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Here, type must be a valid C data type including char, w_char, int, float, double, bool, or
any user-defined object; and variable_list may consist of one or more identifier names separated
by commas. Some valid declarations are shown here:
The line int i, j, k; declares and defines the variables i, j and k; which instruct the

int i, j, k;

char c, ch;
float f, salary;

double d;
compiler to create variables named i, j, and k of type int.
Variables can be initialized (assigned an initial value) in their declaration. The initializer
consists of an equal sign followed by a constant expression as follows:

type variable_name = value;

Some examples are:

extern int d = 3, f = 5; // declaration of d and f.
int d = 3, f = 5; // definition and initializing d and f.

byte z = 22; // definition and initializes z.

char x = 'x'; // the variable x has the value 'x'.

For definition without an initializer: variables with static storage duration are implicitly
initialized with NULL (all bytes have the value 0); the initial value of all other variables are
undefined.

Variable Declaration in C
A variable declaration provides assurance to the compiler that there exists a variable with
the given type and name so that the compiler can proceed for further compilation without
requiring the complete detail about the variable. A variable declaration has its meaning at the time
of compilation only, the compiler needs actual variable declaration at the time of linking the
program.
A variable declaration is useful when you are using multiple files and you define your
variable in one of the files which will be available at the time of linking the program. You will use
the keyword extern to declare a variable at any place. Though you can declare a variable multiple
times in your C program, it can be defined only once in a file, a function, or a block of code.

Example
Try the following example, where variables have been declared at the top, but they have
been defined and initialized inside the main function:

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