C initialization --- the C programming language

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 squote = '\'';

long day = 1000L * 60L * 24L; /* milliseconds/day */

For external and static variables, the initializer must be a constant expression; the initialization is done once, conceptually before the grogram begins execution.

For automatic and register variables, the initializer is not restricted to being a constant: it may be any expression involving previoously defined values, even function calls.

In effect, initializations 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 number specified, the missing elements 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’).