A pointer is a variable that contains the address of a variable. Pointers are much used in C, partly because they are sometimes the only way to express a computation, and partly because they usually lead to more compact and efficient code than can be obtained in other ways. Pointers and arrays are closely related; this chapter also explores this relationship and shows how to exploit it.
Pointers have been lumped with the goto statement as a marvelous way to
create impossible-to-understand programs. This is certainly true when they are
used carelessly, and it is easy to create pointers that point somewhere
unexpected. With discipline, however, pointers can also be used to achieve
clarity and simplicity. This is the aspect that we will try to illustrate.
The main change in ANSI C is to make explicit the rules about how pointers can
be manipulated, in effect mandating what good programmers already practice and
good compilers already enforce. In addition, the type void * (pointer to
void) replaces char * as the proper type for a generic pointer.
The unary operator & gives the address of an object, so the statement
p = &c;assigns the address of c to the variable p, and p is said to "point to"
c. The & operator only applies to objects in memory: variables and array
elements. It cannot be applied to expressions, constants, or register
variables.
The unary operator * is the indirection or dereferencing operator; when
applied to a pointer, it accesses the object the pointer points to. Suppose
that x and y are integers and ip is a pointer to int. This artificial
sequence shows how to declare a pointer and how to use & and *:
int x = 1, y = 2, z[10];
int *ip; /* ip is a pointer to int */
ip = &x; /* ip now points to x */
y = *ip; /* y is now 1 */
*ip = 0; /* x is now 0 */
ip = &z;[0]; /* ip now points to z[0] */The declaration of x, y, and z are what we've seen all along. The
declaration of the pointer ip,
int *ip;is intended as a mnemonic; it says that the expression *ip is an int. The
syntax of the declaration for a variable mimics the syntax of expressions in
which the variable might appear. This reasoning applies to function
declarations as well. For example,
double *dp, atof(char *);says that in an expression *dp and atof(s) have values of double, and
that the argument of atof is a pointer to char.
You should also note the implication that a pointer is constrained to point to a particular kind of object: every pointer points to a specific data type. (There is one exception: a "pointer to void" is used to hold any type of pointer but cannot be dereferenced itself. We'll come back to it in Section 5.11.)
If ip points to the integer x, then *ip can occur in any context where
x could, so
*ip = *ip + 10;increments *ip by 10.
The unary operators * and & bind more tightly than arithmetic operators, so
the assignment
y = *ip + 1takes whatever ip points at, adds 1, and assigns the result to y, while
*ip += 1increments what ip points to, as do
++*ipand
(*ip)++The parentheses are necessary in this last example; without them, the
expression would increment ip instead of what it points to, because unary
operators like * and ++ associate right to left.
Finally, since pointers are variables, they can be used without dereferencing.
For example, if iq is another pointer to int,
iq = ipcopies the contents of ip into iq, thus making iq point to whatever ip
pointed to.
Since C passes arguments to functions by value, there is no direct way for the
called function to alter a variable in the calling function. For instance, a
sorting routine might exchange two out-of-order arguments with a function
called swap. It is not enough to write
swap(a, b);where the swap function is defined as
void swap(int x, int y) /* WRONG */
{
int temp;
temp = x;
x = y;
y = temp;
}Because of call by value, swap can't affect the arguments a and b in the
routine that called it. The function above swaps copies of a and b.
The way to obtain the desired effect is for the calling program to pass pointers to the values to be changed:
swap(&a;, &b;);Since the operator & produces the address of a variable, &a is a pointer to
a. In swap itself, the parameters are declared as pointers, and the operands are accessed indirectly through them.
void swap(int *px, int *py) /* interchange *px and *py */
{
int temp;
temp = *px;
*px = *py;
*py = temp;
}Pictorially:
Pointer arguments enable a function to access and change objects in the
function that called it. As an example, consider a function getint that
performs free-format input conversion by breaking a stream of characters into
integer values, one integer per call. getint has to return the value it found
and also signal end of file when there is no more input. These values have to
be passed back by separate paths, for no matter what value is used for EOF,
that could also be the value of an input integer.
One solution is to have getint return the end of file status as its function
value, while using a pointer argument to store the converted integer back in
the calling function. This is the scheme used by scanf as well; see Section 7.4.
The following loop fills an array with integers by calls to getint:
int n, array[SIZE], getint(int *);
for (n = 0; n < SIZE && getint(&array;[n]) != EOF; n++)
;Each call sets array[n] to the next integer found in the input and increments
n. Notice that it is essential to pass the address of array[n] to getint.
Otherwise there is no way for getint to communicate the converted integer
back to the caller.
Our version of getint returns EOF for end of file, zero if the next input is not a number, and a positive value if the input contains a valid number.
#include <ctype.h>
int getch(void);
void ungetch(int);
/* getint: get next integer from input into *pn */
int getint(int *pn)
{
int c, sign;
while (isspace(c = getch())) /* skip white space */
;
if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
ungetch(c); /* it is not a number */
return 0;
}
sign = (c == '-') ? -1 : 1;
if (c == '+' || c == '-')
c = getch();
for (*pn = 0; isdigit(c), c = getch())
*pn = 10 * *pn + (c - '0');
*pn *= sign;
if (c != EOF)
ungetch(c);
return c;
}Throughout getint, *pn is used as an ordinary int variable. We have also
used getch and ungetch (described in Section 4.3) so the one extra
character that must be read can be pushed back onto the input.
Exercise 5-1. As written, getint treats a + or - not followed by a
digit as a valid representation of zero. Fix it to push such a character back
on the input.
Exercise 5-2. Write getfloat, the floating-point analog of getint. What
type does getfloat return as its function value?
In C, there is a strong relationship between pointers and arrays, strong enough that pointers and arrays should be discussed simultaneously. Any operation that can be achieved by array subscripting can also be done with pointers. The pointer version will in general be faster but, at least to the uninitiated, somewhat harder to understand.
The declaration
int a[10];defines an array of size 10, that is, a block of 10 consecutive objects named
a[0], a[1], ...,a[9].
