Declarations and definitions in C

Guys what is the difference between declarations and definitions of varibles,functions, structures?
I understand

Code:

int c;
int d;

are examples of declarations and int c=4; is an initilisation but how does variable definition differ?

Same with structures

Code:

struct autopart
{
char id;
float price;
}

//is the structure declaration.What is the definition of a structure.

for functions

Code:

int sum2digits(int a,int b);  //the prototype

lets say...

Code:

int sum2digits(int a, int b)  //what is this line called?
{
return a+b;
}

what are the definitions and declarations.

The last one if a function definition. The function before is a declaration - telling the compiler it exists.
I'm not 100% if to call declaring or defining variables, but I'd go for declaring them. You can also initialize them as you say.

edit: got them mixed up.

Definitions are when you do this: struct X { int x,y; }; and declarations are when you make instances, like this: struct X x; int y; char a; and an initialization like you wrote is also a declaration.

Basically, declarations make instances for you to work with, and definitions make types.

Code:

int sum2digits(int a, int b) //what is this line called?
{
return a+b;
}

function signature, I guess. It doesn't really have a name since it isn't expressed in individuality except for a prototype.

That is a function definition; as contrary to a function declaration which is only the prototype with a ; at the end and no body. Exists to tell the compiler a function exists (and some IDEs also extract information from prototypes as to what is expect from the function, like what arguments it takes - Visual Studio is one!).

Code:

struct X { int x,y; };

That is without doubt a declaration. It's a blueprint of the struct - it does not implement its definition.

drat... it's so confusing...

edit: I couldn't find the word definition anywhere in my C99 draft. interesting.

I suppose you could say that you declare what a function or a data type (struct) is, but you must define its behavior (code/implementation).

Although it is for C++, Stroustrup's glossary could come in handy:

declaration - an introduction of a name into a scope. The type of the name must be specified. If the declaration also specifies the entity to which the name refers, the declaration is also a definition.

definition - a declaration that specifies the entity to which the declared name refers.

Originally Posted by K&R-II, A8, p210

Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions.

So basically, if it takes up memory space, then it's a definition.

So basically, a prototype or a struct is not a definition; it's a declaration.
Variables that can reserve space for a said type, is a definition, then.
Implementation of function, refering to a prototype or a declaration, would also be a definition due to code, which would also take space in memory.

That's what I gather.