difference between declaration and definition

I have been studying c language in depth and i could not find the difference between declaration and definition.

my lecturers said when a variable is decleared space will be alloted for that variable. But some here say that only during definition the space is alloted.

int a=6;
The above statement is called a declaration and there is no definition following that can somebody say why?

extern int a=6;
The above statement is called a declaration and why is it not called as a definition?

clarify my doubt
thank you.

re: difference between declaration and definition

The general rule is that a definition allocates memory whereas a declaration just says something exists.

So,

is a definiton. An int has been created containing the value 6.

is a declaration. It says there is an int named a somewhere else.

So these are declarations:

while these are definitions:

re: difference between declaration and definition

Oops. I forgot about this:


This is a definition. A variable has to be created to contain the 6. In this case the variable is sharable with other implementation files due to the extern.

Note that:

is a declaration since no variable had to be created.

re: difference between declaration and definition

thank you for your reply
so, when you assign a variable it becomes a definition.
is it correct?
can we say something like that

re: difference between declaration and definition

You can have declarations and definitions appropriate to functions also.
consider:
Declarations:
/*a declaration of the sqrt() function which takes an arguement of type double and returns a double...... note the semi-colon.
or*/
/*declaration of sqrt () function which has a variable 'x' passed to it of type double.(preferred) and returns a double......note the semi-colon.
Definition:*/

re: difference between declaration and definition

Not quite. The apparent assignments in whodgson's examples are actually initializers. (An initializer is sufficient, but not necessary, for a definition.) It doesn't matter if a variable is declared or defined; you can assign to it either way.