to me that arrays were really just constant

A:   An array name is “constant” in that it cannot be assigned to, but
an array is _not_ a pointer.

21.  I came across some “joke” code containing the “expression”
5[“abcdef”] .  How can this be legal C?

A:   Yes, array subscripting is commutative in C.  The array
subscripting operation a[e] is defined as being equivalent to

22.  My compiler complained when I passed a two-dimensional array to a
routine expecting a pointer to a pointer.

A:   The rule by which arrays decay into pointers is not applied
recursively.  An array of arrays (i.e. a two-dimensional array in
C) decays into a pointer to an array, not a pointer to a pointer.

23.  How do I declare a pointer to an array?

A:   Usually, you don’t want to.  Consider using a pointer to one of the
array’s elements instead.

24.  How can I dynamically allocate a multidimensional array?

A:   It is usually best to allocate an array of pointers, and then
initialize each pointer to a dynamically-allocated “row.”  See the
full list for code samples.

25.  I have a char * pointer that happens to point to some ints, and I
want to step it over them.  Why doesn’t “((int *)p)++;” work?

A:   In C, a cast operator is a conversion operator, and by definition
it yields an rvalue, which cannot be assigned to, or incremented
with ++.

Section 3. Order of Evaluation

26.  Under my compiler, the code “int i = 7; printf(“%dn”, i++ * i++);”
prints 49.  Regardless of the order of evaluation, shouldn’t it
print 56?

A:   The operations implied by the postincrement and postdecrement
operators ++ and — are performed at some time after the operand’s
former values are yielded and before the end of the expression, but
not necessarily immediately after, or before other parts of the
expression are evaluated.

27.  But what about the &&, ||, and comma operators?

A:   There is a special exception for those operators, (as well as ?: );
left-to-right evaluation is guaranteed.

Section 4. ANSI C

28.  What is the “ANSI C Standard?”

A:   In 1983, the American National Standards Institute commissioned a
committee, X3J11, to standardize the C language.  After a long,
arduous process, the committee’s work was finally ratified as an
American National Standard, X3.159-1989, on December 14, 1989, and
published in the spring of 1990.  The Standard has also been
adopted as ISO/IEC 9899:1990.

29.  How can I get a copy of the Standard?

A:   Copies are available from the American National Standards Institute
in New York, or from Global Engineering Documents in Irvine, CA.
See the unabridged list for addresses.

30.  Does anyone have a tool for converting old-style C programs to ANSI
C, or for automatically generating prototypes?

A:   See the full list for details.

31.  What’s the difference between “char const *p” and “char * const p”?

A:   The former is a pointer to a constant character; the latter is a
constant pointer to a character.

32.  My ANSI compiler complains about a mismatch when it sees

extern int func(float);

int func(x)
float x;

A:   You have mixed the new-style prototype declaration
“extern int func(float);” with the old-style definition
“int func(x) float x;”.  The problem can be fixed by using either
new-style (prototype) or old-style syntax consistently.

33.  I’m getting strange syntax errors inside code which I’ve #ifdeffed

A:   Under ANSI C, #ifdeffed-out text must still consist of “valid
preprocessing tokens.”  This means that there must be no
unterminated comments or quotes (i.e. no single apostrophes), and
no newlines inside quotes.

34.  Can I declare main as void, to shut off these annoying “main
returns no value” messages?

A:   No.

35.  Why does the ANSI Standard not guarantee more than six monocase
characters of external identifier significance?

A:   The problem is older linkers which cannot be forced (by mere words
in a Standard) to upgrade.

36.  Whatever happened to noalias?

A:   It was deleted from the final versions of the standard because of
widespread complaint and the near-impossibility of defining it

37.  What are #pragmas and what are they good for?

A:   The #pragma directive provides a single, well-defined “escape
hatch” which can be used for extensions.

Section 5. C Preprocessor

38.  How can I write a generic macro to swap two values?

A:   There is no good answer to this question.  The best all-around
solution is probably to forget about using a macro.

