C Programming chapter 5 2018-12-19T12:15:15+00:00

C Programming Chapter 5

Topics :- (Basics of Structures, Structures and Functions, Arrays of Structures, Pointers to Structures, Self-referential Structures, Table Lookup, Typedef, Unions, Bit-fields,Standard Input and Output, Formatted Output – printf, Variable-length Argument Lists, Formatted Input – Scanf, File Access, Error Handling – Stderr and Exit, Line Input and Output, Miscellaneous Functions )


A structure is a collection of one or more variables, possibly of different types, grouped together under a single name for convenient handling. (Structures are called “records” in some languages, notably Pascal.) Structures help to organize complicated data, particularly in large programs, because they permit a group of related variables to be treated as a unit instead of as separate entities.

One traditional example of a structure is the payroll record: an employee is described by a set of attributes such as name, address, social security number, salary, etc. Some of these in turn could be structures: a name has several components, as does an address and even a salary. Another example, more typical for C, comes from graphics: a point is a pair of coordinate, a rectangle is a pair of points, and so on. The main change made by the ANSI standard is to define structure assignment – structures may be copied and assigned to, passed to functions, and returned by functions. This has been supported by most compilers for many years, but the properties are now precisely defined. Automatic structures and arrays may now also be initialized.

Basics of Structures

Let us create a few structures suitable for graphics. The basic object is a point, which we will assume has an x coordinate and a y coordinate, both integers.

The two components can be placed in a structure declared like this:

struct  point  {

      int  x;

      int  y;


The keyword struct introduces a structure declaration, which is a list of declarations enclosed in braces. An optional name called a structure tag may follow the word struct (as with point here). The tag names this kind of structure, and can be used subsequently as a shorthand for the part of the declaration in braces.

The variables named in a structure are called members. A structure member or tag and an ordinary (i.e., non-member) variable can have the same name without conflict, since they can always be distinguished by context. Furthermore, the same member names may occur in different structures, although as a matter of style one would normally use the same names only for closely related objects. A struct declaration defines a type. The right brace that terminates the list of members may be followed by a list of variables, just as for any basic type. That is,

struct  {    }  x,  y,  z;

is syntactically analogous to

int  x,  y,  z;

in the sense that each statement declares x, y and z to be variables of the named type and causes space to be set aside for them. A structure declaration that is not followed by a list of variables reserves no storage; it merely describes a template or shape of a structure. If the declaration is tagged, however, the tag can be used later in definitions of instances of the structure. For example, given the declaration of point above,

struct  point  pt;

defines a variable pt which is a structure of type struct point. A structure can be initialized by following its definition with a list of initializers, each a constant expression, for the members:

struct  maxpt  =  {  320,  200  };

An automatic structure may also be initialized by assignment or by calling a function that returns a structure of the right type. A member of a particular structure is referred to in an expression by a construction of the form


The structure member operator “.” connects the structure name and the member name. To print the coordinates of the point pt, for instance,

printf(“%d,%d”,  pt.x,  pt.y);

or to compute the distance from the origin (0,0) to pt,

double  dist,  sqrt(double);

dist  =  sqrt((double)pt.x  *  pt.x  +  (double)pt.y  *  pt.y);

Structures can be nested. One representation of a rectangle is a pair of points that denote the diagonally opposite corners:

struct  rect  {

      struct  point  pt1;

      struct  point  pt2;


The rect structure contains two point structures. If we declare screen as

struct  rect  screen;



refers to the x coordinate of the pt1 member of screen.

Structures and Functions

The only legal operations on a structure are copying it or assigning to it as a unit, taking its address with &, and accessing its members. Copy and assignment include passing arguments to functions and returning values from functions as well. Structures may not be compared. A structure may be initialized by a list of constant member values; an automatic structure may also be initialized by an assignment.

Let us investigate structures by writing some functions to manipulate points and rectangles. There are at least three possible approaches: pass components separately, pass an entire structure, or pass a pointer to it. Each has its good points and bad points. The first function, makepoint, will take two integers and return a point structure:

/* makepoint: make a point from x and y components */ struct point makepoint(int x, int y) {

       struct  point  temp;

       temp.x  =  x;

       temp.y  =  y;

       return  temp;


Notice that there is no conflict between the argument name and the member with the same name; indeed the re-use of the names stresses the relationship. makepoint can now be used to initialize any structure dynamically, or to provide structure arguments to a function:

struct  rect  screen;

struct  point  middle;

struct  point  makepoint(int,  int);

screen.pt1  =  makepoint(0,0);

screen.pt2  =  makepoint(XMAX,  YMAX);

middle = makepoint((screen.pt1.x + screen.pt2.x)/2, (screen.pt1.y + screen.pt2.y)/2);

The next step is a set of functions to do arithmetic on points. For instance,

/*  addpoints: add  two  points  */

struct  addpoint(struct  point  p1,  struct  point  p2)


          p1.x  +=  p2.x;

          p1.y  +=  p2.y;

         return  p1;


Here both the arguments and the return value are structures. We incremented the components in p1 rather than using an explicit temporary variable to emphasize that structure parameters are passed by value like any others. As another example, the function ptinrect tests whether a point is inside a rectangle, where we have adopted the convention that a rectangle includes its left and bottom sides but not its top and right sides:

/* ptinrect: return 1 if p in r, 0 if not */ int ptinrect(struct point p, struct rect r) {

            return p.x >= r.pt1.x && p.x < r.pt2.x

           && p.y >= r.pt1.y && p.y < r.pt2.y;


This assumes that the rectangle is presented in a standard form where the pt1 coordinates are less than the pt2 coordinates. The following function returns a rectangle guaranteed to be in canonical form:

        #define min(a, b) ((a) < (b) ? (a) : (b))

   #define max(a, b) ((a) > (b) ? (a) : (b))
   /* canonrect: canonicalize coordinates of rectangle */
   struct rect canonrect(struct rect r)
       struct rect temp;
       temp.pt1.x = min(r.pt1.x, r.pt2.x);
       temp.pt1.y = min(r.pt1.y, r.pt2.y);
       temp.pt2.x = max(r.pt1.x, r.pt2.x);
       temp.pt2.y = max(r.pt1.y, r.pt2.y);
       return temp;


If a large structure is to be passed to a function, it is generally more efficient to pass a pointer than to copy the whole structure. Structure pointers are just like pointers to ordinary variables. The declaration

struct  point  *pp;

says that pp is a pointer to a structure of type struct point. If pp points to a point structure, *pp is the structure, and (*pp).x and (*pp).y are the members. To use pp, we might write, for example,

struct  point  origin,  *pp;

pp  =  &origin;

printf(“origin  is  (%d,%d)\n”,  (*pp).x,  (*pp).y);

The parentheses are necessary in (*pp).x because the precedence of the structure member operator . is higher then *. The expression *pp.x means *(pp.x), which is illegal here because x is not a pointer.

Pointers to structures are so frequently used that an alternative notation is provided as a shorthand. If p is a pointer to a structure, then


refers to the particular member. So we could write instead

printf(“origin  is  (%d,%d)\n”,  pp->x,  pp->y);

Both . and -> associate from left to right, so if we have

struct  rect  r,  *rp  =  &r;

then these four expressions are equivalent:





The structure operators . and ->, together with () for function calls and [] for subscripts, are at the top of the precedence hierarchy and thus bind very tightly. For example, given the declaration

struct  {

    int  len;

    char  *str;

}  *p;



increments len, not p, because the implied parenthesization is ++(p->len). Parentheses can be used to alter binding: (++p)->len increments p before accessing len, and (p++)->len increments p afterward. (This last set of parentheses is unnecessary.) In the same way, *p->str fetches whatever str points to; *p->str++ increments str after accessing whatever it points to (just like *s++); (*p->str)++ increments whatever str points to; and *p++->str increments p after accessing whatever str points to.

