Declarations are used to introduce the identifiers used in a program and to specify their important attributes, such as type, storage class, and identifier name. A declaration that also causes storage to be reserved for an object or that includes the body of a function, is called a definition.

Section 4.1 covers general declaration syntax rules, Section 4.2 discusses initialization, and Section 4.3 describes external declarations.

The following kinds of identifiers can be declared. See the associated section for information on specific declaration and initialization syntax. Functions are discussed in Chapter 5.

• Simple objects ( Section 4.4)
• Enumeration constants ( Section 4.5)
• Pointers ( Section 4.6)
• Arrays ( Section 4.7)
• Structure and union members ( Section 4.8)
• Tags ( Section 4.10)

The general syntax of a declaration is as follows:

declaration:

declaration-specifiers init-declarator-listopt;

declaration-specifiers:

storage-class-specifier declaration-specifiersopt type-specifier declaration-specifiersopt type-qualifier declaration-specifiersopt

init-declarator-list:

init-declarator init_declarator-list , init-declarator

init-declarator:

declarator declarator = initializer

Note the following items about the general syntax of a declaration:

• The storage-class-specifier, type-qualifier, and type-specifier can be listed in any order. All are optional, but, except for function declarations, at least one such specifier or qualifier must be present. Placing the storage-class-specifier anywhere but at the beginning of the declaration is an obsolete style.
• Storage-class keywords are auto , static , extern , and register .
• Type qualifiers are const and volatile .
• The declarator is the name of the object or function being declared. A declarator can be as simple as a single identifier, or can be a complex construction declaring an array, structure, pointer, union, or function (such as *x , tree() , and treebar[10] ).
  A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. If the nested sequence of declarators in a full declarator contains a variable-length array type, the type specified by the full declarator is said to be variably modified.
• Initializers are optional and provide the initial value of an object. Initializers can be a single value or a brace-enclosed list of values, depending on the type of object being declared.
• A declaration determines the beginning of an identifier's scope.
• An identifier's linkage is determined by the declaration's placement and its specified storage class.

Consider the following example:

volatile static int data = 10;

This declaration shows a qualified type (a data type with a type qualifier -- in this case, int qualified by volatile ), a storage class ( static ), a declarator ( data ), and an initializer ( 10 ). This declaration is also a definition, because storage is reserved for the data object data .

The previous example is simple to interpret, but complex declarations are more difficult. See your platform-specific Compaq C documentation for more information about interpreting C declarations.

The following semantic rules apply to declarations:

• Empty declarations are illegal; declarations must contain at least one declarator, or specify a structure tag, union tag, or the members of an enumeration.
• Each declarator declares one identifier. There is no limit to the number of declarators in a declaration.
• At most, one storage-class specifier can be used in each object declaration. If none is provided, the auto storage class is assigned to objects declared inside a function definition, and the extern class is assigned to objects declared outside of a function definition.
• The only allowable (and optional) storage class for declaration of a function with block scope is extern .
• If no type-specifier is present, the default is signed int .
• A declarator is usable only over a certain range of the program, determined by the declarator's scope. The duration of its storage allocation is dependent on its storage class. See Section 2.3 for more information on scope and Section 2.10 for more information on storage classes.
• The usefulness of an identifier can be limited by its visibility, which can be hidden in some parts of the program. See Section 2.4 for more information on visibility.
• All declarations in the same scope that refer to the same object or function must have compatible types.
• If an object has no linkage, there can be no more than one declaration of the object with the same scope and in the same name space. Objects without linkage must have their type completed by the end of the declaration, or by the final initializer (if it has one). Section 2.8 describes linkage.

Storage Allocation

Storage is allocated to a data object in the following circumstances:

• If the object has no linkage, storage is allocated upon declaration of the object. If a block scope object with auto or register storage class is declared, storage is deallocated at the end of the block.
• If the object has internal linkage, storage is allocated upon the first definition of the object.
• If the object has external linkage, storage is allocated upon initialization of the object, which must occur only once for each object. If an object has only a tentative definition (see Section 2.9), the compiler acts as though there were a file scope definition of the object with an initializer of zero. Section 2.8 describes linkage in detail.

