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Pointer Declarations

A "pointer declaration" names a pointer variable and specifies the type of the object to which the variable points. A variable declared as a pointer holds a memory address.

Syntax

declarator :
pointer opt direct-declarator
direct-declarator :
identifier
( declarator )
direct-declarator [ constant-expression opt ]
direct-declarator ( parameter-type-list )
direct-declarator ( identifier-list opt )
pointer :
* type-qualifier-list opt
* type-qualifier-list opt pointer
type-qualifier-list :
type-qualifier
type-qualifier-list type-qualifier

The type-specifier gives the type of the object, which can be any basic, structure, or union type. Pointer variables can also point to functions, arrays, and other pointers. (For information on declaring and interpreting more complex pointer types, refer to Interpreting More Complex Declarators.)

By making the type-specifier void, you can delay specification of the type to which the pointer refers. Such an item is referred to as a "pointer to void" and is written as void *. A variable declared as a pointer to void can be used to point to an object of any type. However, to perform most operations on the pointer or on the object to which it points, the type to which it points must be explicitly specified for each operation. (Variables of type char * and type void * are assignment-compatible without a type cast.) Such conversion can be accomplished with a type cast (see Type-Cast Conversions for more information).

The type-qualifier can be either const or volatile, or both. These specify, respectively, that the pointer cannot be modified by the program itself (const), or that the pointer can legitimately be modified by some process beyond the control of the program (volatile). (See Type Qualifiers for more information on const and volatile.)

The declarator names the variable and can include a type modifier. For example, if declarator represents an array, the type of the pointer is modified to be a pointer to an array.

You can declare a pointer to a structure, union, or enumeration type before you define the structure, union, or enumeration type. You declare the pointer by using the structure or union tag as shown in the examples below. Such declarations are allowed because the compiler does not need to know the size of the structure or union to allocate space for the pointer variable.

Examples

The following examples illustrate pointer declarations.

char *message; /* Declares a pointer variable named message */

The message pointer points to a variable with char type.

int *pointers[10];  /* Declares an array of pointers */

The pointers array has 10 elements; each element is a pointer to a variable with int type.

int (*pointer)[10]; /* Declares a pointer to an array of 10 elements */

The pointer variable points to an array with 10 elements. Each element in this array has int type.

int const *x;      /* Declares a pointer variable, x,
                      to a constant value */ 

The pointer x can be modified to point to a different int value, but the value to which it points cannot be modified.

const int some_object = 5 ;
int other_object = 37;
int *const y = &fixed_object;
const volatile *const z = &some_object;
int *const volatile w = &some_object;

The variable y in these declarations is declared as a constant pointer to an int value. The value it points to can be modified, but the pointer itself must always point to the same location: the address of fixed_object. Similarly, z is a constant pointer, but it is also declared to point to an int whose value cannot be modified by the program. The additional specifier volatile indicates that although the value of the const int pointed to by z cannot be modified by the program, it could legitimately be modified by a process running concurrently with the program. The declaration of w specifies that the program cannot change the value pointed to and that the program cannot modify the pointer.

struct list *next, *previous; /* Uses the tag for list */

This example declares two pointer variables, next and previous, that point to the structure type list. This declaration can appear before the definition of the list structure type (see the next example), as long as the list type definition has the same visibility as the declaration.

struct list 
{
    char *token;
    int count;
    struct list *next;
} line;

The variable line has the structure type named list. The list structure type has three members: the first member is a pointer to a char value, the second is an int value, and the third is a pointer to another list structure.

struct id 
{
    unsigned int id_no;
    struct name *pname;
} record;

The variable record has the structure type id. Note that pname is declared as a pointer to another structure type named name. This declaration can appear before the name type is defined.

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