The notation a[i] refers to the i-th element of the array. If pa is a
pointer to an integer, declared as
int *pa;then the assignment
pa = &a;[0];sets pa to point to element zero of a; that is, pa contains the address
of a[0].
Now the assignment
x = *pa;will copy the contents of a[0] into x.
If pa points to a particular element of an array, then by definition pa+1
points to the next element, pa+i points i elements after pa, and pa-i
points i elements before. Thus, if pa points to a[0],
*(pa+1)refers to the contents of a[1], pa+i is the address of a[i], and
*(pa+i) is the contents of a[i].
These remarks are true regardless of the type or size of the variables in the
array a. The meaning of "adding 1 to a pointer," and by extension, all
pointer arithmetic, is that pa+1 points to the next object, and pa+i points
to the i-th object beyond pa.
The correspondence between indexing and pointer arithmetic is very close. By definition, the value of a variable or expression of type array is the address of element zero of the array. Thus after the assignment
pa = &a;[0];pa and a have identical values. Since the name of an array is a synonym for
the location of the initial element, the assignment pa=&a[0] can also be written as
pa = a;Rather more surprising, at first sight, is the fact that a reference to a[i]
can also be written as *(a+i). In evaluating a[i], C converts it to
*(a+i) immediately; the two forms are equivalent. Applying the operator &
to both parts of this equivalence, it follows that &a[i] and a+i are also
identical: a+i is the address of the i-th element beyond a. As the other
side of this coin, if pa is a pointer, expressions might use it with a
subscript; pa[i] is identical to *(pa+i). In short, an array-and-index
expression is equivalent to one written as a pointer and offset.
There is one difference between an array name and a pointer that must be kept
in mind. A pointer is a variable, so pa=a and pa++ are legal. But an array
name is not a variable; constructions like a=pa and a++ are illegal.
When an array name is passed to a function, what is passed is the location of
the initial element. Within the called function, this argument is a local
variable, and so an array name parameter is a pointer, that is, a variable
containing an address. We can use this fact to write another version of
strlen, which computes the length of a string.
/* strlen: return length of string s */
int strlen(char *s)
{
int n;
for (n = 0; *s != '\0', s++)
n++;
return n;
}Since s is a pointer, incrementing it is perfectly legal; s++ has no effect
on the character string in the function that called strlen, but merely
increments strlen's private copy of the pointer. That means that calls like
strlen("hello, world"); /* string constant */
strlen(array); /* char array[100]; */
strlen(ptr); /* char *ptr; */all work.
As formal parameters in a function definition,
char s[];and
char *s;are equivalent; we prefer the latter because it says more explicitly that the variable is a pointer. When an array name is passed to a function, the function can at its convenience believe that it has been handed either an array or a pointer, and manipulate it accordingly. It can even use both notations if it seems appropriate and clear.
It is possible to pass part of an array to a function, by passing a pointer to
the beginning of the subarray. For example, if a is an array,
f(&a;[2])and
f(a+2)both pass to the function f the address of the subarray that starts at
a[2]. Within f, the parameter declaration can read
f(int arr[]) { ... }or
f(int *arr) { ... }So as far as f is concerned, the fact that the parameter refers to part of a
larger array is of no consequence.
If one is sure that the elements exist, it is also possible to index backwards
in an array; p[-1], p[-2], and so on are syntactically legal, and refer to
the elements that immediately precede p[0]. Of course, it is illegal to refer
to objects that are not within the array bounds.
If p is a pointer to some element of an array, then p++ increments p to
point to the next element, and p+=i increments it to point i elements beyond where it currently does. These and similar constructions are the simples forms of pointer or address arithmetic.
C is consistent and regular in its approach to address arithmetic; its
integration of pointers, arrays, and address arithmetic is one of the strengths
of the language. Let us illustrate by writing a rudimentary storage allocator.
There are two routines. The first, alloc(n), returns a pointer to n
consecutive character positions, which can be used by the caller of alloc for
storing characters. The second, afree(p), releases the storage thus acquired
so it can be re-used later. The routines are "rudimentary" because the calls to
afree must be made in the opposite order to the calls made on alloc. That
is, the storage managed by alloc and afree is a stack, or last-in,
first-out. The standard library provides analogous functions called malloc
and free that have no such restrictions; in Section 8.7 we will show how they
can be implemented.
The easiest implementation is to have alloc hand out pieces of a large
character array that we will call allocbuf. This array is private to alloc
and afree. Since they deal in pointers, not array indices, no other routine
need know the name of the array, which can be declared static in the source
file containing alloc and afree, and thus be invisible outside it. In
practical implementations, the array may well not even have a name; it might
instead be obtained by calling malloc or by asking the operating system for a
pointer to some unnamed block of storage.
The other information needed is how much of allocbuf has been used. We use a
pointer, called allocp, that points to the next free element. When alloc is
asked for n characters, it checks to see if there is enough room left in
allocbuf. If so, alloc returns the current value of allocp (i.e., the
beginning of the free block), then increments it by n to point to the next
free area. If there is no room, alloc returns zero. afree(p) merely sets
allocp to p if p is inside allocbuf.
#define ALLOCSIZE 10000 /* size of available space */
static char allocbuf[ALLOCSIZE]; /* storage for alloc */
static char *allocp = allocbuf; /* next free position */
char *alloc(int n) /* return pointer to n characters */
{
if (allocbuf + ALLOCSIZE - allocp >= n) { /* it fits */
allocp += n;
return allocp - n; /* old p */
} else /* not enough room */
return 0;
}
void afree(char *p) /* free storage pointed to by p */
{
if (p >= allocbuf && p < allocbuf + ALLOCSIZE)
allocp = p;
}In general a pointer can be initialized just as any other variable can, though normally the only meaningful values are zero or an expression involving the address of previously defined data of appropriate type. The declaration
static char *allocp = allocbuf;defines allocp to be a character pointer and initializes it to point to the
beginning of allocbuf, which is the next free position when the program
starts. This could also have been written
static char *allocp = &allocbuf;[0];since the array name is the address of the zeroth element.
