Difference between the definition and declaration of a variable in c?

Answer 1

definition defines the memory area ( allocates the memory ) for the variable and the declaration tells about the signature of the variable ( type and size to be considered). definition occures once through the program( memory is allocated once ), but the declaration can occur many times.

OR For a variable, the definition is the statement that actually allocates memory. For example, the statement:

long int var;

is a definition. On the other hand, an extern reference to the same variable:

extern long int var;

is a declaration, since this statement doesn't cause any memory to be allocated. Here's another example of a declaration:

typedef MyType short;

Second Answer

Declaration can come only once and definition can come many times in the program.

Let's take an example,

Line 1: #include

Line 2:

Line 3: void main()

Line 4: {

Line 5: int x; //declaration

Line 6:

Line 7: x=10; //definition

Line 8: }

In the above program, on the line 5 what appears is declaration and on line 7 the definition is there.

Now if we declare the variable x again in the program we will get the error saying that the variable is already declared.

Whereas if we assign some new value to variable x, say 20

i.e.

x=20;

this is totally fine and we can assign any number of values in the same program. This means that the declaration is only one throughout the program but definitions can be many.