Initializers provide an initial value for objects, and follow this syntax:

initializer:

assignment-expr { initializer-list } { initializer-list, }

initializer-list:

designation-opt initializer initializer-list, designation-opt initializer

designation:

designator-list =

designator-list:

designator designator-list designator

designator:

[ constant-expr ] . identifier

Initialization of objects of each type is discussed in the following sections, but a few universal constraints apply to all initializations in C:

• The number of initializers cannot exceed the number of objects to be initialized. Initializers can number less than the number of objects to be initialized, in which case the remaining objects are initialized to zero.
• Constant expressions must be used in an initializer for an object that has static storage duration, or in an initializer list for an object that has an aggregate or union type.
• If an identifier's declaration has block scope, and the identifier has external or internal linkage, the declaration of the identifier cannot include an initializer.
• If an object that has static storage duration is not explicitly initialized, it is initialized implicitly as if every member with an arithmetic type were assigned 0, and every member with a pointer type were assigned a null pointer constant. If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.
• The initializer for a scalar object must be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression. The same type constraints and conversions apply as for simple assignment.
• If an aggregate object contains members that are aggregates or unions, or if the first member of a union is an aggregate or union, the initialization rules apply recursively to the aggregate members or contained unions. If an initializer list is used for an aggregate member or contained union, the initializers in that list initialize the members of the aggregate member or contained union. Otherwise, only enough initializers from the list are used to account for the object; any remaining members in the list are left to initialize the next member of the aggregate object. For example:

  struct t1 { int i; double d; }; union t2 { int i; double d; }; struct t3 { struct t1 s; union t2 u; }; struct t3 st[] = { /* complete initializer */ 1, 2, 0, 4, 0, 0, 7, 0, 0 };

  
  Given the previous declarations, the variable st is an array of 3 structures. Its initial contents are:

  s u ------ - st[0]: 1, 2.0, 0 st[1]: 4, 0.0, 0 st[2]: 7, 0.0, 0

  
  This variable can also be defined in the following ways---all four initializers are equivalent:

  struct t3 st[] = { /* partial initializer */ 1, 2, 0, 4, 0, 0, 7 }; struct t3 st[] = { /* nested and complete initializers */ {1, 2, 0}, {4, 0, 0}, {7, 0, 0} }; struct t3 st[] = { /* nested and partial initializers */ {1, 2}, {4}, {7} };

  
  For initialization of arrays, structures, and unions, see Sections 4.7.1, 4.8.4, and 4.8.5.

• For a description of initializers with designations for arrays and structures, see Section 4.9.
• Variant structures and unions are initialized just like normal structures and unions. See Section 4.8.4 and Section 4.8.5 for more information.

C has historically allowed initializers to be optionally surrounded by extra braces (to improve formatting clarity, for instance). These initializers are parsed differently depending on the type of parser used. Compaq C uses the parsing technique specified by the ANSI standard, known as the top-down parse. Programs depending on a bottom-up parse of partially braced initializers can yield unexpected results. The compiler generates a warning message when it encounters unnecessary braces in common C compatibility mode or when the error-checking compiler option is specified on the command line.

4.3 External Declarations

An object declaration outside of a function is called an external declaration. Contrast this with an internal declaration, which is a declaration made inside a function or block; the declaration is internal to that function or block, and is visible only to that function or block. The compiler recognizes an internally declared identifier from the point of the declaration to the end of the block.

If an object's declaration has file scope and an initializer, the declaration is also an external definition for the object. A C program consists of a sequence of external definitions of objects and functions.