The test
if (allocbuf + ALLOCSIZE - allocp >= n) { /* it fits */checks if there's enough room to satisfy a request for n characters. If there
is, the new value of allocp would be at most one beyond the end of
allocbuf. If the request can be satisfied, alloc returns a pointer to the
beginning of a block of characters (notice the declaration of the function
itself). If not,alloc must return some signal that there is no space left. C
guarantees that zero is never a valid address for data, so a return value of
zero can be used to signal an abnormal event, in this case no space.
Pointers and integers are not interchangeable. Zero is the sole exception: the
constant zero may be assigned to a pointer, and a pointer may be compared with
the constant zero. The symbolic constant NULL is often used in place of zero,
as a mnemonic to indicate more clearly that this is a special value for a
pointer. NULL is defined in <stdio.h>. We will use NULL henceforth.
Tests like
if (allocbuf + ALLOCSIZE - allocp >= n) { /* it fits */and
if (p >= allocbuf && p < allocbuf + ALLOCSIZE)show several important facets of pointer arithmetic. First, pointers may be
compared under certain circumstances. If p and q point to members of the
same array, then relations like ==, !=, <, >=, etc., work properly. For example,
p < qis true if p points to an earlier element of the array than q does. Any
pointer can be meaningfully compared for equality or inequality with zero. But
the behavior is undefined for arithmetic or comparisons with pointers that do
not point to members of the same array. (There is one exception: the address of
the first element past the end of an array can be used in pointer arithmetic.)
Second, we have already observed that a pointer and an integer may be added or subtracted. The construction
p + nmeans the address of the n-th object beyond the one p currently points to.
This is true regardless of the kind of object p points to; n is scaled
according to the size of the objects p points to, which is determined by the
declaration of p. If an int is four bytes, for example, the int will be
scaled by four.
Pointer subtraction is also valid: if p and q point to elements of the same
array, and p<q, then q-p+1 is the number of elements from p to q
inclusive. This fact can be used to write yet another version of strlen:
/* strlen: return length of string s */
int strlen(char *s)
{
char *p = s;
while (*p != '\0')
p++;
return p - s;
}In its declaration, p is initialized to s, that is, to point to the first
character of the string. In the while loop, each character in turn is
examined until the '\0' at the end is seen. Because p points to characters,
p++ advances p to the next character each time, and p-s gives the number
of characters advanced over, that is, the string length. (The number of
characters in the string could be too large to store in an int. The header
<stddef.h> defines a type ptrdiff_t that is large enough to hold the signed
difference of two pointer values. If we were being cautious, however, we would
use size_t for the return value of strlen, to match the standard library
version. size_t is the unsigned integer type returned by the sizeof
operator.
Pointer arithmetic is consistent: if we had been dealing with floats, which
occupy more storage that chars, and if p were a pointer to float, p++
would advance to the next float. Thus we could write another version of
alloc that maintains floats instead of chars, merely by changing char to
float throughout alloc and afree. All the pointer manipulations
automatically take into account the size of the objects pointed to.
The valid pointer operations are assignment of pointers of the same type,
adding or subtracting a pointer and an integer, subtracting or comparing two
pointers to members of the same array, and assigning or comparing to zero. All
other pointer arithmetic is illegal. It is not legal to add two pointers, or to
multiply or divide or shift or mask them, or to add float or double to
them, or even, except for void *, to assign a pointer of one type to a
pointer of another type without a cast.
A string constant, written as
"I am a string"is an array of characters. In the internal representation, the array is
terminated with the null character '\0' so that programs can find the end.
The length in storage is thus one more than the number of characters between
the double quotes.
Perhaps the most common occurrence of string constants is as arguments to functions, as in
printf("hello, world\n");When a character string like this appears in a program, access to it is through
a character pointer; printf receives a pointer to the beginning of the
character array. That is, a string constant is accessed by a pointer to its
first element.
String constants need not be function arguments. If pmessage is declared as
char *pmessage;then the statement
pmessage = "now is the time";assigns to pmessage a pointer to the character array. This is not a string
copy; only pointers are involved. C does not provide any operators for
processing an entire string of characters as a unit.
There is an important difference between these definitions:
char amessage[] = "now is the time"; /* an array */
char *pmessage = "now is the time"; /* a pointer */amessage is an array, just big enough to hold the sequence of characters and
'\0' that initializes it. Individual characters within the array may be
changed but amessage will always refer to the same storage. On the other
hand, pmessage is a pointer, initialized to point to a string constant; the
pointer may subsequently be modified to point elsewhere, but the result is
undefined if you try to modify the string contents.
We will illustrate more aspects of pointers and arrays by studying versions of
two useful functions adapted from the standard library. The first function is
strcpy(s,t), which copies the string t to the string s. It would be nice
just to say s=t but this copies the pointer, not the characters. To copy the
characters, we need a loop. The array version first:
/* strcpy: copy t to s; array subscript version */
void strcpy(char *s, char *t)
{
int i;
i = 0;
while ((s[i] = t[i]) != '\0')
i++;
}For contrast, here is a version of strcpy with pointers:
/* strcpy: copy t to s; pointer version */
void strcpy(char *s, char *t)
{
int i;
i = 0;
while ((*s = *t) != '\0') {
s++;
t++;
}
}Because arguments are passed by value, strcpy can use the parameters s and
t in any way it pleases. Here they are conveniently initialized pointers,
which are marched along the arrays a character at a time, until the '\0' that
terminates t has been copied into s.
In practice, strcpy would not be written as we showed it above. Experienced C
programmers would prefer
/* strcpy: copy t to s; pointer version 2 */
void strcpy(char *s, char *t)
{
while ((*s++ = *t++) != '\0')
;
}This moves the increment of s and t into the test part of the loop. The
value of *t++ is the character that t pointed to before t was
incremented; the postfix ++ doesn't change t until after this character has
been fetched. In the same way, the character is stored into the old s
position before s is incremented. This character is also the value that is
compared against '\0' to control the loop. The net effect is that characters
are copied from t to s, up and including the terminating '\0'.
