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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] |
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;
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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();
};
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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 */
};
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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' */
};
};
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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 Compaq 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' */
};
};
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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 */
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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;
struct
{
unsigned : 4;
unsigned index : 9;
unsigned :19;
} bits;
} hash_pointer;
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The void type is an incomplete type that cannot be completed.
The void type has three important uses:
The following example shows how void is used to define a function, with no parameters, that does not return a value:
void message(void)
{
printf ("Stop making sense!");
}
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The next example shows a function prototype for a function that accepts a pointer to any object as its first and second argument:
void memcopy (void *dest, void *source, int length); |
A pointer to the void type has the same representation and alignment requirements as a pointer to a character type. The void * type is a derived type based on void .
The void type can also be used in a cast expression to explicitly discard or ignore a value. For example:
int tree(void);
void main()
{
int i;
for (; ; (void)tree()){...} /* void cast is valid */
for (; (void)tree(); ;){...} /* void cast is NOT valid, because the */
/* value of the second expression in a */
/* for statement is used */
for ((void)tree(); ;) {...} /* void cast is valid */
}
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A
void
expression has no value, and cannot be used in any context where a
value is required.
3.6 Enumerated Types
An enumerated type is used to specify the possible values of an object
from a predefined list.
Elements of the list are called enumeration constants.
The main use of enumerated types is to explicitly show the symbolic
names, and therefore the intended purpose, of objects whose values can
be represented with integer values.
Objects of enumerated type are interpreted as objects of type
signed int
, and are compatible with objects of other integral types.
The compiler automatically assigns integer values to each of the enumeration constants, beginning with 0. The following example declares an enumerated object background_color with a list of enumeration constants:
enum colors { black, red, blue, green, white } background_color;
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Later in the program, a value can be assigned to the object background_color :
background_color = white; |
In this example, the compiler automatically assigns the integer values as follows: black = 0, red = 1, blue = 2, green = 3, and white = 4. Alternatively, explicit values can be assigned during the enumerated type definition:
enum colors { black = 5, red = 10, blue, green = 7, white = green+2 };
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Here, black equals the integer value 5, red = 10, blue = 11, green = 7, and white = 9. Note that blue equals the value of the previous constant ( red ) plus one, and green is allowed to be out of sequential order.
Because the ANSI C standard is not strict about assignment to
enumerated types, any assigned value not in the predefined list is
accepted without complaint.
3.7 Type Qualifiers
There are four type qualifiers:
Type qualifiers were introduced by the ANSI C standard to, in part, give you greater control over the compiler's optimizations. The const and volatile type qualifiers can be applied to any type. The __restrict type qualifier can be applied only to pointer types.
Note that because the __restrict type qualifier is not part of the 1989 ANSI C standard, this keyword has double leading underscores. The next version (9X) of the C standard is expected to adopt the keyword restrict with the same semantics described in this section.
The use of const gives you a method of controlling write access to an object, and eliminates potential side effects across function calls involving that object. This is because a side effect is an alteration of an object's storage and const prohibits such alteration.
Use volatile to qualify an object that can be changed by other processes or hardware. The use of volatile disables optimizations with respect to referencing the object. If an object is volatile qualified, it may be changed between the time it is initialized and any subsequent assignments. Therefore, it cannot be optimized.
Function parameters, however, do not all share the type qualification of one parameter. For example:
int f( const int a, int b) /* a is const qualified; b is not */ |
When using a type qualifier with an array identifier, the elements of the array are qualified, not the array type itself.
The following declarations and expressions show the behavior when type qualifiers modify an array or structure type:
const struct s { int mem; } cs = { 1 };
struct s ncs; /* ncs is modifiable */
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; /* array of array of const */
/* int's */
int *pi;
const int *pci;
ncs = cs; /* Valid */
cs = ncs; /* Invalid, cs is const-qualified */
pi = &ncs.mem; /* Valid */
pi = &cs.mem; /* Violates type constraints for = operator */
pci = &cs.mem; /* Valid */
pi = a[0]; /* Invalid; a[0] has type "const int *" */
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Use the const type qualifier to qualify an object whose value cannot be changed. Objects qualified by the const keyword cannot be modified. This means that an object declared as const cannot serve as the operand in an operation that changes its value; for example, the ++ and -- operators are not allowed on objects qualified with const . Using the const qualifier on an object protects it from the side effects caused by operations that alter storage.
The declaration of const -qualified objects can be slightly more complicated than that for nonqualified types. Here are some examples, with explanatory comments:
const int x = 44; /* const qualification of int type --
the value of x cannot be modified */
const int *z; /* Pointer to a constant integer --
The value in the location pointed
to by z cannot be modified */
int * const ptr; /* A constant pointer -- a pointer
which will always point to the
same location */
const int *const p; /* A constant pointer to a constant
integer -- neither the pointer or
the integer can be modified */
const const int y; /* Illegal - redundant use of const */
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The following rules apply to the const type qualifier:
const struct employee {
char *name;
int birthdate; /* name, birthdate, job_code, and salary are */
int job_code; /* treated as though declared with const. */
float salary;
} a, b; /* All members of a and b are const-qualified*/
struct employee2 {
char *name;
const int birthdate; /* Only this member is qualified */
int job_code;
float salary;
} c, d;
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const int i = 0;
int j = 1;
const int *p = &i; /* Explicit const specifier required */
int *q = &j;
*p = 1; /* Error -- attempt to modify a const-
qualified object through a pointer */
*q = 1; /* OK */
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Any object whose type includes the volatile type qualifier indicates that the object should not be subject to compiler optimizations altering references to, or modifications of, the object.
volatile objects are especially prone to side effects. (See Section 2.5.) |
Optimizations that are defeated by using the volatile specifier can be categorized as follows:
An object without the volatile specifier does not compel the compiler to perform these optimizations; it indicates that the compiler has the freedom to apply the optimizations depending on program context and compiler optimization level.
The volatile qualifier forces the compiler to allocate memory for the volatile object, and to always access the object from memory. This qualifier is often used to declare that an object can be accessed in some way not under the compiler's control. Therefore, an object qualified by the volatile keyword can be modified or accessed in ways by other processes or hardware, and is especially vulnerable to side effects.
The following rules apply to the use of the volatile qualifier:
volatile volatile int x; |
volatile struct employee {
char *name;
int birthdate; /* name, birthdate, job_code, and salary are */
int job_code; /* treated as though declared with volatile. */
float salary;
} a,b; /* All members of a and b are volatile-qualified */
struct employee2 {
char *name;
volatile int birthdate; /* Only this member is qualified */
int job_code;
float salary;
} c, d;
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const int *intptr; volatile int x; intptr = &x; |
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