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// function declaration

int func();

int main()
{

// function call
int i = func();

}

// function definition
int func()

{
return 0;

}

Lvalues and Rvalues in C
There are two kinds of expressions in C:
 lvalue : Expressions that refer to a memory location are called "lvalue"
expressions. An lvalue may appear as either the left-hand or right-hand side of an assignment.
 rvalue : The term rvalue refers to a data value that is stored at some
address in memory. An rvalue is an expression that cannot have a value assigned to it which
means an rvalue may appear on the right-hand side but not on the left-hand side of an assignment.
Variables are lvalues and so they may appear on the left-hand side of an assignment.
Numeric literals are rvalues and so they may not be assigned and cannot appear on the left-hand
side. Take a look at the following valid and invalid statements:

int g = 20; // valid statement
10 = 20; // invalid statement; would generate compile-time error

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