As the final abbreviation, observe that a comparison against '\0' is redundant, since the question is merely whether the expression is zero. So the function would likely be written as
/* strcpy: copy t to s; pointer version 3 */
void strcpy(char *s, char *t)
{
while (*s++ = *t++)
;
}Although this may seem cryptic at first sight, the notational convenience is considerable, and the idiom should be mastered, because you will see it frequently in C programs.
The strcpy in the standard library (<string.h>) returns the target string
as its function value.
The second routine that we will examine is strcmp(s,t), which compares the
character strings s and t, and returns negative, zero or positive if s is
lexicographically less than, equal to, or greater than t. The value is
obtained by subtracting the characters at the first position where s and t
disagree.
/* strcmp: return <0 if s<t, 0 if s==t, >0 if s>t */
int strcmp(char *s, char *t)
{
int i;
for (i = 0; s[i] == t[i]; i++)
if (s[i] == '\0')
return 0;
return s[i] - t[i];
}The pointer version of strcmp:
/* strcmp: return <0 if s<t, 0 if s==t, >0 if s>t */
int strcmp(char *s, char *t)
{
for ( ; *s == *t; s++, t++)
if (*s == '\0')
return 0;
return *s - *t;
}Since ++ and -- are either prefix or postfix operators, other combinations
of * and ++ and -- occur, although less frequently. For example,
*--pdecrements p before fetching the character that p points to. In fact, the pair of expressions
*p++ = val; /* push val onto stack */
val = *--p; /* pop top of stack into val */are the standard idiom for pushing and popping a stack; see Section 4.3.
The header <string.h> contains declarations for the functions mentioned in
this section, plus a variety of other string-handling functions from the
standard library.
Exercise 5-3. Write a pointer version of the function strcat that we
showed in Chapter 2: strcat(s,t) copies the string t to the end of s.
Exercise 5-4. Write the function strend(s,t), which returns 1 if the
string t occurs at the end of the string s, and zero otherwise.
Exercise 5-5. Write versions of the library functions strncpy, strncat,
and strncmp, which operate on at most the first n characters of their
argument strings. For example, strncpy(s,t,n) copies at most n characters
of t to s. Full descriptions are in Appendix B.
Exercise 5-6. Rewrite appropriate programs from earlier chapters and
exercises with pointers instead of array indexing. Good possibilities include
getline (Chapters 1 and 4), atoi, itoa, and their variants (Chapters 2,
3, and 4), reverse (Chapter 3), and strindex and getop (Chapter 4).
Since pointers are variables themselves, they can be stored in arrays just as
other variables can. Let us illustrate by writing a program that will sort a
set of text lines into alphabetic order, a stripped-down version of the UNIX
program sort.
In Chapter 3, we presented a Shell sort function that would sort an array of integers, and in Chapter 4 we improved on it with a quicksort. The same algorithms will work, except that now we have to deal with lines of text, which are of different lengths, and which, unlike integers, can't be compared or moved in a single operation. We need a data representation that will cope efficiently and conveniently with variable-length text lines.
This is where the array of pointers enters. If the lines to be sorted are
stored end-to-end in one long character array, then each line can be accessed
by a pointer to its first character. The pointers themselves can bee stored in
an array. Two lines can be compared by passing their pointers to strcmp. When
two out-of-order lines have to be exchanged, the pointers in the pointer array
are exchanged, not the text lines themselves.
This eliminates the twin problems of complicated storage management and high overhead that would go with moving the lines themselves.
The sorting process has three steps:
- read all the lines of input
- sort them
- print them in order
As usual, it's best to divide the program into functions that match this natural division, with the main routine controlling the other functions. Let us defer the sorting step for a moment, and concentrate on the data structure and the input and output.
The input routine has to collect and save the characters of each line, and
build an array of pointers to the lines. It will also have to count the number
of input lines, since that information is needed for sorting and printing.
Since the input function can only cope with a finite number of input lines, it
can return some illegal count like -1 if too much input is presented.
The output routine only has to print the lines in the order in which they appear in the array of pointers.
#include <stdio.h>
#include <string.h>
#define MAXLINES 5000 /* max #lines to be sorted */
char *lineptr[MAXLINES]; /* pointers to text lines */
int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);
void qsort(char *lineptr[], int left, int right);
/* sort input lines */
main()
{
int nlines; /* number of input lines read */
if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
qsort(lineptr, 0, nlines-1);
writelines(lineptr, nlines);
return 0;
} else {
printf("error: input too big to sort\n");
return 1;
}
}
#define MAXLEN 1000 /* max length of any input line */
int getline(char *, int);
char *alloc(int);
/* readlines: read input lines */
int readlines(char *lineptr[], int maxlines)
{
int len, nlines;
char *p, line[MAXLEN];
nlines = 0;
while ((len = getline(line, MAXLEN)) > 0)
if (nlines >= maxlines || p = alloc(len) == NULL)
return -1;
else {
line[len-1] = '\0'; /* delete newline */
strcpy(p, line);
lineptr[nlines++] = p;
}
return nlines;
}
/* writelines: write output lines */
void writelines(char *lineptr[], int nlines)
{
int i;
for (i = 0; i < nlines; i++)
printf("%s\n", lineptr[i]);
}The function getline is from Section 1.9.
The main new thing is the declaration for lineptr:
char *lineptr[MAXLINES]says that lineptr is an array of MAXLINES elements, each element of which
is a pointer to a char. That is, lineptr[i] is a character pointer, and
*lineptr[i] is the character it points to, the first character of the i-th
saved text line.
Since lineptr is itself the name of an array, it can be treated as a pointer
in the same manner as in our earlier examples, and writelines can be written instead as
/* writelines: write output lines */
void writelines(char *lineptr[], int nlines)
{
while (nlines-- > 0)
printf("%s\n", *lineptr++);
}Initially, *lineptr points to the first line; each element advances it to the
next line pointer while nlines is counted down.
