6.7 Typedef

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;

in the sort program of Chapter 5.

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.

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