Arrays of Structures

Consider writing a program to count the occurrences of each C keyword. We need an array of character strings to hold the names, and an array of integers for the counts. One possibility is to use two parallel arrays, keyword and keycount, as in

char  *keyword[NKEYS];

int  keycount[NKEYS];

But the very fact that the arrays are parallel suggests a different organization, an array of structures. Each keyword is a pair:

char  *word;

int  cout;

and there is an array of pairs. The structure declaration

struct  key  {

char  *word;

int  count;

}  keytab[NKEYS];

declares a structure type key, defines an array keytab of structures of this type, and sets aside storage for them. Each element of the array is a structure. This could also be written

struct  key  {

char  *word;

int  count;


struct  key  keytab[NKEYS];

Since the structure keytab contains a constant set of names, it is easiest to make it an external variable and initialize it once and for all when it is defined. The structure initialization is analogous to earlier ones – the definition is followed by a list of initializers enclosed in braces:

struct key {
       char *word;
       int count;
   } keytab[] = {
       "auto", 0,
      "break", 0,
       "case", 0,
       "char", 0
      "const", 0,
      "continue", 0,
     "default", 0,
      /* ... */
     "unsigned", 0,
     "void", 0,
     "volatile", 0,
    "while", 0         


The initializers are listed in pairs corresponding to the structure members. It would be more precise to enclose the initializers for each “row” or structure in braces, as in

{  “auto”,  0  },

{ “break”, 0 }, { “case”, 0 },

but inner braces are not necessary when the initializers are simple variables or character strings, and when all are present. As usual, the number of entries in the array keytab will be computed if the initializers are present and the [] is left empty. The keyword counting program begins with the definition of keytab. The main routine reads the input by repeatedly calling a function getword that fetches one word at a time. Each word is looked up in keytab with a version of the binary search function. The list of keywords must be sorted in increasing order in the table.

   #include  < stdio.h >
   #include  < string.h >
   #include  < ctype.h >
   #define MAXWORD 100
   int getword(char *, int);
   int binsearch(char *, struct key *, int);
   /* count C keywords */
       int n;
       char word[MAXWORD];
       while (getword(word, MAXWORD) != EOF)
         if (isalpha(word[0]))
               if ((n = binsearch(word, keytab, NKEYS)) >= 0)
       for (n = 0; n < NKEYS; n++)
           if (keytab[n].count > 0)
               printf("%4d %s\n",
                   keytab[n].count, keytab[n].word);

return 0; }

   /* binsearch:  find word in tab[0]...tab[n-1] */
   int binsearch(char *word, struct key tab[], int n)
       int cond;
       int low, high, mid;
       low = 0;
       high = n - 1;
       while (low <= high) {
           mid = (low+high) / 2;
           if ((cond = strcmp(word, tab[mid].word)) < 0)
               high = mid - 1;
           else if (cond > 0)
               low = mid + 1;
               return mid;


return -1; }

We will show the function getword in a moment; for now it suffices to say that each call to getword finds a word, which is copied into the array named as its first argument. The quantity NKEYS is the number of keywords in keytab. Although we could count this by hand, it’s a lot easier and safer to do it by machine, especially if the list is subject to change. One possibility would be to terminate the list of initializers with a null pointer, then loop along keytab until the end is found.But this is more than is needed, since the size of the array is completely determined at compile time. The size of the array is the size of one entry times the number of entries, so the number of entries is just

size of keytab  /  size of struct  key

C provides a compile-time unary operator called sizeof that can be used to compute the size of any object. The expressions

sizeof  object


sizeof  (type  name)

yield an integer equal to the size of the specified object or type in bytes. (Strictly, sizeof produces an unsigned integer value whose type, size_t, is defined in the header .) An object can be a variable or array or structure. A type name can be the name of a basic type like int or double, or a derived type like a structure or a pointer.

In our case, the number of keywords is the size of the array divided by the size of one element.

This computation is used in a #define statement to set the value of NKEYS:

#define  NKEYS  (sizeof  keytab  /  sizeof(struct  key))

Another way to write this is to divide the array size by the size of a specific element:

#define  NKEYS  (sizeof  keytab  /  sizeof(keytab[0]))

This has the advantage that it does not need to be changed if the type changes.

A sizeof can not be used in a #if line, because the preprocessor does not parse type names. But the expression in the #define is not evaluated by the preprocessor, so the code here is legal.Now for the function getword. We have written a more general getword than is necessary for this program, but it is not complicated. getword fetches the next “word” from the input, where a word is either a string of letters and digits beginning with a letter, or a single non-white space character. The function value is the first character of the word, or EOF for end of file, or the character itself if it is not alphabetic.

/* getword:  get next word or character from input */
   int getword(char *word, int lim)
       int c, getch(void);
       void ungetch(int);
       char *w = word;
       while (isspace(c = getch()))
       if (c != EOF)
           *w++ = c;
       if (!isalpha(c)) {
           *w = '\0';

return c; }

       for ( ; --lim > 0; w++)
           if (!isalnum(*w = getch())) {

break; }

*w = ‘\0’;

       return word[0];

getword uses the getch and ungetch. When the collection of an alphanumeric token stops, getword has gone one character too far. The call to ungetch pushes that character back on the input for the next call. getword also uses isspace to skip whitespace, isalpha to identify letters, and isalnum to identify letters and digits; all are from the standard header .

Pointers to Structures

To illustrate some of the considerations involved with pointers to and arrays of structures, let us write the keyword-counting program again, this time using pointers instead of array indices. The external declaration of keytab need not change, but main and binsearch do need modification.

   #include  < stdio.h >
   #include  < string.h >
   #include  < ctype.h >
   #define MAXWORD 100
   int getword(char *, int);
   struct key *binsearch(char *, struct key *, int);
   /* count C keywords; pointer version */
char word[MAXWORD];
struct key *p;
while (getword(word, MAXWORD) != EOF)
    if (isalpha(word[0]))
        if ((p=binsearch(word, keytab, NKEYS)) != NULL)
for (p = keytab; p < keytab + NKEYS; p++) if (p->count > 0)
        printf("%4d %s\n", p->count, p->word);
return 0;


   /* binsearch: find word in tab[0]...tab[n-1] */
   struct key *binsearch(char *word, struck key *tab, int n)
       int cond;
       struct key *low = &tab[0];
       struct key *high = &tab[n];
       struct key *mid;
       while (low < high) { mid = low + (high-low) / 2; if ((cond = strcmp(word, mid->word)) < 0)
               high = mid;
           else if (cond > 0)
               low = mid + 1;
               return mid;


       return NULL;

There are several things worthy of note here. First, the declaration of binsearch must indicate that it returns a pointer to struct key instead of an integer; this is declared both in the function prototype and in binsearch. If binsearch finds the word, it returns a pointer to it; if it fails, it returns NULLSecond, the elements of keytab are now accessed by pointers. This requires significant changes in binsearch. The initializers for low and high are now pointers to the beginning and just past the end of the table. The computation of the middle element can no longer be simply

mid  =  (low+high)  /  2   /*  WRONG  */

because the addition of pointers is illegal. Subtraction is legal, however, so high-low is the number of elements, and thus

mid  =  low  +  (high-low)  /  2

sets mid to the element halfway between low and highThe most important change is to adjust the algorithm to make sure that it does not generate an illegal pointer or attempt to access an element outside the array. The problem is that &tab[-1] and &tab[n] are both outside the limits of the array tab. The former is strictly illegal, and it is illegal to dereference the latter. The language definition does guarantee, however, that pointer arithmetic that involves the first element beyond the end of an array (that is, &tab[n]) will work correctly. In main we wrote

for  (p  =  keytab;  p  <  keytab  +  NKEYS;  p++)

If p is a pointer to a structure, arithmetic on p takes into account the size of the structure, so p++ increments p by the correct amount to get the next element of the array of structures, and the test stops the loop at the right time.