Any definition reserves storage for the entity being declared. For example:

float fvalue = 15.0; /* external definition */ main () { int ivalue = 15; /* internal definition */ }

External data declarations and external function definitions take the same form as any data or function declaration (see Chapter 5 for standard function declaration syntax), and must follow these rules:

• The storage class of an object externally declared can be left unspecified, or it can be declared as extern or static (see Section 2.10). If it is unspecified, the default is the extern storage class, and linkage for the declared object is external. The type specifier may also be omitted, in which case the default type is int . Note that the storage-class-specifier, type-qualifier, and type-specifier cannot all be omitted from a declaration.
• If an object with external linkage is declared or used in an expression, there must be only one external definition for the identifier somewhere in the program. If the same object is declared more than once externally, the declarations must agree in type and linkage. (See Section 2.8.)
• If one or more of the declarations incompletely specify the object's type, and there exists one declaration of the object with completed type, all the declarations are taken to be in agreement with the completed type.
• The scope of external declarations persist to the end of the file in which they are declared, while internal declarations persist only to the end of the block in which they were declared. Data objects to be used within only one block should be declared in that block. The syntax for external definitions is the same as for all definitions. Function definitions can only occur at the external level.
• Externally declared auto and register objects are not permitted. Internally declared auto and register objects are not automatically initialized and, if not explicitly initialized, have the irrelevant value previously stored at their address. All static objects are automatically initialized to 0, if not explicitly initialized.

The first declaration of an identifier in a compilation unit must specify, explicitly or by the omission of the static keyword, whether the identifier is internal or external. For each object, there can be only one definition. Multiple declarations of the same object may be made, as long as there are no conflicting or duplicate definitions for the same object.

An external object may be defined with either an explicit initialization or a tentative definition. A declaration of an object with file scope, without an initializer, and with a storage-class specifier other than static is a tentative definition. The compiler will treat a tentative definition as the object's only definition unless a complete definition for the object is found. As with all declarations, storage is not actually allocated until the object is defined.

If a compilation unit contains more than one tentative definition for an object, and no external definition for the object, the compiler treats the definition as if there were a file scope declaration of the object with an initializer of zero, with composite type as of the end of the compilation unit. See Section 2.7 for a definition of composite type.

If the declaration of an object is a tentative definition and has internal linkage, the declared type must not be an incomplete type. See Section 2.9 for examples of tentative definitions.

4.4 Declaring Simple Objects

Simple objects are objects with one of the basic data types. Therefore, a simple object can have an integral or floating-point type. Like all objects, simple objects are named storage locations whose values can change throughout the execution of the program. All simple objects used in a program must be declared.

A simple object declaration can be composed of the following items:

• Optional data-type specifier keywords
• Optional type-qualifier keywords ( const or volatile ). For example:

  const int *p; /* const qualifies the integer p points to */ int *const p; /* const qualifies the pointer p */

• An optional storage-class keyword. If the storage-class keyword is omitted, there is a default storage class that depends on the location of the declaration in the program. The positions of the storage-class keywords and the data-type keywords are interchangeable, but placing the storage-class keyword anywhere but at the beginning of the declaration is an obsolete construction.
• Declarators, which list the names of the declared objects.
• Initializers giving the initial value of a simple object. An initializer for a simple object consists of an equal sign (=) followed by a single expression.

An initializer for a simple object consists of an equal sign (=) followed by a single constant expression. For example:

int x = 10; float y = ((12 - 2) + 25);

Here, the declaration both declares and defines the object x as an integer value initially equal to 10, and declares and defines the floating-point value y with an initial value of 35.

Without an initializer, the initial value of an auto object is undefined. A static object without explicit initialization is automatically initialized to 0. (If the object is a static array or structure, all members are initialized to 0.)

A block scope identifier with external or internal linkage (that is, declared using the extern or static keywords) cannot include an initializer in the declaration, because it is initialized elsewhere.

4.4.2 Declaring Integer Objects

Integer objects can be declared with the int , long , short , signed , and unsigned keywords. char can also be used, but only for small values. The following statements are examples of integer declarations:

int x; /* Declares an integer variable x */ int y = 10; /* Declares an integer variable y */ /* and sets y's initial value to 10 */

Some of the keywords can be used together to explicitly state the allowed value range. For example:

unsigned long int a; signed long; /* Synonymous with "signed long int" */ unsigned int;

Consider the range of values an integer object must be capable of representing when selecting the integral data type for the object. See Chapter 3 for more information on the size and range of integral data types.

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