Initialization

Initialization has been mentioned in passing many times so far, but always peripherally to some other topic. This section summarizes some of the rules, now that we have discussed the various storage classes.

In the absence of explicit initialization, external and static variables are guaranteed to be initialized to zero; automatic and register variables have undefined (i.e., garbage) initial values.

Scalar variables may be initialized when they are defined, by following the name with an equals sign and an expression:

int x = 1; 
char squota = '\''; 
long day = 1000L * 60L * 60L * 24L; /* milliseconds/day */

For external and static variables, the initializer must be a constant expression; the initialization is done once, conceptually before the program begins execution. For automatic and register variables, the initializer is not restricted to being a constant: it may be any expression involving previously defined values, even function calls. For example, the initialization of the binary search program in Section 3.3 could be written as

int binsearch(int x, int v[], int n) { 

    int low = 0; 
    int high = n - 1; 
    int mid; 
    ... 
} 

instead of:

int low, high, mid; 
low = 0; 
high = n - 1;

In effect, initialization of automatic variables are just shorthand for assignment statements. Which form to prefer is largely a matter of taste. We have generally used explicit assignments, because initializers in declarations are harder to see and further away from the point of use.

An array may be initialized by following its declaration with a list of initializers enclosed in braces and separated by commas. For example, to initialize an array days with the number of days in each month:

int days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

When the size of the array is omitted, the compiler will compute the length by counting the initializers, of which there are 12 in this case.

If there are fewer initializers for an array than the specified size, the others will be zero for external, static and automatic variables. It is an error to have too many initializers. There is no way to specify repetition of an initializer, nor to initialize an element in the middle of an array without supplying all the preceding values as well.

Character arrays are a special case of initialization; a string may be used instead of the braces and commas notation:

char pattern[] = "ould"; 

is a shorthand for the longer but equivalent:

char pattern[] = { 'o', 'u', 'l', 'd', '\0' }; 

In this case, the array size is five (four characters plus the terminating '\0').

Note

When using a string literal like "ould", an important side effect is that it will always be stored in static memory and last for the entire duration of the program.

On the contrary, using the second approach, it exists solely on the stack for the duration of the scope it was declared in.

In both cases, the actual array exists on the stack, but the memory for the string literal is still statically allocated.