Don’t assume, however, that the size of a structure is the sum of the sizes of its members. Because of alignment requirements for different objects, there may be unnamed “holes” in a structure. Thus, for instance, if a char is one byte and an int four bytes, the structure

struct  {

char  c;

int  i;


might well require eight bytes, not five. The sizeof operator returns the proper value. Finally, an aside on program format: when a function returns a complicated type like a structure pointer, as in

struct  key  *binsearch(char  *word,  struct  key  *tab,  int  n)

the function name can be hard to see, and to find with a text editor. Accordingly an alternate style is sometimes used:

struct  key  *

binsearch(char  *word,  struct  key  *tab,  int  n)

This is a matter of personal taste; pick the form you like and hold to it.

 Self-referential Structures

Suppose we want to handle the more general problem of counting the occurrences of all the words in some input. Since the list of words isn’t known in advance, we can’t conveniently sort it and use a binary search. Yet we can’t do a linear search for each word as it arrives, to see if it’s already been seen; the program would take too long. (More precisely, its running time is likely to grow quadratically with the number of input words.) How can we organize the data to copy efficiently with a list or arbitrary words? One solution is to keep the set of words seen so far sorted at all times, by placing each word into its proper position in the order as it arrives. This shouldn’t be done by shifting words in a linear array, though – that also takes too long. Instead we will use a data structure called a binary tree.

The tree contains one “node” per distinct word; each node contains

  • A pointer to the text of the word,
  • A count of the number of occurrences,
  • A pointer to the left child node,
  • A pointer to the right child node.

No node may have more than two children; it might have only zero or one. The nodes are maintained so that at any node the left subtree contains only words that are lexicographically less than the word at the node, and the right subtree contains only words that are greater. This is the tree for the sentence “now is the time for all good men to come to the aid of their party”, as built by inserting each word as it is encountered:

To find out whether a new word is already in the tree, start at the root and compare the new word to the word stored at that node. If they match, the question is answered affirmatively. If the new record is less than the tree word, continue searching at the left child, otherwise at the right child. If there is no child in the required direction, the new word is not in the tree, and in fact the empty slot is the proper place to add the new word. This process is recursive, since the search from any node uses a search from one of its children. Accordingly, recursive routines for insertion and printing will be most natural.

Going back to the description of a node, it is most conveniently represented as a structure with four components:

struct  tnode  {  /*  the  tree  node:  */

char  *word ;         /*  points  to  the  text  */

int  count;              /*  number  of  occurrences  */

struct  tnode  *left;                /*  left  child  */

struct  tnode  *right;          /*  right  child  */


This recursive declaration of a node might look chancy, but it’s correct. It is illegal for a structure to contain an instance of itself, but

struct  tnode  *left;

declares left to be a pointer to a tnode, not a tnode itself.

Occasionally, one needs a variation of self-referential structures: two structures that refer to each other. The way to handle this is:

struct  t  {

/*  p  points  to  an  s  */

struct  s  *p;


struct  s  {

/*  q  points  to  a  t  */

struct  t  *q;


The code for the whole program is surprisingly small, given a handful of supporting routines like getword that we have already written. The main routine reads words with getword and installs them in the tree with addtree.

   #include  < stdio.h >
   #include  < string.h >
   #include  < ctype.h >
   #define MAXWORD 100
   struct tnode *addtree(struct tnode *, char *);
   void treeprint(struct tnode *);
   int getword(char *, int);
   /* word frequency count */
       struct tnode *root;
       char word[MAXWORD];
       root = NULL;
       while (getword(word, MAXWORD) != EOF)
           if (isalpha(word[0]))
               root = addtree(root, word);

return 0; }

The function addtree is recursive. A word is presented by main to the top level (the root) of the tree. At each stage, that word is compared to the word already stored at the node, and is percolated down to either the left or right subtree by a recursive call to adtree. Eventually, the word either matches something already in the tree (in which case the count is incremented), or a null pointer is encountered, indicating that a node must be created and added to the tree. If a new node is created, addtree returns a pointer to it, which is installed in the parent node.

struct tnode *talloc(void);
   char *strdup(char *);
   /* addtree:  add a node with w, at or below p */
   struct treenode *addtree(struct tnode *p, char *w)

int cond;

       if (p == NULL) {     /* a new word has arrived */
           p = talloc();    /* make a new node */
           p->word = strdup(w);
           p->count = 1;
           p->left = p->right = NULL;
       } else if ((cond = strcmp(w, p->word)) == 0)
           p->count++;      /* repeated word */
       else if (cond < 0)   /* less than into left subtree */
           p->left = addtree(p->left, w);
       else             /* greater than into right subtree */
           p->right = addtree(p->right, w);
       return p;


Storage for the new node is fetched by a routine talloc, which returns a pointer to a free space suitable for holding a tree node, and the new word is copied into a hidden space by strdup. (We will discuss these routines in a moment.) The count is initialized, and the two children are made null. This part of the code is executed only at the leaves of the tree, when a new node is being added. We have (unwisely) omitted error checking on the values returned by strdup and talloctreeprint prints the tree in sorted order; at each node, it prints the left subtree (all the words less than this word), then the word itself, then the right subtree (all the words greater). If you feel shaky about how recursion works, simulate treeprint as it operates on the tree shown above.

/* treeprint: in-order print of tree p */

void treeprint(struct tnode *p) {

if  (p  !=  NULL)  {


     printf(“%4d %s\n”, p->count, p->word);




A practical note: if the tree becomes “unbalanced” because the words don’t arrive in random order, the running time of the program can grow too much. As a worst case, if the words are already in order, this program does an expensive simulation of linear search. There are generalizations of the binary tree that do not suffer from this worst-case behavior, but we will not describe them here.

Before leaving this example, it is also worth a brief digression on a problem related to storage allocators. Clearly it’s desirable that there be only one storage allocator in a program, even though it allocates different kinds of objects. But if one allocator is to process requests for, say, pointers to chars and pointers to struct tnodes, two questions arise. First, how does it meet the requirement of most real machines that objects of certain types must satisfy alignment restrictions (for example, integers often must be located at even addresses)? Second, what declarations can cope with the fact that an allocator must necessarily return different kinds of pointers?Alignment requirements can generally be satisfied easily, at the cost of some wasted space, by ensuring that the allocator always returns a pointer that meets all alignment restrictions.

The question of the type declaration for a function like malloc is a vexing one for any language that takes its type-checking seriously. In C, the proper method is to declare that malloc returns a pointer to void, then explicitly coerce the pointer into the desired type with a cast. malloc and related routines are declared in the standard header . Thus talloc can be written as

   #include  < stdio.h >
   /* talloc:  make a tnode */
   struct tnode *talloc(void)
       return (struct tnode *) malloc(sizeof(struct tnode));

strdup merely copies the string given by its argument into a safe place, obtained by a call on malloc:

   char *strdup(char *s)   /* make a duplicate of s */

char *p;

       p = (char *) malloc(strlen(s)+1); /* +1 for '\0' */
       if (p != NULL)
           strcpy(p, s);
       return p;


malloc returns NULL if no space is available; strdup passes that value on, leaving error-handling to its caller. Storage obtained by calling malloc may be freed for re-use by calling free.

