3.4 Derived Types

There are five derived types in C:

The following sections describe these derived types.

A derived type is formed by using one or more basic types in combination. Using derived types, an infinite variety of new types can be formed. The array and structure types are collectively called the aggregate types. Note that the aggregate types do not include union types, but a union may contain an aggregate member.

3.4.1 Function Type

A function type describes a function that returns a value of a specified type. If the function returns no value, it should be declared as "function returning void " as follows:

void function1 ();

In the following example, the data type for the function is "function returning int ":

int uppercase(int lc)
  int uc = lc + 0X20;
  return uc;

Chapter 4 discusses declarations in general. Chapter 5 covers functions specifically, including their declarations, parameters, and argument passing.

3.4.2 Pointer Type

A pointer type describes a value that represents the address of an object of a stated type. A pointer is stored as an integral value that references the address of the target object. Pointer types are derived from other types, called the referenced type of the pointer. For example:

int *p;          /*  p is a pointer to an int type                 */
double *q();     /*  q is a function returning a pointer to an
                     object of type double                         */
int (*r)[5];     /*  r is a pointer to an array of five elements   */
                 /*  (r holds the address to the first element of
                     the array)                                    */
const char s[6]; /*  s is a const-qualified array of 6 elements    */

The pointer itself can have any storage class, but the object addressed by the pointer cannot have the register storage class or be a bit field. Pointers to qualified or unqualified versions of compatible types have the same representation and alignment requirements as the target type. Pointers to other types need not have the same representation or alignment requirements.

The construction void * designates a generic "pointer to void " type. The void * construction can be used to point to an object of any type, and it is most useful when a pointer is needed to point to the address of objects with different or unknown types (such as in a function prototype). A pointer to void can also be converted to or from a pointer of any other type, and has the same representation and alignment requirements as a pointer to a character type.

A pointer to the address 0 (zero) is called a null pointer. Null pointers are often used to indicate that no more members of a list exist (for example, when using pointers to show the next member of the list). Dereferencing a null pointer with the * or subscripting operators leads to unpredictable and usually very unfavorable results.

See Chapter 4 for details on the syntax of pointer declarations.

3.4.3 Array Type

An array type can be formed from any valid completed type. Completion of an array type requires that the number and type of array members be explicitly or implicitly specified. The member types can be completed in the same or a different compilation unit. Arrays cannot be of void or function type, since the void type cannot be completed and function types are not object types requiring storage.

Typically, arrays are used to perform operations on some homogeneous set of values. The size of the array type is determined by the data type of the array and the number of elements in the array. Each element in an array has the same type. For example, the following definition creates an array of four characters:

char x[] = "Hi!"   /*  Declaring an array x   */;

Each of the elements has the size of a char object, 8 bits. The size of the array is determined by its initialization; in the previous example, the array has three explicit elements plus one null character. Four elements of 8 bits each results in an array with a size of 32 bits.

An array is allocated contiguously in memory, and cannot be empty (that is, have no members). An array can have only one dimension. To create an array of "two dimensions," declare an array of arrays, and so on.

It is possible to declare an array of unknown size; this sort of declaration is called an incomplete array declaration, because the size is not specified. The following example shows an incomplete declaration:

int x[];

The size of an array declared in this manner must be specified elsewhere in the program. (See Section 4.7 for more information on declaring incomplete arrays and initializing arrays.)

Character strings (string literals) are stored in the form of an array of char or wchar_t type, and are terminated by the null character (\0).

An array in C has only one dimension. An array of arrays can be declared, however, to create a multidimensional array. The elements of these arrays are stored in increasing addresses so that the rightmost subscript varies most rapidly. This is called row- major order, and is analogous to a car's odometer. For example, in an array of two arrays declared as int a[2][3]; the elements are stored in this order:

a[0][0], a[0][1], a[0][2], a[1][0], a[1][1], a[1][2]

3.4.4 Structure Type

A structure type is a sequentially allocated nonempty set of objects, called members. Structures let you group heterogeneous data. They are much like records in Pascal. Unlike arrays, the elements of a structure need not be of the same data type. Also, elements of a structure are accessed by name, not by subscript. The following example declares a structure employee , with two structure variables (ed and mary ) of the structure type employee :

struct employee { char name[30]; int age; int empnumber; };
struct employee ed, mary;