39.  I have some old code that tries to construct identifiers with a
macro like “#define Paste(a, b) a/**/b”, but it doesn’t work any

A:   Try the ANSI token-pasting operator ##.

40.  What’s the best way to write a multi-statement cpp macro?

A:   #define Func() do {stmt1; stmt2; … } while(0)   /* (no trailing ; ) */

41.  How can I write a cpp macro which takes a variable number of

A:   One popular trick is to define the macro with a single argument,
and call it with a double set of parentheses, which appear to the
preprocessor to indicate a single argument:

#define DEBUG(args) {printf(“DEBUG: “); printf args;}

if(n != 0) DEBUG((“n is %dn”, n));

Section 6. Variable-Length Argument Lists

42.  How can I write a function that takes a variable number of

A:   Use the <stdarg.h> header.

43.  How can I write a function that takes a format string and a
variable number of arguments, like printf, and passes them to
printf to do most of the work?

A:   Use vprintf, vfprintf, or vsprintf.

44.  How can I discover how many arguments a function was actually
called with?

A:   Any function which takes a variable number of arguments must be
able to determine from the arguments themselves how many of them
there are.

45.  How can I write a function which takes a variable number of
arguments and passes them to some other function (which takes a
variable number of arguments)?

A:   In general, you cannot.

Section 7. Lint

46.  I just typed in this program, and it’s acting strangely.  Can you
see anything wrong with it?

A:   Try running lint first.

47.  How can I shut off the “warning: possible pointer alignment
problem” message lint gives me for each call to malloc?

A:   It may be easier simply to ignore the message, perhaps in an
automated way with grep -v.

48.  Where can I get an ANSI-compatible lint?

A:   See the unabridged list for two commercial products.

Section 8. Memory Allocation

49.  Why doesn’t the code “char *answer; gets(answer);” work?

A:   The pointer variable “answer” has not been set to point to any
valid storage.  The simplest way to correct this fragment is to use
a local array, instead of a pointer.

50.  I can’t get strcat to work.  I tried “char *s1 = “Hello, “,
*s2 = “world!”, *s3 = strcat(s1, s2);” but I got strange results.

A:   Again, the problem is that space for the concatenated result is not
properly allocated.

51.  But the man page for strcat says that it takes two char *’s as
arguments.  How am I supposed to know to allocate things?

A:   In general, when using pointers you _always_ have to consider
memory allocation, at least to make sure that the compiler is doing
it for you.

52.  You can’t use dynamically-allocated memory after you free it, can

A:   No.  Some early man pages implied otherwise, but the claim is no
longer valid.

53.  How does free() know how many bytes to free?

A:   The malloc/free package remembers the size of each block it
allocates and returns.

54.  Is it legal to pass a null pointer as the first argument to

A:   ANSI C sanctions this usage, but several earlier implementations do
not support it.

55.  Is it safe to use calloc’s zero-fill guarantee for pointer and
floating-point values?

A:   No.

56.  What is alloca and why is its use discouraged?

A:   alloca allocates memory which is automatically freed when the
function which called alloca returns.  alloca cannot be written
portably, is difficult to implement on machines without a stack,
and fails under certain conditions if implemented simply.

Section 9. Structures

57.  I heard that structures could be assigned to variables and passed
to and from functions, but K&R I says not.

A:   These operations are supported by all modern compilers.

58.  How does struct passing and returning work?

A:   If you really need to know, see the unabridged list.

59.  I have a program which works correctly, but dumps core after it
finishes.  Why?

A:   Check to see if a structure type declaration just before main is
missing its trailing semicolon, causing the compiler to believe
that main returns a struct.  See also question 101.

60.  Why can’t you compare structs?

A:   There is no reasonable way for a compiler to implement struct
comparison which is consistent with C’s low-level flavor.

61.  I came across some code that declared a structure with the last
member an array of one element, and then did some tricky allocation
to make the array act like it had several elements.  Is this legal
and/or portable?