Table Lookup

In this section we will write the innards of a table-lookup package, to illustrate more aspects of structures. This code is typical of what might be found in the symbol table management routines of a macro processor or a compiler. For example, consider the #define statement. When a line like

#define IN 1

is encountered, the name IN and the replacement text 1 are stored in a table. Later, when the name IN appears in a statement like

state  =  IN;

it must be replaced by 1.

There are two routines that manipulate the names and replacement texts. install(s,t) records the name s and the replacement text t in a table; s and t are just character strings. lookup(s) searches for s in the table, and returns a pointer to the place where it was found, or NULL if it wasn’t there.The algorithm is a hash-search – the incoming name is converted into a small non-negative integer, which is then used to index into an array of pointers. An array element points to the beginning of a linked list of blocks describing names that have that hash value. It is NULL if no names have hashed to that value.

 A block in the list is a structure containing pointers to the name, the replacement text, and the next block in the list. A null next-pointer marks the end of the list.

struct  nlist  { .   /*  table  entry:  */

       struct  nlist  *next;     /*  next  entry  in  chain  */

      char  *name;    /*  defined  name  */

       char  *defn;  /*  replacement  text  */


The pointer array is just

#define  HASHSIZE  101

static  struct  nlist  *hashtab[HASHSIZE];       /*  pointer  table  */

The hashing function, which is used by both lookup and install, adds each character value in the string to a scrambled combination of the previous ones and returns the remainder modulo the array size. This is not the best possible hash function, but it is short and effective.

/* hash: form hash value for string s */ unsigned hash(char *s) {

unsigned  hashval;

   for  (hashval  =  0;  *s  !=  ‘\0’;  s++)

        hashval  =  *s  +  31  *  hashval;

       return  hashval  %  HASHSIZE;


Unsigned arithmetic ensures that the hash value is non-negative. The hashing process produces a starting index in the array hashtab; if the string is to be found anywhere, it will be in the list of blocks beginning there. The search is performed by lookup. If lookup finds the entry already present, it returns a pointer to it; if not, it returns NULL.

/*  lookup: look  for  s  in  hashtab  */

struct  nlist  *lookup(char  *s)


     struct  nlist  *np;

         for  (np  =  hashtab[hash(s)]; np  !=  NULL;  np  =  np->next)

             if  (strcmp(s,  np->name)  ==  0)

                     return np;      /* found */

                    return NULL;    /* not found */


The for loop in lookup is the standard idiom for walking along a linked list:

for  (ptr  =  head;  ptr  !=  NULL;  ptr  =  ptr->next)

install uses lookup to determine whether the name being installed is already present; if so, the new definition will supersede the old one. Otherwise, a new entry is created. install returns NULL if for any reason there is no room for a new entry.

   struct nlist *lookup(char *);
   char *strdup(char *);
   /* install:  put (name, defn) in hashtab */
   struct nlist *install(char *name, char *defn)
       struct nlist *np;
       unsigned hashval;
       if ((np = lookup(name)) == NULL) { /* not found */
           np = (struct nlist *) malloc(sizeof(*np));
           if (np == NULL || (np->name = strdup(name)) == NULL)
               return NULL;
           hashval = hash(name);
           np->next = hashtab[hashval];
           hashtab[hashval] = np;
       } else       /* already there */
           free((void *) np->defn);   /*free previous defn */
       if ((np->defn = strdup(defn)) == NULL)
           return NULL;

return np; }


C provides a facility called typedef for creating new data type names. For example, the declaration

typedef  int  Length;

makes the name Length a synonym for int. The type Length can be used in declarations, casts, etc., in exactly the same ways that the int type can be:

Length  len,  maxlen;

Length  *lengths[];

Similarly, the declaration

typedef  char  *String;

makes String a synonym for char * or character pointer, which may then be used in declarations and casts:

String p, lineptr[MAXLINES], alloc(int);

int strcmp(String, String);

p = (String) malloc(100);

Notice that the type being declared in a typedef appears in the position of a variable name, not right after the word typedef. Syntactically, typedef is like the storage classes extern, static, etc. We have used capitalized names for typedefs, to make them stand out. As a more complicated example, we could make typedefs for the tree nodes shown earlier in this chapter:

typedef  struct  tnode  *Treeptr;

         typedef  struct  tnode  {          /*  the  tree  node:  */

                char  *word;          /*  points  to  the  text  */

                int  count;             /*  number  of  occurrences  */

               struct  tnode  *left;           /*  left  child  */

              struct tnode *right;        /* right child */

 } Treenode;

This creates two new type keywords called Treenode (a structure) and Treeptr (a pointer to the structure). Then the routine talloc could become

Treeptr  talloc(void)


return  (Treeptr)  malloc(sizeof(Treenode));


It must be emphasized that a typedef declaration does not create a new type in any sense; it merely adds a new name for some existing type. Nor are there any new semantics: variables declared this way have exactly the same properties as variables whose declarations are spelled out explicitly. In effect, typedef is like #define, except that since it is interpreted by the compiler, it can cope with textual substitutions that are beyond the capabilities of the preprocessor. For example,

typedef  int  (*PFI)(char  *,  char  *);

creates the type PFI, for “pointer to function (of two char * arguments) returning int,” which can be used in contexts like

PFI  strcmp,  numcmp;

Besides purely aesthetic issues, there are two main reasons for using typedefs. The first is to parameterize a program against portability problems. If typedefs are used for data types that may be machine-dependent, only the typedefs need change when the program is moved. One common situation is to use typedef names for various integer quantities, then make an appropriate set of choices of short, int, and long for each host machine. Types like size_t and ptrdiff_t from the standard library are examples. The second purpose of typedefs is to provide better documentation for a program – a type called Treeptr may be easier to understand than one declared only as a pointer to a complicated structure.


A union is a variable that may hold (at different times) objects of different types and sizes, with the compiler keeping track of size and alignment requirements. Unions provide a way to manipulate different kinds of data in a single area of storage, without embedding any machine-dependent information in the program. They are analogous to variant records in pascal. As an example such as might be found in a compiler symbol table manager, suppose that a constant may be an int, a float, or a character pointer. The value of a particular constant must be stored in a variable of the proper type, yet it is most convenient for table management if the value occupies the same amount of storage and is stored in the same place regardless of its type. This is the purpose of a union – a single variable that can legitimately hold any of one of several types. The syntax is based on structures:

union  u_tag  {

    int  ival;

   float  fval;

   char  *sval;

}  u;

The variable u will be large enough to hold the largest of the three types; the specific size is implementation-dependent. Any of these types may be assigned to u and then used in expressions, so long as the usage is consistent: the type retrieved must be the type most recently stored. It is the programmer’s responsibility to keep track of which type is currently stored in a union; the results are implementation-dependent if something is stored as one type and extracted as another. Syntactically, members of a union are accessed as




just as for structures. If the variable utype is used to keep track of the current type stored in u, then one might see code such as

if  (utype  ==  INT)

        printf(“%d\n”,  u.ival);

if  (utype  ==  FLOAT)

      printf(“%f\n”,  u.fval);

if  (utype  ==  STRING)

     printf(“%s\n”,  u.sval);


    printf(“bad  type  %d  in  utype\n”,  utype);

Unions may occur within structures and arrays, and vice versa. The notation for accessing a member of a union in a structure (or vice versa) is identical to that for nested structures. For example, in the structure array defined by

struct  {

    char  *name;

    int  flags;

   int  utype;

union  {

   int  ival;

  float  fval;

  char  *sval;

   }  u;

}  symtab[NSYM];

the member ival is referred to as


and the first character of the string sval by either of



In effect, a union is a structure in which all members have offset zero from the base, the structure is big enough to hold the “widest” member, and the alignment is appropriate for all of the types in the union. The same operations are permitted on unions as on structures: assignment to or copying as a unit, taking the address, and accessing a member.A union may only be initialized with a value of the type of its first member; thus union u described above can only be initialized with an integer value.The storage allocator shows how a union can be used to force a variable to be aligned on a particular kind of storage boundary.