With input and output under control, we can proceed to sorting. The quicksort
from Chapter 4 needs minor changes: the declarations have to be modified, and
the comparison operation must be done by calling strcmp. The algorithm
remains the same, which gives us some confidence that it will still work.
/* qsort: sort v[left]...v[right] into increasing order */
void qsort(char *v[], int left, int right)
{
int i, last;
void swap(char *v[], int i, int j);
if (left >= right) /* do nothing if array contains */
return; /* fewer than two elements */
swap(v, left, (left + right)/2);
last = left;
for (i = left+1; i <= right; i++)
if (strcmp(v[i], v[left]) < 0)
swap(v, ++last, i);
swap(v, left, last);
qsort(v, left, last-1);
qsort(v, last+1, right);
}Similarly, the swap routine needs only trivial changes:
/* swap: interchange v[i] and v[j] */
void swap(char *v[], int i, int j)
{
char *temp;
temp = v[i];
v[i] = v[j];
v[j] = temp;
}Since any individual element of v (alias lineptr) is a character pointer,
temp must be also, so one can be copied to the other.
Exercise 5-7. Rewrite readlines to store lines in an array supplied by
main, rather than calling alloc to maintain storage. How much faster is the
program?
C provides rectangular multi-dimensional arrays, although in practice they are much less used than arrays of pointers. In this section, we will show some of their properties.
Consider the problem of date conversion, from day of the month to day of the
year and vice versa. For example, March 1 is the 60th day of a non-leap year,
and the 61st day of a leap year. Let us define two functions to do the
conversions: day_of_year converts the month and day into the day of the year,
and month_day converts the day of the year into the month and day. Since this
latter function computes two values, the month and day arguments will be
pointers:
month_day(1988, 60, &m;, &d;)setsm to 2 and d to 29 (February 29th).
These functions both need the same information, a table of the number of days in each month (:thirty days hath September ...:). Since the number of days per month differs for leap years and non-leap years, it's easier to separate them into two rows of a two-dimensional array than to keep track of what happens to February during computation. The array and the functions for performing the transformations are as follows:
static char daytab[2][13] = {
{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
{0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};
/* day_of_year: set day of year from month & day */
int day_of_year(int year, int month, int day)
{
int i, leap;
leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
for (i = 1; i < month; i++)
day += daytab[leap][i];
return day;
}
/* month_day: set month, day from day of year */
void month_day(int year, int yearday, int *pmonth, int *pday)
{
int i, leap;
leap = year%4 == 0 && year%100 != 0 || year%400 == 0;
for (i = 1; yearday > daytab[leap][i]; i++)
yearday -= daytab[leap][i];
*pmonth = i;
*pday = yearday;
}Recall that the arithmetic value of a logical expression, such as the one for
leap, is either zero (false) or one (true), so it can be used as a subscript
of the array daytab.
The array daytab has to be external to both day_of_year and month_day, so
they can both use it. We made it char to illustrate a legitimate use of
char for storing small non-character integers.
daytab is the first two-dimensional array we have dealt with. In C, a
two-dimensional array is really a one-dimensional array, each of whose elements
is an array. Hence subscripts are written as
daytab[i][j] /* [row][col] */rather than
daytab[i,j] /* WRONG */Other than this notational distinction, a two-dimensional array can be treated in much the same way as in other languages. Elements are stored by rows, so the rightmost subscript, or column, varies fastest as elements are accessed in storage order.
An array is initialized by a list of initializers in braces; each row of a
two-dimensional array is initialized by a corresponding sub-list. We started
the array daytab with a column of zero so that month numbers can run from the natural 1 to 12 instead of 0 to 11. Since space is not at a premium here, this is clearer than adjusting the indices.
If a two-dimensional array is to be passed to a function, the parameter
declaration in the function must include the number of columns; the number of
rows is irrelevant, since what is passed is, as before, a pointer to an array
of rows, where each row is an array of 13 ints. In this particular case, it
is a pointer to objects that are arrays of 13 ints. Thus if the array
daytab is to be passed to a function f, the declaration of f would be:
f(int daytab[2][13]) { ... }It could also be
f(int daytab[][13]) { ... }since the number of rows is irrelevant, or it could be
f(int (*daytab)[13]) { ... }which says that the parameter is a pointer to an array of 13 integers. The
parentheses are necessary since brackets [] have higher precedence than *.
Without parentheses, the declaration
int *daytab[13]is an array of 13 pointers to integers. More generally, only the first dimension (subscript) of an array is free; all the others have to be specified.
Section 5.12 has a further discussion of complicated declarations.
Exercise 5-8. There is no error checking in day_of_year or month_day.
Remedy this defect.
Consider the problem of writing a function month_name(n), which returns a
pointer to a character string containing the name of the n-th month. This is
an ideal application for an internal static array. month_name contains a
private array of character strings, and returns a pointer to the proper one
when called. This section shows how that array of names is initialized.
The syntax is similar to previous initializations:
/* month_name: return name of n-th month */
char *month_name(int n)
{
static char *name[] = {
"Illegal month",
"January", "February", "March",
"April", "May", "June",
"July", "August", "September",
"October", "November", "December"
};
return (n < 1 || n > 12) ? name[0] : name[n];
}The declaration of name, which is an array of character pointers, is the same
as lineptr in the sorting example. The initializer is a list of character
strings; each is assigned to the corresponding position in the array. The
characters of the i-th string are placed somewhere, and a pointer to them is
stored in name[i]. Since the size of the array name is not specified, the
compiler counts the initializers and fills in the correct number.
Newcomers to C are sometimes confused about the difference between a two-dimensional array and an array of pointers, such as name in the example above. Given the definitions
int a[10][20];
int *b[10];then a[3][4] and b[3][4] are both syntactically legal references to a
single int. But a is a true two-dimensional array: 200 int-sized
locations have been set aside, and the conventional rectangular subscript
calculation 20 * row +col is used to find the element a[row,col]. For b,
however, the definition only allocates 10 pointers and does not initialize
them; initialization must be done explicitly, either statically or with code.