A:   The ANSI C standard allows it, but only implicitly.

62.  How can I determine the byte offset of a field within a structure?

A:   ANSI C defines the offsetof macro, which should be used if

63.  How can I access structure fields by name at run time?

A:   Build a table of names and offsets, using the offsetof() macro.

Section 10. Declarations

64.  How do you decide which integer type to use?

A:   If you might need large values, use long.  Otherwise, if space is
very important, use short.  Otherwise, use int.

65.  I can’t seem to define a linked list node which contains a pointer
to itself.

A:   Structs in C can certainly contain pointers to themselves; the
discussion and example in section 6.5 of K&R make this clear.
Problems arise if an attempt is made to define (and use) a typedef
in the midst of such a declaration; avoid this.

66.  How do I declare an array of pointers to functions returning
pointers to functions returning pointers to characters?

A:   char *(*(*a[5])())();
Using a chain of typedefs, or the cdecl program, makes these
declarations easier.

67.  So where can I get cdecl?

A:   Several public-domain versions are available.  See the full list
for details.

68.  How do I initialize a pointer to a function?

A:   Use something like “extern int func(); int (*fp)() = func;” .

69.  I’ve seen different methods used for calling through pointers to

A:   The extra parentheses and explicit * are now officially optional,
although some older implementations require them.

Section 11. Boolean Expressions and Variables

70.  What is the right type to use for boolean values in C?  Why isn’t
it a standard type?  Should #defines or enums be used for the true
and false values?

A:   C does not provide a standard boolean type, because picking one
involves a space/time tradeoff which is best decided by the
programmer.  The choice between #defines and enums is arbitrary and
not terribly interesting.

71.  Isn’t #defining TRUE to be 1 dangerous, since any nonzero value is
considered “true” in C?  What if a built-in boolean or relational
operator “returns” something other than 1?

A:   It is true (sic) that any nonzero value is considered true in C,
but this applies only “on input”, i.e. where a boolean value is
expected.  When a boolean value is generated by a built-in
operator, it is guaranteed to be 1 or 0.  (This is _not_ true for
some library routines such as isalpha.)

72.  What is the difference between an enum and a series of preprocessor

A:   At the present time, there is little difference.  The ANSI standard
states that enumerations are compatible with integral types.

Section 12. Operating System Dependencies

73.  How can I read a single character from the keyboard without waiting
for a newline?

A:   Contrary to popular belief and many people’s wishes, this is not a
C-related question.  How to do so is a function of the operating
system in use.

74.  How can I find out if there are characters available for reading
(and if so, how many)?  Alternatively, how can I do a read that
will not block if there are no characters available?

A:   These, too, are entirely operating-system-specific.

75.  How can my program discover the complete pathname to the executable
file from which it was invoked?

A:   argv[0] may contain all or part of the pathname.  You may be able
to duplicate the command language interpreter’s search path logic
to locate the executable.

76.  How can a process change an environment variable in its caller?

A:   In general, it cannot.

77.  How can a file be shortened in-place without completely clearing or
rewriting it?

A:   There are various ways to do this, but there is no truly portable

Section 13. Stdio

78.  Why does errno contain ENOTTY after a call to printf?

A:   Don’t worry about it.  It is only meaningful for a program to
inspect the contents of errno after an error has occurred.

79.  My program’s prompts and intermediate output don’t always show up
on the screen, especially when I pipe the output through another

A:   It is best to use an explicit fflush(stdout) whenever output should
definitely be visible.

80.  When I read from the keyboard with scanf(), it seems to hang until
I type one extra line of input.

A:   scanf() was designed for free-format input, which is seldom what
you want when reading from the keyboard.

81.  How can I recover the file name given an open file descriptor?

A:   This problem is, in general, insoluble.  It is best to remember the
names of open files yourself.

Section 14. Style

82.  Is the code “if(!strcmp(s1, s2))” good style?

A:   No.

83.  What’s the best style for code layout in C?

A:   There is no one “best style,” but see the full list for a few

84.  Where can I get the “Indian Hill Style Guide” and other coding

A:   See the unabridged list.