When storage space is at a premium, it may be necessary to pack several objects into a single machine word; one common use is a set of single-bit flags in applications like compiler symbol tables. Externally-imposed data formats, such as interfaces to hardware devices, also often require the ability to get at pieces of a word. Imagine a fragment of a compiler that manipulates a symbol table. Each identifier in a program has certain information associated with it, for example, whether or not it is a keyword, whether or not it is external and/or static, and so on. The most compact way to encode such information is a set of one-bit flags in a single char or int.The usual way this is done is to define a set of “masks” corresponding to the relevant bit positions, as in

#define  KEYWORD 01

#define  EXTRENAL  02

#define  STATIC 04


enum  {  KEYWORD  =  01,  EXTERNAL  =  02,  STATIC  =  04  };

The numbers must be powers of two. Then accessing the bits becomes a matter of “bit-fiddling” with the shifting, masking, and complementing operators. Certain idioms appear frequently:

flags  |=  EXTERNAL  |  STATIC;

turns on the EXTERNAL and STATIC bits in flags, while

flags &= ~(EXTERNAL | STATIC);

turns them off, and

if  ((flags  &  (EXTERNAL  |  STATIC))  ==  0) 

is true if both bits are off.

Although these idioms are readily mastered, as an alternative C offers the capability of defining and accessing fields within a word directly rather than by bitwise logical operators. A bit-field, or field for short, is a set of adjacent bits within a single implementation-defined storage unit that we will call a “word.” For example, the symbol table #defines above could be replaced by the definition of three fields:

struct  {

     unsigned  int  is_keyword  :  1;

     unsigned  int  is_extern :  1;

   unsigned  int  is_static :  1;

}  flags;

This defines a variable table called flags that contains three 1-bit fields. The number following the colon represents the field width in bits. The fields are declared unsigned int to ensure that they are unsigned quantities.

Individual fields are referenced in the same way as other structure members: flags.is_keyword, flags.is_extern, etc. Fields behave like small integers, and may participate in arithmetic expressions just like other integers. Thus the previous examples may be written more naturally as

flags.is_extern  =  flags.is_static  =  1;

to turn the bits on;

flags.is_extern  =  flags.is_static  =  0;

to turn them off; and

if  (flags.is_extern  ==  0  &&  flags.is_static  ==  0)  

to test them. Almost everything about fields is implementation-dependent. Whether a field may overlap a word boundary is implementation-defined. Fields need not be names; unnamed fields (a colon and width only) are used for padding. The special width 0 may be used to force alignment at the next word boundary. Fields are assigned left to right on some machines and right to left on others. This means that although fields are useful for maintaining internally-defined data structures, the question of which end comes first has to be carefully considered when picking apart externally-defined data; programs that depend on such things are not portable. Fields may be declared only as ints; for portability, specify signed or unsigned explicitly. They are not arrays and they do not have addresses, so the & operator cannot be applied on them.

Input and Output

Input and output are not part of the C language itself, so we have not emphasized them in our presentation thus far. Nonetheless, programs interact with their environment in much more complicated ways than those we have shown before. In this chapter we will describe the standard library, a set of functions that provide input and output, string handling, storage management, mathematical routines, and a variety of other services for C programs. We will concentrate on input and output The ANSI standard defines these library functions precisely, so that they can exist in compatible form on any system where C exists. Programs that confine their system interactions to facilities provided by the standard library can be moved from one system to another without change.The properties of library functions are specified in more than a dozen headers; we have already seen several of these, including , , and . We will not present the entire library here, since we are more interested in writing C programs that use it.

Standard Input and Output

As we said, the library implements a simple model of text input and output. A text stream consists of a sequence of lines; each line ends with a newline character. If the system doesn’t operate that way, the library does whatever necessary to make it appear as if it does. For instance, the library might convert carriage return and linefeed to newline on input and back again on output. The simplest input mechanism is to read one character at a time from the standard input, normally the keyboard, with getchar:

int  getchar(void)

getchar returns the next input character each time it is called, or EOF when it encounters end of file. The symbolic constant EOF is defined in . The value is typically -1, bus tests should be written in terms of EOF so as to be independent of the specific value. In many environments, a file may be substituted for the keyboard by using the < convention for input redirection: if a program prog uses getchar, then the command line

prog  <infile

causes prog to read characters from infile instead. The switching of the input is done in such a way that prog itself is oblivious to the change; in particular, the string “<infile” is not included in the command-line arguments in argv. Input switching is also invisible if the input comes from another program via a pipe mechanism: on some systems, the command line

otherprog  |  prog

runs the two programs otherprog and prog, and pipes the standard output of otherprog into the standard input for prog.

The function

int  putchar(int)

is used for output: putchar(c) puts the character c on the standard output, which is by default the screen. putchar returns the character written, or EOF is an error occurs. Again, output can usually be directed to a file with >filename: if prog uses putchar,

prog  >outfile

will write the standard output to outfile instead. If pipes are supported,

prog  |  anotherprog

puts the standard output of prog into the standard input of anotherprogOutput produced by printf also finds its way to the standard output. Calls to putchar and printf may be interleaved – output happens in the order in which the calls are made. Each source file that refers to an input/output library function must contain the line

#include   <stdio.h>

before the first reference. When the name is bracketed by < and > a search is made for the header in a standard set of places (for example, on UNIX systems, typically in the directory /usr/include).

Many programs read only one input stream and write only one output stream; for such programs, input and output with getchar, putchar, and printf may be entirely adequate, and is certainly enough to get started. This is particularly true if redirection is used to connect the output of one program to the input of the next. For example, consider the program lower, which converts its input to lower case:

#include   <stdio.h>

#include   <ctype.h>

main() /* lower: convert input to lower case*/ {

           int  c

                 while  ((c  =  getchar())  !=  EOF)


                       return  0;


The function tolower is defined in ; it converts an upper case letter to lower case, and returns other characters untouched. As we mentioned earlier, “functions” like getchar and putchar in &lt;stdio.h&gt;  and tolower in &lt;ctype.h&gt; are often macros, thus avoiding the overhead of a function call per character. Regardless of how the functions are implemented on a given machine, programs that use them are shielded from knowledge of the character set.