Assuming that each element of b does point to a twenty-element array, then
there will be 200 ints set aside, plus ten cells for the pointers. The
important advantage of the pointer array is that the rows of the array may be
of different lengths. That is, each element of b need not point to a
twenty-element vector; some may point to two elements, some to fifty, and some
to none at all.
Although we have phrased this discussion in terms of integers, by far the most
frequent use of arrays of pointers is to store character strings of diverse
lengths, as in the function month_name. Compare the declaration and picture
for an array of pointers:
char *name[] = { "Illegal month", "Jan", "Feb", "Mar" };
with those for a two-dimensional array:
Exercise 5-9. Rewrite the routines day_of_year and month_day with
pointers instead of indexing.
In environments that support C, there is a way to pass command-line arguments
or parameters to a program when it begins executing. When main is called, it
is called with two arguments. The first (conventionally called argc, for
argument count) is the number of command-line arguments the program was invoked
with; the second (argv, for argument vector) is a pointer to an array of
character strings that contain the arguments, one per string. We customarily
use multiple levels of pointers to manipulate these character strings.
The simplest illustration is the program echo, which echoes its command-line
arguments on a single line, separated by blanks. That is, the command
echo hello, world
prints the output
hello, world
By convention, argv[0] is the name by which the program was invoked, so
argc is at least 1. If argc is 1, there are no command-line arguments after
the program name. In the example above, argc is 3, and argv[0], argv[1],
and argv[2] are "echo", "hello, ", and "world" respectively. The first
optional argument is argv[1] and the last is argv[argc-1]; additionally,
the standard requires that argv[argc] be a null pointer.
The first version of echo treats argv as an array of character pointers:
#include <stdio.h>
/* echo command-line arguments; 1st version */
main(int argc, char *argv[])
{
int i;
for (i = 1; i < argc; i++)
printf("%s%s", argv[i], (i < argc-1) ? " " : "");
printf("\n");
return 0;
}Since argv is a pointer to an array of pointers, we can manipulate the
pointer rather than index the array. This next variant is based on incrementing
argv, which is a pointer to pointer to char, while argc is counted down:
#include <stdio.h>
/* echo command-line arguments; 2nd version */
main(int argc, char *argv[])
{
while (--argc > 0)
printf("%s%s", *++argv, (argc > 1) ? " " : "");
printf("\n");
return 0;
}Since argv is a pointer to the beginning of the array of argument strings,
incrementing it by 1 (++argv) makes it point at the original argv[1]
instead of argv[0]. Each successive increment moves it along to the next
argument; *argv is then the pointer to that argument. At the same time,
argc is decremented; when it becomes zero, there are no arguments left to
print.
Alternatively, we could write the printf statement as
printf((argc > 1) ? "%s " : "%s", *++argv);This shows that the format argument of printf can be an expression too.
As a second example, let us make some enhancements to the pattern-finding
program from Section 4.1. If you recall, we wired the search pattern deep into
the program, an obviously unsatisfactory arrangement. Following the lead of the
UNIX program grep, let us enhance the program so the pattern to be matched is
specified by the first argument on the command line.
#include <stdio.h>
#include <string.h>
#define MAXLINE 1000
int getline(char *line, int max);
/* find: print lines that match pattern from 1st arg */
main(int argc, char *argv[])
{
char line[MAXLINE];
int found = 0;
if (argc != 2)
printf("Usage: find pattern\n");
else
while (getline(line, MAXLINE) > 0)
if (strstr(line, argv[1]) != NULL) {
printf("%s", line);
found++;
}
return found;
}The standard library function strstr(s,t) returns a pointer to the first
occurrence of the string t in the string s, or NULL if there is none. It
is declared in <string.h>.
The model can now be elaborated to illustrate further pointer constructions. Suppose we want to allow two optional arguments. One says "print all the lines except those that match the pattern;" the second says "precede each printed line by its line number."
A common convention for C programs on UNIX systems is that an argument that
begins with a minus sign introduces an optional flag or parameter. If we choose
-x (for "except") to signal the inversion, and -n ("number") to request
line numbering, then the command
find -x -npattern
will print each line that doesn't match the pattern, preceded by its line number.
Optional arguments should be permitted in any order, and the rest of the program should be independent of the number of arguments that we present. Furthermore, it is convenient for users if option arguments can be combined, as in
find -nx pattern
Here is the program:
#include <stdio.h>
#include <string.h>
#define MAXLINE 1000
int getline(char *line, int max);
/* find: print lines that match pattern from 1st arg */
main(int argc, char *argv[])
{
char line[MAXLINE];
long lineno = 0;
int c, except = 0, number = 0, found = 0;
while (--argc > 0 && (*++argv)[0] == '-')
while (c = *++argv[0])
switch (c) {
case 'x':
except = 1;
break;
case 'n':
number = 1;
break;
default:
printf("find: illegal option %c\n", c);
argc = 0;
found = -1;
break;
}
if (argc != 1)
printf("Usage: find -x -n pattern\n");
else
while (getline(line, MAXLINE) > 0) {
lineno++;
if ((strstr(line, *argv) != NULL) != except) {
if (number)
printf("%ld:", lineno);
printf("%s", line);
found++;
}
}
return found;
}argc is decremented and argv is incremented before each optional argument.
At the end of the loop, if there are no errors, argc tells how many arguments
remain unprocessed and argv points to the first of these. Thus argc should
be 1 and *argv should point at the pattern. Notice that *++argv is a
pointer to an argument string, so (*++argv)[0] is its first character. (An
alternate valid form would be **++argv.) Because [] binds tighter than *
and ++, the parentheses are necessary; without them the expression would be
taken as *++(argv[0]). In fact, that is what we have used in the inner loop,
where the task is to walk along a specific argument string. In the inner loop,
the expression *++argv[0] increments the pointer argv[0]!
It is rare that one uses pointer expressions more complicated than these; in such cases, breaking them into two or three steps will be more intuitive.
Exercise 5-10. Write the program expr, which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument. For example,
expr 2 3 4 + *
evaluates 2 * (3+4).