Section 15. Miscellaneous

85.  What can I safely assume about the initial values of variables
which are not explicitly initialized?

A:   Variables with “static” duration start out as 0, as if the
programmer had initialized them.  Variables with “automatic”
duration, and dynamically-allocated memory, start out containing
garbage (with the exception of calloc).

86.  I’m trying to sort an array of strings with qsort, using strcmp as
the comparison function, but it’s not working.

A:   You’ll have to write a “helper” comparison function which takes two
generic pointer arguments, casts them to char **, and dereferences
them, yielding char *’s which can be usefully compared.

87.  Now I’m trying to sort an array of structures with qsort.  My
comparison routine takes pointers to structures, but the compiler
complains it’s of the wrong type for qsort.  How can I cast the
function pointer to shut off the warning?

A:   The casts must be in the comparison function, which must be
declared as accepting “generic pointers” (void * or char *).

88.  Can someone tell me how to write itoa?

A:   Just use sprintf.

89.  How can I convert a struct tm or a string into a time_t?

A:   The ANSI mktime routine converts a struct tm to a time_t.  No
standard routine exists to parse strings.

90.  How can I write data files which can be read on other machines with
different data formats?

A:   The best solution is to use text files.

91.  I seem to be missing the system header file <sgtty.h>.  Can someone
send me a copy?

A:   You cannot just pick up a copy of someone else’s header file and
expect it to work, since the definitions within header files are
frequently system-dependent.  Contact your vendor.

92.  How can I call Fortran (BASIC, Pascal, ADA, lisp) functions from C?

A:   The answer is entirely dependent on the machine and the specific
calling sequences of the various compilers in use.

93.  Does anyone know of a program for converting Pascal (Fortran, lisp,
“Old” C, …) to C?

A:   Several public-domain programs are available, namely ptoc, p2c, and
f2c.  See the full list for details.

94.  Where can I get copies of all these public-domain programs?

A:   See the regular postings in the comp.sources.unix and
comp.sources.misc newsgroups for information.

95.  When will the next Obfuscated C Contest be held?  How can I get a
copy of the previous winning entries?

A:   See the full list, or send email to judges@toad.com .

96.  Why don’t C comments nest?  Are they legal inside quoted strings?

A:   Nested comments would cause more harm than good.  The character
sequences /* and */ are not special within double-quoted strings.

97.  How can I make this code more efficient?

A:   Efficiency is not important nearly as often as people tend to think
it is.  Most of the time, by simply paying attention to good
algorithm choices, perfectly acceptable results can be achieved.

98.  Are pointers really faster than arrays?  Do function calls really
slow things down?

A:   Precise answers to these and many similar questions depend of
course on the processor and compiler in use.

99.  My floating-point calculations are acting strangely and giving me
different answers on different machines.

A:   See the full list for a brief explanation, or any good programming
book for a better one.

100. I’m having trouble with a Turbo C program which crashes and says
something like “floating point not loaded.”

A:   Some compilers for small machines, including Turbo C, attempt to
leave out floating point support if it looks like it will not be
needed.  The programmer must occasionally insert a dummy explicit
floating-point call to force loading of floating-point support.

101. This program crashes before it even runs!

A:   Look for very large, local arrays.
(See also question 59.)

102. Does anyone have a C compiler test suite I can use?

A:   Plum Hall, among others, sells one.

103. Where can I get a YACC grammar for C?

A:   See the ANSI Standard, or the unabridged list.

104. How do you pronounce “char”?

A:   Like the English words “char,” “care,” or “car” (your choice).

105. Where can I get extra copies of this list?

A:   For now, just pull it off the net; the unabridged version is
normally posted on the first of each month, with an Expiration:
line which should keep it around all month.

Steve Summit

This article is Copyright 1988, 1990, 1991 by Steve Summit.
It may be freely redistributed so long as the author’s name, and this
notice, are retained.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.