Formatted Output – printf

The output function printf translates internal values to characters. We have used printf informally in previous chapters. The description here covers most typical uses but is not complete.

int  printf(char  *format,  arg1,  arg2,  …);

printf converts, formats, and prints its arguments on the standard output under control of the format. It returns the number of characters printed. The format string contains two types of objects: ordinary characters, which are copied to the output stream, and conversion specifications, each of which causes conversion and printing of the next successive argument to printf. Each conversion specification begins with a % and ends with a conversion character. Between the % and the conversion character there may be, in order:

  • A minus sign, which specifies left adjustment of the converted argument.
  • A number that specifies the minimum field width. The converted argument will be printed in a field at least this wide. If necessary it will be padded on the left (or right, if left adjustment is called for) to make up the field width.
  • A period, which separates the field width from the precision.
  • A number, the precision, that specifies the maximum number of characters to be printed from a string, or the number of digits after the decimal point of a floating-point value, or the minimum number of digits for an integer.
  • An h if the integer is to be printed as a short, or l (letter ell) if as a long.

Conversion characters are shown in Table  If the character after the % is not a conversion specification, the behavior is undefined.

                               Table  Basic Printf Conversions


Argument type; Printed As


int; decimal number


int; unsigned octal number (without a leading zero)


int; unsigned hexadecimal number (without a leading 0x or 0X), using abcdef or ABCDEF for 10, …,15.


int; unsigned decimal number


int; single character


char *; print characters from the string until a ‘\0’ or the number of characters given by the precision.


double; [-]m.dddddd, where the number of d’s is given by the precision (default 6).


double; [-]m.dddddde+/-xx or [-]m.ddddddE+/-xx, where the number of d’s is given by the precision (default 6).


double; use %e or %E if the exponent is less than -4 or greater than or equal to the precision; otherwise use %f. Trailing zeros and a trailing decimal point are not printed.


void *; pointer (implementation-dependent representation).


no argument is converted; print a %

A width or precision may be specified as *, in which case the value is computed by converting the next argument (which must be an int). For example, to print at most max characters from a string s,

printf(“%.*s”,  max,  s);

Most of the format conversions have been illustrated in earlier chapters. One exception is the precision as it relates to strings. The following table shows the effect of a variety of specifications in printing “hello, world” (12 characters). We have put colons around each field so you can see it extent.


:hello,  world:


:hello,  world:


:hello,  wor:


:hello,  world:


:hello,  world:



:hello,  world



hello,  wor:


:hello,  wor


A warning: printf uses its first argument to decide how many arguments follow and what their type is. It will get confused, and you will get wrong answers, if there are not enough arguments of if they are the wrong type. You should also be aware of the difference between these two calls:

printf(s);   /*  FAILS  if  s  contains  %  */

 printf(“%s”, s);     /*  SAFE  */

The function sprintf does the same conversions as printf does, but stores the output in a string:

int  sprintf(char  *string,  char  *format,  arg1,  arg2,  …);

sprintf formats the arguments in arg1, arg2, etc., according to format as before, but places the result in string instead of the standard output; string must be big enough to receive the result.

Variable-length Argument Lists

This section contains an implementation of a minimal version of printf, to show how to write a function that processes a variable-length argument list in a portable way. Since we are mainly interested in the argument processing, minprintf will process the format string and arguments but will call the real printf to do the format conversions.

The proper declaration for printf is

int  printf(char  *fmt,  …)

where the declaration means that the number and types of these arguments may vary. The declaration can only appear at the end of an argument list. Our minprintf is declared as

void  minprintf(char  *fmt,  …)

since we will not return the character count that printf does. The tricky bit is how minprintf walks along the argument list when the list doesn’t even have a name. The standard header contains a set of macro definitions that define how to step through an argument list. The implementation of this header will vary from machine to machine, but the interface it presents is uniform. The type va_list is used to declare a variable that will refer to each argument in turn; in minprintf, this variable is called ap, for “argument pointer.” The macro va_start initializes ap to point to the first unnamed argument. It must be called once before ap is used. There must be at least one named argument; the final named argument is used by va_start to get started.

Each call of va_arg returns one argument and steps ap to the next; va_arg uses a type name to determine what type to return and how big a step to take. Finally, va_end does whatever cleanup is necessary. It must be called before the program returns.

These properties form the basis of our simplified printf:

#include  <stdarg.h>

   /* minprintf: minimal printf with variable argument list */
   void minprintf(char *fmt, ...)
       va_list ap; /* points to each unnamed arg in turn */
       char *p, *sval;
       int ival;
       double dval;
       va_start(ap, fmt); /* make ap point to 1st unnamed arg */
       for (p = fmt; *p; p++) {
           if (*p != '%') {

continue; }

           switch (*++p) {
           case 'd':
               ival = va_arg(ap, int);
               printf("%d", ival);
          case 'f':
              dval = va_arg(ap, double);
              printf("%f", dval);
         case 's':
              for (sval = va_arg(ap, char *); *sval; sval++)




       va_end(ap); /* clean up when done */

Formatted Input – Scanf

The function scanf is the input analog of printf, providing many of the same conversion facilities in the opposite direction.

int  scanf(char  *format,  …)

scanf reads characters from the standard input, interprets them according to the specification in format, and stores the results through the remaining arguments. The format argument is described below; the other arguments, each of which must be a pointer, indicate where the corresponding converted input should be stored. As with printf, this section is a summary of the most useful features, not an exhaustive list.

scanf stops when it exhausts its format string, or when some input fails to match the control specification. It returns as its value the number of successfully matched and assigned input items. This can be used to decide how many items were found. On the end of file, EOF is returned; note that this is different from 0, which means that the next input character does not match the first specification in the format string. The next call to scanf resumes searching immediately after the last character already converted.

There is also a function sscanf that reads from a string instead of the standard input:

int  sscanf(char  *string,  char  *format,  arg1,  arg2,  …)

It scans the string according to the format in format and stores the resulting values through arg1, arg2, etc. These arguments must be pointers.

The format string usually contains conversion specifications, which are used to control conversion of input. The format string may contain:

  • Blanks or tabs, which are not ignored.
  • Ordinary characters (not %), which are expected to match the next non-white space character of the input stream.
  • Conversion specifications, consisting of the character %, an optional assignment suppression character *, an optional number specifying a maximum field width, an optional h, l or L indicating the width of the target, and a conversion character.

A conversion specification directs the conversion of the next input field. Normally the result is places in the variable pointed to by the corresponding argument. If assignment suppression is indicated by the * character, however, the input field is skipped; no assignment is made. An input field is defined as a string of non-white space characters; it extends either to the next white space character or until the field width, is specified, is exhausted. This implies that scanf will read across boundaries to find its input, since newlines are white space. (White space characters are blank, tab, newline, carriage return, vertical tab, and formfeed.)

The conversion character indicates the interpretation of the input field. The corresponding argument must be a pointer, as required by the call-by-value semantics of C. Conversion characters are shown in Table

Table : Basic Scanf Conversions


Input Data; Argument type


decimal integer; int *


integer; int *. The integer may be in octal (leading 0) or hexadecimal (leading 0x or 0X).


octal integer (with or without leading zero); int *


unsigned decimal integer; unsigned int *


hexadecimal integer (with or without leading 0x or 0X); int *


characters; char *. The next input characters (default 1) are placed at the indicated spot. The normal skip-over white space is suppressed; to read the next non-white space character, use %1s


character string (not quoted); char *, pointing to an array of characters long enough for the string and a terminating ‘\0’ that will be added.


floating-point number with optional sign, optional decimal point and optional exponent;

float *


literal %; no assignment is made.

The conversion characters d, i, o, u, and x may be preceded by h to indicate that a pointer to short rather than int appears in the argument list, or by l (letter ell) to indicate that a pointer to long appears in the argument list.