Exercise 5-11. Modify the program entab and detab (written as exercises
in Chapter 1) to accept a list of tab stops as arguments. Use the default tab
settings if there are no arguments.
Exercise 5-12. Extend entab and detab to accept the shorthand
entab -m +n
to mean tab stops every n columns, starting at column m. Choose convenient
(for the user) default behavior.
Exercise 5-13. Write the program tail, which prints the last n lines of
its input. By default, n is set to 10, let us say, but it can be changed by
an optional argument so that
tail -n
prints the last n lines. The program should behave rationally no matter how
unreasonable the input or the value of n. Write the program so it makes the
best use of available storage; lines should be stored as in the sorting program
of Section 5.6, not in a two-dimensional array of fixed size.
In C, a function itself is not a variable, but it is possible to define
pointers to functions, which can be assigned, placed in arrays, passed to
functions, returned by functions, and so on. We will illustrate this by
modifying the sorting procedure written earlier in this chapter so that if the
optional argument -n is given, it will sort the input lines numerically
instead of lexicographically.
A sort often consists of three parts - a comparison that determines the ordering of any pair of objects, an exchange that reverses their order, and a sorting algorithm that makes comparisons and exchanges until the objects are in order. The sorting algorithm is independent of the comparison and exchange operations, so by passing different comparison and exchange functions to it, we can arrange to sort by different criteria. This is the approach taken in our new sort.
Lexicographic comparison of two lines is done by strcmp, as before; we will
also need a routine numcmp that compares two lines on the basis of numeric
value and returns the same kind of condition indication as strcmp does. These
functions are declared ahead of main and a pointer to the appropriate one is
passed to qsort. We have skimped on error processing for arguments, so as to
concentrate on the main issues.
#include <stdio.h>
#include <string.h>
#define MAXLINES 5000 /* max #lines to be sorted */
char *lineptr[MAXLINES]; /* pointers to text lines */
int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);
void qsort(void *lineptr[], int left, int right,
int (*comp)(void *, void *));
int numcmp(char *, char *);
/* sort input lines */
main(int argc, char *argv[])
{
int nlines; /* number of input lines read */
int numeric = 0; /* 1 if numeric sort */
if (argc > 1 && strcmp(argv[1], "-n") == 0)
numeric = 1;
if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
qsort((void**) lineptr, 0, nlines-1,
(int (*)(void*,void*))(numeric ? numcmp : strcmp));
writelines(lineptr, nlines);
return 0;
} else {
printf("input too big to sort\n");
return 1;
}
}In the call to qsort, strcmp and numcmp are addresses of functions. Since
they are known to be functions, the & is not necessary, in the same way that
it is not needed before an array name.
We have written qsort so it can process any data type, not just character
strings. As indicated by the function prototype, qsort expects an array of
pointers, two integers, and a function with two pointer arguments. The generic
pointer type void * is used for the pointer arguments. Any pointer can be
cast to void * and back again without loss of information, so we can call
qsort by casting arguments to void *. The elaborate cast of the function
argument casts the arguments of the comparison function. These will generally
have no effect on actual representation, but assure the compiler that all is
well.
/* qsort: sort v[left]...v[right] into increasing order */
void qsort(void *v[], int left, int right,
int (*comp)(void *, void *))
{
int i, last;
void swap(void *v[], int, int);
if (left >= right) /* do nothing if array contains */
return; /* fewer than two elements */
swap(v, left, (left + right)/2);
last = left;
for (i = left+1; i <= right; i++)
if ((*comp)(v[i], v[left]) < 0)
swap(v, ++last, i);
swap(v, left, last);
qsort(v, left, last-1, comp);
qsort(v, last+1, right, comp);
}The declarations should be studied with some care. The fourth parameter of
qsort is
int (*comp)(void *, void *)which says that comp is a pointer to a function that has two void *
arguments and returns an int.
The use of comp in the line
if ((*comp)(v[i], v[left]) < 0)is consistent with the declaration: comp is a pointer to a function, *comp
is the function, and
(*comp)(v[i], v[left])is the call to it. The parentheses are needed so the components are correctly associated; without them,
int *comp(void *, void *) /* WRONG */says that comp is a function returning a pointer to an int, which is very
different.
We have already shown strcmp, which compares two strings. Here is numcmp,
which compares two strings on a leading numeric value, computed by calling
atof:
#include <stdlib.h>
/* numcmp: compare s1 and s2 numerically */
int numcmp(char *s1, char *s2)
{
double v1, v2;
v1 = atof(s1);
v2 = atof(s2);
if (v1 < v2)
return -1;
else if (v1 > v2)
return 1;
else
return 0;
}The swap function, which exchanges two pointers, is identical to what we
presented earlier in the chapter, except that the declarations are changed to
void *.
void swap(void *v[], int i, int j;)
{
void *temp;
temp = v[i];
v[i] = v[j];
v[j] = temp;
}A variety of other options can be added to the sorting program; some make challenging exercises.
Exercise 5-14. Modify the sort program to handle a -r flag, which
indicates sorting in reverse (decreasing) order. Be sure that -r works with
-n.
Exercise 5-15. Add the option -f to fold upper and lower case together,
so that case distinctions are not made during sorting; for example, a and A
compare equal.
Exercise 5-16. Add the -d (:directory order") option, which makes
comparisons only on letters, numbers and blanks. Make sure it works in
conjunction with -f.
Exercise 5-17. Add a field-searching capability, so sorting may be done on
fields within lines, each field sorted according to an independent set of
options. (The index for this book was sorted with -df for the index category
and -n for the page numbers.)
C is sometimes castigated for the syntax of its declarations, particularly ones that involve pointers to functions. The syntax is an attempt to make the declaration and the use agree; it works well for simple cases, but it can be confusing for the harder ones, because declarations cannot be read left to right, and because parentheses are over-used. The difference between
int *f(); /* f: function returning pointer to int */and
int (*pf)(); /* pf: pointer to function returning int */ illustrates the problem: * is a prefix operator and it has lower precedence
than (), so parentheses are necessary to force the proper association.