Structure members can have any type except an incomplete type, such as the void type or a function type. Structures can contain pointers to objects of their own type, but they cannot contain an object of their own type as a member; such an object would have an incomplete type. For example:

struct employee {
  char name[30];
  struct employee div1;       /*  This is invalid. */
  int *f();

The following example, however, is valid:

struct employee {
  char name[30];
  struct employee *div1;/*  Member can contain pointer to employee
                            structure.                             */
  int (*f)();           /*  Pointer to a function returning int    */

The name of a declared structure member must be unique within the structure, but it can be used in another nested or unnested structure or name spaces to refer to a different object. For example:

struct {
  int a;
  struct {
    int a;  /* This 'a' refers to a different object
               than the previous 'a'               */

Chapter 4 contains more examples on structures and their declarations.

The compiler assigns storage for structure members in the order of member declaration, with increasing memory addresses for subsequent members. The first member always begins at the starting address of the structure itself. Subsequent members are aligned per the alignment unit, which may differ depending on the member sizes in the structure. A structure may contain padding (unused bits) so that members of an array of such structures are properly aligned, and the size of the structure is the amount of storage necessary for all members plus any padded space needed to meet alignment requirements. See your system's DEC C documentation for platform-specific information about structure alignment and representation.

A pragma is available to change the alignment of a structure on one platform to match that of structures on other platforms. See Section B.29 for more information on this pragma.

3.4.5 Union Type

A union type can store objects of different types at the same location in memory. The different union members can occupy the same location at different times in the program. The declaration of a union includes all members of the union, and lists the possible object types the union can hold. The union can hold any one member at a time-subsequent assignments of other members to the union overwrite the existing object in the same storage area.

Unions can be named with any valid identifier. An empty union cannot be declared, nor can a union contain an instance of itself. A member of a union cannot have a void , function, or incomplete type. Unions can contain pointers to unions of their type.

Another way to look at a union is as a single object that can represent objects of different types at different times. Unions let you use objects whose type and size can change as the program progresses, without using machine-dependent constructions. Some other languages call this concept a variant record.

The syntax for defining unions is very similar to that for structures. Each union type definition creates a unique type. Names of union members must be unique within the union, but they can be duplicated in other nested or unnested unions or name spaces. For example:

union {
  int a;
  union {
    int a;  /* This 'a' refers to a different object
               than the previous 'a'                */

The size of a union is the amount of storage necessary for its largest member, plus any padding needed to meet alignment requirements.

Once a union is defined, a value can be assigned to any of the objects declared in the union declaration. For example:

union name {
  double dvalue;
  struct x { int value1; int value2; };
  float fvalue;
} alberta;
alberta.dvalue = 3.141596; /* Assigns the value of pi to the union object */

Here, alberta can hold a double , struct , or float value. The programmer has responsibility for tracking the current type of object contained in the union. An assignment expression can be used to change the type of value held in the union.

Undefined behavior results when a union is used to store a value of one type, and then the value is accessed through another type. For example:

    Assume that `node' is a typedef_name for objects for which
    information has been entered into a hash table;

    `hash_entry' is a structure describing an entry in the hash table.
    The member `hash_value' is a pointer to the relevant `node'.
typedef struct hash_entry
   struct hash_entry *next_hash_entry;
   node   *hash_value;
   /* ... other information may be present ... */
} hash_entry;

extern hash_entry *hash_table [512];

    `hash_pointer' is a union whose members are a pointer to a
    `node' and a structure containing three bit fields that
    overlay the pointer value.  Only the second bit field is
    being used, to extract a value from the middle
    of the pointer to be used as an index into the hash table.
    Note that nine bits gives a range of values from 0 to 511;
    hence, the size of `hash_table' above.
typedef union
   node *node_pointer;
    unsigned : 4;
    unsigned  index : 9;
    unsigned :19;
   } bits;
} hash_pointer;

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