As a first example, the rudimentary calculator can be written with scanf to do the input conversion:

#include   <stdio.h>

main() /*  rudimentary  calculator  */


          double  sum,  v;

          sum  =  0;

          while  (scanf(“%lf”,  &v)  ==  1)

                                     printf(“\t%.2f\n”,  sum  +=  v);

           return  0;


Suppose we want to read input lines that contain dates of the form

25  Dec  1988

The scanf statement is

int  day,  year;

char  monthname[20];

scanf(“%d  %s  %d”,  &day,  monthname,  &year);

No & is used with monthname, since an array name is a pointer. Literal characters can appear in the scanf format string; they must match the same characters in the input. So we could read dates of the form mm/dd/yy with the scanf statement:

int  day,  month,  year;

scanf(“%d/%d/%d”,  &month,  &day,  &year);

scanf ignores blanks and tabs in its format string. Furthermore, it skips over white space (blanks, tabs, newlines, etc.) as it looks for input values. To read input whose format is not fixed, it is often best to read a line at a time, then pick it apart with scanf. For example, suppose we want to read lines that might contain a date in either of the forms above. Then we could write

while  (getline(line,  sizeof(line))  >  0)  {

        if  (sscanf(line,  “%d  %s  %d”,  &day,  monthname,  &year)  ==  3)

                printf(“valid:  %s\n”,  line);  /*  25  Dec  1988  form  */

        else  if  (sscanf(line,  “%d/%d/%d”,  &month,  &day,  &year)  ==  3)

                   printf(“valid:  %s\n”,  line);  /*  mm/dd/yy  form  */


                   printf(“invalid:  %s\n”,  line);  /*  invalid  form  */


Calls to scanf can be mixed with calls to other input functions. The next call to any input function will begin by reading the first character not read by scanfA final warning: the arguments to scanf and sscanf must be pointers. By far the most common error is writing

scanf(“%d”,  n);

instead of

scanf(“%d”,  &n);

This error is not generally detected at compile time.

File Access

The examples so far have all read the standard input and written the standard output, which are automatically defined for a program by the local operating system. The next step is to write a program that accesses a file that is not already connected to the program. One program that illustrates the need for such operations is cat, which concatenates a set of named files into the standard output. cat is used for printing files on the screen, and as a general-purpose input collector for programs that do not have the capability of accessing files by name. For example, the command

cat  x.c  y.c

prints the contents of the files x.c and y.c (and nothing else) on the standard output. The question is how to arrange for the named files to be read – that is, how to connect the external names that a user thinks of to the statements that read the data. The rules are simple. Before it can be read or written, a file has to be opened by the library function fopen. fopen takes an external name like x.c or y.c, does some housekeeping and negotiation with the operating system (details of which needn’t concern us), and returns a pointer to be used in subsequent reads or writes of the file.

This pointer, called the file pointer, points to a structure that contains information about the file, such as the location of a buffer, the current character position in the buffer, whether the file is being read or written, and whether errors or end of file have occurred. Users don’t need to know the details, because the definitions obtained from include a structure declaration called FILE. The only declaration needed for a file pointer is exemplified by

FILE  *fp;

FILE  *fopen(char  *name,  char  *mode);

This says that fp is a pointer to a FILE, and fopen returns a pointer to a FILE. Notice that FILE is a type name, like int, not a structure tag; it is defined with a typedefThe call to fopen in a program is

fp  =  fopen(name,  mode);

The first argument of fopen is a character string containing the name of the file. The second argument is the mode, also a character string, which indicates how one intends to use the file. Allowable modes include read (“r”), write (“w”), and append (“a”). Some systems distinguish between text and binary files; for the latter, a “b” must be appended to the mode string.

If a file that does not exist is opened for writing or appending, it is created if possible. Opening an existing file for writing causes the old contents to be discarded, while opening for appending preserves them. Trying to read a file that does not exist is an error, and there may be other causes of error as well, like trying to read a file when you don’t have permission. If there is any error, fopen will return NULL. (The error can be identified more precisely.)The next thing needed is a way to read or write the file once it is open. getc returns the next character from a file; it needs the file pointer to tell it which file.

int  getc(FILE  *fp)

getc returns the next character from the stream referred to by fp; it returns EOF for end of file or error. putc is an output function:

int  putc(int  c,  FILE  *fp)

putc writes the character c to the file fp and returns the character written, or EOF if an error occurs. Like getchar and putchar, getc and putc may be macros instead of functions. When a C program is started, the operating system environment is responsible for opening three files and providing pointers for them. These files are the standard input, the standard output, and the standard error; the corresponding file pointers are called stdin, stdout, and stderr, and are declared in . Normally stdin is connected to the keyboard and stdout and stderr are connected to the screen, but stdin and stdout may be redirected to files or pipes.

getchar and putchar can be defined in terms of getc, putc, stdin, and stdout as follows:

#define  getchar() getc(stdin)

#define  putchar(c) putc((c),  stdout)

For formatted input or output of files, the functions fscanf and fprintf may be used. These are identical to scanf and printf, except that the first argument is a file pointer that specifies the file to be read or written; the format string is the second argument.

int  fscanf(FILE  *fp,  char  *format,  …)

int  fprintf(FILE  *fp,  char  *format,  …)

With these preliminaries out of the way, we are now in a position to write the program cat to concatenate files. The design is one that has been found convenient for many programs. If there are command-line arguments, they are interpreted as filenames, and processed in order. If there are no arguments, the standard input is processed.

#include  <stdio.h>
/* cat:  concatenate files, version 1 */
main(int argc, char *argv[])
    FILE *fp;
    void filecopy(FILE *, FILE *)
    if (argc == 1) /* no args; copy standard input */
        filecopy(stdin, stdout);
       while(--argc > 0)


    if ((fp = fopen(*++argv, "r")) == NULL) {
        printf("cat: can't open %s\n, *argv);
        return 1;
    } else {
       filecopy(fp, stdout);

} return 0;

    /* filecopy:  copy file ifp to file ofp */
    void filecopy(FILE *ifp, FILE *ofp)

int c;

        while ((c = getc(ifp)) != EOF)
            putc(c, ofp);


The file pointers stdin and stdout are objects of type FILE *. They are constants, however, not variables, so it is not possible to assign to them.

The function

int  fclose(FILE  *fp)

is the inverse of fopen, it breaks the connection between the file pointer and the external name that was established by fopen, freeing the file pointer for another file. Since most operating systems have some limit on the number of files that a program may have open simultaneously, it’s a good idea to free the file pointers when they are no longer needed, as we did in cat. There is also another reason for fclose on an output file – it flushes the buffer in which putc is collecting output. fclose is called automatically for each open file when a program terminates normally. (You can close stdin and stdout if they are not needed. They can also be reassigned by the library function freopen.)

Error Handling – Stderr and Exit

The treatment of errors in cat is not ideal. The trouble is that if one of the files can’t be accessed for some reason, the diagnostic is printed at the end of the concatenated output. That might be acceptable if the output is going to a screen, but not if it’s going into a file or into another program via a pipeline. To handle this situation better, a second output stream, called stderr, is assigned to a program in the same way that stdin and stdout are. Output written on stderr normally appears on the screen even if the standard output is redirected. Let us revise cat to write its error messages on the standard error.