Although truly complicated declarations rarely arise in practice, it is
important to know how to understand them, and, if necessary, how to create
them. One good way to synthesize declarations is in small steps with typedef,
which is discussed in Section 6.7. As an alternative, in this section we will
present a pair of programs that convert from valid C to a word description and
back again. The word description reads left to right.
The first, dcl, is the more complex. It converts a C declaration into a word
description, as in these examples:
| Declaration | Type |
|---|---|
char **argv |
argv: pointer to char |
int (*daytab)[13] |
daytab: pointer to array[13] of int |
int *daytab[13] |
daytab: array[13] of pointer to int |
void *comp() |
comp: function returning pointer to void |
void (*comp)() |
comp: pointer to function returning void |
char (*(*x())[])() |
x: function returning pointer to array[] of pointer to function returning char |
char (*(*x[3])())[5] |
x: array[3] of pointer to function returning pointer to array[5] of char |
dcl is based on the grammar that specifies a declarator, which is spelled out precisely in Appendix A, Section 8.5; this is a simplified form:
dcl: optional *'s direct-dcl
direct-dcl name
(dcl)
direct-dcl()
direct-dcl[optional size]
In words, a dcl is a direct-dcl, perhaps preceded by *'s. A direct-dcl
is a name, or a parenthesized dcl, or a direct-dcl followed by parentheses,
or a direct-dcl followed by brackets with an optional size.
This grammar can be used to parse functions. For instance, consider this declarator:
(*pfa[])()pfa will be identified as a name and thus as a direct-dcl. Then pfa[] is
also a direct-dcl. Then *pfa[] is recognized as a dcl, so (*pfa[]) is a
direct-dcl. Then (*pfa[])() is a direct-dcl and thus a dcl. We can also
illustrate the parse with a tree like this (where direct-dcl has been
abbreviated to dir-dcl):
The heart of the dcl program is a pair of functions, dcl and dirdcl, that
parse a declaration according to this grammar. Because the grammar is
recursively defined, the functions call each other recursively as they
recognize pieces of a declaration; the program is called a recursive-descent
parser.
/* dcl: parse a declarator */
void dcl(void)
{
int ns;
for (ns = 0; gettoken() == '*'; ) /* count *'s */
ns++;
dirdcl();
while (ns-- > 0)
strcat(out, " pointer to");
}
/* dirdcl: parse a direct declarator */
void dirdcl(void)
{
int type;
if (tokentype == '(') { /* ( dcl ) */
dcl();
if (tokentype != ')')
printf("error: missing )\n");
} else if (tokentype == NAME) /* variable name */
strcpy(name, token);
else
printf("error: expected name or (dcl)\n");
while ((type=gettoken()) == PARENS || type == BRACKETS)
if (type == PARENS)
strcat(out, " function returning");
else {
strcat(out, " array");
strcat(out, token);
strcat(out, " of");
}
}Since the programs are intended to be illustrative, not bullet-proof, there are
significant restrictions on dcl. It can only handle a simple data type line
char or int. It does not handle argument types in functions, or qualifiers
like const. Spurious blanks confuse it. It doesn't do much error recovery, so
invalid declarations will also confuse it. These improvements are left as
exercises.
Here are the global variables and the main routine:
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define MAXTOKEN 100
enum { NAME, PARENS, BRACKETS };
void dcl(void);
void dirdcl(void);
int gettoken(void);
int tokentype; /* type of last token */
char token[MAXTOKEN]; /* last token string */
char name[MAXTOKEN]; /* identifier name */
char datatype[MAXTOKEN]; /* data type = char, int, etc. */
char out[1000];
main() /* convert declaration to words */
{
while (gettoken() != EOF) { /* 1st token on line */
strcpy(datatype, token); /* is the datatype */
out[0] = '\0';
dcl(); /* parse rest of line */
if (tokentype != '\n')
printf("syntax error\n");
printf("%s: %s %s\n", name, out, datatype);
}
return 0;
}The function gettoken skips blanks and tabs, then finds the next token in the
input; a "token" is a name, a pair of parentheses, a pair of brackets perhaps including a number, or any other single character.
int gettoken(void) /* return next token */
{
int c, getch(void);
void ungetch(int);
char *p = token;
while ((c = getch()) == ' ' || c == '\t')
;
if (c == '(') {
if ((c = getch()) == ')') {
strcpy(token, "()");
return tokentype = PARENS;
} else {
ungetch(c);
return tokentype = '(';
}
} else if (c == '[') {
for (*p++ = c; (*p++ = getch()) != ']'; )
;
*p = '\0';
return tokentype = BRACKETS;
} else if (isalpha(c)) {
for (*p++ = c; isalnum(c = getch()); )
*p++ = c;
*p = '\0';
ungetch(c);
return tokentype = NAME;
} else
return tokentype = c;
}getch and ungetch are discussed in Chapter 4.
Going in the other direction is easier, especially if we do not worry about
generating redundant parentheses. The program undcl converts a word
description like "x is a function returning a pointer to an array of pointers
to functions returning char," which we will express as
x () * [] * () charto
char (*(*x())[])()The abbreviated input syntax lets us reuse the gettoken function. undcl
also uses the same external variables as dcl does.
/* undcl: convert word descriptions to declarations */
main()
{
int type;
char temp[MAXTOKEN];
while (gettoken() != EOF) {
strcpy(out, token);
while ((type = gettoken()) != '\n')
if (type == PARENS || type == BRACKETS)
strcat(out, token);
else if (type == '*') {
sprintf(temp, "(*%s)", out);
strcpy(out, temp);
} else if (type == NAME) {
sprintf(temp, "%s %s", token, out);
strcpy(out, temp);
} else
printf("invalid input at %s\n", token);
}
return 0;
}Exercise 5-18. Make dcl recover from input errors.
Exercise 5-19. Modify undcl so that it does not add redundant parentheses
to declarations.
Exercise 5-20. Expand dcl to handle declarations with function argument
types, qualifiers like const, and so on.