Initialization gives an object an initial value when its definition is reached. The behavior differs between automatic objects and objects with static storage duration.

#include <stdio.h>

int global;

void test()
{
    int automatic;
    static int persistent;
    
    printf("automatic = %d\n", automatic);
    printf("persistent = %d\n", persistent);
}

int main()
{
    int initialized = 10;

    printf("initialized = %d\n", initialized);
    printf("global = %d\n", global);

    test();

    return 0;
}

Example Output #

The values of uninitialized automatic objects are indeterminate, so automatic must not be read. The static objects are initialized to zero.

initialized = 10
global = 0
persistent = 0

Explanation #

An object can be initialized when it is defined:

int initialized = 10;

The object starts with the value 10.

An automatic object defined without an initializer:

int automatic;

has an indeterminate value. Reading such a value is generally not valid and can result in undefined behavior.

Objects with static storage duration that are not explicitly initialized are initialized to zero:

static int persistent;

The same applies to an uninitialized file-scope object:

int global;

Initialization Behavior #

Initialization and assignment are different operations. Initialization occurs when an object is created, while assignment changes the value of an already existing object:

int number = 10;  /* Initialization */

number = 20;      /* Assignment */