#include   <stdio.h>

   /* cat:  concatenate files, version 2 */
   main(int argc, char *argv[])
       FILE *fp;
       void filecopy(FILE *, FILE *);
       char *prog = argv[0];  /* program name for errors */
       if (argc == 1 ) /* no args; copy standard input */
           filecopy(stdin, stdout);
           while (--argc > 0)
               if ((fp = fopen(*++argv, "r")) == NULL) {
                   fprintf(stderr, "%s: can't open %s\n",
                           prog, *argv);
               } else {
                   filecopy(fp, stdout);
       if (ferror(stdout)) {
           fprintf(stderr, "%s: error writing stdout\n", prog);





The program signals errors in two ways. First, the diagnostic output produced by fprintf goes to stderr, so it finds its way to the screen instead of disappearing down a pipeline or into an output file. We included the program name, from argv[0], in the message, so if this program is used with others, the source of an error is identified.

Second, the program uses the standard library function exit, which terminates program execution when it is called. The argument of exit is available to whatever process called this one, so the success or failure of the program can be tested by another program that uses this one as a sub-process. Conventionally, a return value of 0 signals that all is well; non-zero values usually signal abnormal situations. exit calls fclose for each open output file, to flush out any buffered output.Within main, return expr is equivalent to exit(expr). exit has the advantage that it can be called from other functions, and that calls to it can be found with a pattern-searching program.The function ferror returns non-zero if an error occurred on the stream fp.

int  ferror(FILE  *fp)

Although output errors are rare, they do occur (for example, if a disk fills up), so a production program should check this as well.The function feof(FILE *) is analogous to ferror; it returns non-zero if end of file has occurred on the specified file.

int  feof(FILE  *fp)

We have generally not worried about exit status in our small illustrative programs, but any serious program should take care to return sensible, useful status values.

Line Input and Output

The standard library provides an input and output routine fgets that is similar to the getline function that we have used in earlier chapters:

char  *fgets(char  *line,  int  maxline,  FILE  *fp)

fgets reads the next input line (including the newline) from file fp into the character array line; at most maxline-1 characters will be read. The resulting line is terminated with ‘\0’. Normally fgets returns line; on end of file or error it returns NULL. (Our getline returns the line length, which is a more useful value; zero means end of file.) For output, the function fputs writes a string (which need not contain a newline) to a file:

int  fputs(char  *line,  FILE  *fp)

It returns EOF if an error occurs, and non-negative otherwise. The library functions gets and puts are similar to fgets and fputs, but operate on stdin and stdout. Confusingly, gets deletes the terminating ‘\n’, and puts adds it. To show that there is nothing special about functions like fgets and fputs, here they are, copied from the standard library on our system:

   /* fgets:  get at most n chars from iop */
   char *fgets(char *s, int n, FILE *iop)
       register int c;
       register char *cs;
       cs = s;
       while (--n > 0 && (c = getc(iop)) != EOF)
           if ((*cs++ = c) == '\n')

*cs = ‘\0’;

       return (c == EOF && cs == s) ? NULL : s;
   /* fputs:  put string s on file iop */
   int fputs(char *s, FILE *iop)

int c;

       while (c = *s++)
           putc(c, iop);
       return ferror(iop) ? EOF : 0;

For no obvious reason, the standard specifies different return values for ferror and fputs.

It is easy to implement our getline from fgets:

/* getline: read a line, return length */

int getline(char *line, int max) {

       if  (fgets(line,  max,  stdin)  ==  NULL)

                return  0;


               return  strlen(line);


Miscellaneous Functions

The standard library provides a wide variety of functions. This section is a brief synopsis of the most useful.

String Operations

We have already mentioned the string functions strlen, strcpy, strcat, and strcmp, found in . In the following, s and t are char *‘s, and c and n are ints.

strcat(s,t)             concatenate t to end of s

strncat(s,t,n)       concatenate n characters of t to end of s

strcmp(s,t)           return negative, zero, or positive for s  <  t, s  ==  t, s  >  t

strncmp(s,t,n)     same as strcmp but only in first n characters

strcpy(s,t)            copy t to s

strncpy(s,t,n)      copy at most n characters of t to s

strlen(s)               return length of s

strchr(s,c)            return pointer to first c in s, or NULL if not present

strrchr(s,c)          return pointer to last c in s, or NULL if not present

Character Class Testing and Conversion

Several functions from perform character tests and conversions. In the following, c is an int that can be represented as an unsigned char or EOF. The function returns int.

isalpha(c) non-zero if c is alphabetic, 0 if not

isupper(c) non-zero if c is upper case, 0 if not

islower(c) non-zero if c is lower case, 0 if not

isdigit(c) non-zero if c is digit, 0 if not

isalnum(c) non-zero if isalpha(c) or isdigit(c), 0 if not

isspace(c) non-zero if c is blank, tab, newline, return, formfeed, vertical tab toupper(c) return c converted to upper case tolower(c) return c converted to lower case


The standard library provides a rather restricted version of the function ungetch ; it is called ungetc.

int  ungetc(int  c,  FILE  *fp)

pushes the character c back onto file fp, and returns either c, or EOF for an error. Only one character of pushback is guaranteed per file. ungetc may be used with any of the input functions like scanf, getc, or getchar.

Command Execution

The function system(char *s) executes the command contained in the character string s, then resumes execution of the current program. The contents of s depend strongly on the local operating system. As a trivial example, on UNIX systems, the statement


causes the program date to be run; it prints the date and time of day on the standard output. system returns a system-dependent integer status from the command executed. In the UNIX system, the status return is the value returned by exit.

Storage Management

The functions malloc and calloc obtain blocks of memory dynamically.

void  *malloc(size_t  n)

returns a pointer to n bytes of uninitialized storage, or NULL if the request cannot be satisfied.

void  *calloc(size_t  n,  size_t  size)

returns a pointer to enough free space for an array of n objects of the specified size, or NULL if the request cannot be satisfied. The storage is initialized to zero.

The pointer returned by malloc or calloc has the proper alignment for the object in question, but it must be cast into the appropriate type, as in

int  *ip;

ip  =  (int  *)  calloc(n,  sizeof(int));

free(p) frees the space pointed to by p, where p was originally obtained by a call to malloc or calloc. There are no restrictions on the order in which space is freed, but it is a ghastly error to free something not obtained by calling malloc or calloc.

It is also an error to use something after it has been freed. A typical but incorrect piece of code is this loop that frees items from a list:

for (p = head; p != NULL; p = p->next)       /* WRONG */                   free(p);

The right way is to save whatever is needed before freeing:

for  (p  =  head;  p  !=  NULL;  p  =  q)  {

       q  =  p->next;



Mathematical Functions

There are more than twenty mathematical functions declared in ; here are some of the more frequently used. Each takes one or two double arguments and returns a double.


sine of x, x in radians


cosine of x, x in radians


arctangent of y/x, in radians


exponential function ex


natural (base e) logarithm of x (x>0)


common (base 10) logarithm of x (x>0)




square root of x (x>0)


absolute value of x

Random Number generation

The function rand() computes a sequence of pseudo-random integers in the range zero to RAND_MAX, which is defined in . One way to produce random floating-point numbers greater than or equal to zero but less than one is

#define  frand()  ((double)  rand()  /  (RAND_MAX+1.0))

(If your library already provides a function for floating-point random numbers, it is likely to have better statistical properties than this one.)

The function srand(unsigned) sets the seed for rand. The portable implementation of rand and srand suggested by the standard.

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This Is A Custom Widget

This Sliding Bar can be switched on or off in theme options, and can take any widget you throw at it or even fill it with your custom HTML Code. Its perfect for grabbing the attention of your viewers. Choose between 1, 2, 3 or 4 columns, set the background color, widget divider color, activate transparency, a top border or fully disable it on desktop and mobile.