Variables declared at the file-scope level can be initialized. If you do not explicitly initialize a variable at the external level, it is initialized to 0 by default.When initializing scalar types, the value of the assignment-expression is assigned to the variable. The conversion rules for assignment apply. (See“Type Conversions” for information on conversion rules.)
declaration : declaration-specifiers init-declarator-list opt ;
declaration-specifiers :
storage-class-specifier declaration-specifiers opt
type-specifier declaration-specifiers opt
type-qualifier declaration-specifiers opt
init-declarator-list :
init-declarator
init-declarator-list , init-declarator
init-declarator :
declarator
declarator = initializer /* For scalar initialization */
initializer : assignment-expression
You can initialize variables of any type, provided that you obey the following rules:
Variables declared at the file-scope level can be initialized. If you do not explicitly initialize a variable at the external level, it is initialized to 0 by default.
A constant expression can be used to initialize any global variable declared with the static storage-class-specifier. Variables declared to be static are initialized when program execution begins. If you do not explicitly initialize a global static variable, it is initialized to 0 by default, and every member that has pointer type is assigned a null pointer.
Variables declared with the auto or register storage-class specifier are initialized each time execution control passes to the block in which they are declared. If you omit an initializer from the declaration of an auto or register variable, the initial value of the variable is undefined. For automatic and register values, the initializer is not restricted to being a constant; it can be any expression involving previously defined values, even function calls.
The initial values for external variable declarations and for all static variables, whether external or internal, must be constant expressions. (Constant expressions are described on topic .) Since the address of any externally declared or static variable is constant, it can be used to initialize an internally declared static pointer variable. However, the address of an auto variable cannot be used as a static initializer because it may be different for each execution of the block. You can use either constant or variable values to initialize auto and register variables.
If the declaration of an identifier has block scope, and the identifier has external linkage, the declaration cannot have an initialization.
The following examples illustrate initializations:
int x = 10;
The integer variable x is initialized to the constant expression 10.
register int *px = 0;
The pointer px is initialized to 0, producing a “null” pointer.
const int c = (3 * 1024);
This example uses a constant expression (3 * 1024) to initialize c to a constant value that cannot be modified because of the const keyword.
int *b = &x;
This statement initializes the pointer b with the address of another variable, x.
int *const a = &z;
The pointer a is initialized with the address of a variable named z. However, since it is specified to be a const, the variable a can only be initialized, never modified. It always points to the same location.
int GLOBAL ;
int function( void )
{
int LOCAL ;
static int *lp = &LOCAL; /* Illegal initialization */
static int *gp = &GLOBAL; /* Legal initialization */
register int *rp = &LOCAL; /* Legal initialization */
}
The global variable GLOBAL is declared at the external level, so it has global lifetime. The local variable LOCAL has auto storage class and only has an address during the execution of the function in which it is declared. Therefore, attempting to initialize the static pointer variable lp with the address of LOCAL is not permitted. The static pointer variable gp can be initialized to the address of GLOBAL because that address is always the same. Similarly, *rp can be initialized because rp is a local variable and can have a nonconstant initializer. Each time the block is entered, LOCAL has a new address, which is then assigned to rp.