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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.
Let us begin with a simplified picture of how memory is organized. A typical machine has an array of consecutively numbered or addressed memory cells that may be manipulated individually or in contiguous groups. One common situation is that any byte can be a char, a pair of one-byte cells can be treated as a short integer, and four adjacent bytes form a long. A pointer is a group of cells (often two or four) that can hold an address. So if c is a char and p is a pointer that points to it, we could represent the situation this way:
The unary operator & gives the address of an object, so the statement
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
The declaration of x y, and z are what we've seen all along. The declaration of the pointer 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,
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 11.)
If ip points to the integer x, then *ip can occur in any context where x could, so
increments *ip by 10.
The unary operators and & bind more tightly than arithmetic operators, so the assignment
takes whatever ip points at, adds 1, and assigns the result to y, while
increments what ip points to, as do
and
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
copies 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
where the swap function is defined as
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:
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.
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
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.
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
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
then the assignment
sets pa to point to element zero of a; that is, pa contains the address of a[0]
Now the assignment
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]
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 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
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.
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
all work.
As formal parameters in a function definition,
and
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,
and
both pass to the function f the address of the subarray that starts at a[2]. Within f, the parameter declaration can read
or
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
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
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
since the array name is the address of the zeroth element.
The test
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 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
is an array of characters. In the internal representation, the array is terminated with the null character 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
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
then the statement
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:
amessage is an array, just big enough to hold the sequence of characters and 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:
For contrast, here is a version of strcpy with pointers:
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 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
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 to control the loop. The net effect is that characters are copied from t to s, up and including the terminating
As the final abbreviation, observe that a comparison against is redundant, since the question is merely whether the expression is zero. So the function would likely be written as
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.
The pointer version of strcmp
Since and are either prefix or postfix operators, other combinations of and and occur, although less frequently. For example,
decrements p before fetching the character that p points to. In fact, the pair of expressions
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- 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 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.
The function getline is from Section 1.9.
The main new thing is the declaration for lineptr
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
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.
Similarly, the swap routine needs only trivial changes:
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:
sets m 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:
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
rather than
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:
It could also be
since the number of rows is irrelevant, or it could be
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
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 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:
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
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:
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
prints the output
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:
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:
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
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.
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
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
Here is the program:
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,
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
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
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 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.
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.
The declarations should be studied with some care. The fourth parameter of qsort is
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
is consistent with the declaration: comp is a pointer to a function, *comp is the function, and
is the call to it. The parentheses are needed so the components are correctly associated; without them,
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
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 *
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 5-1 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 bee 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
and
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:
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:
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 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.
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:
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.
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
to
The abbreviated input syntax lets us reuse the gettoken function. undcl also uses the same external variables as dcl does.
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.
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