Scope rule of functions

Look at the following program

main( )

{

int i = 20 ;

display ( i ) ;

}

display ( int j )

{

int k = 35 ;

printf ( "\n%d", j ) ;

printf ( "\n%d", k ) ;

}

In this program is it necessary to pass the value of the variable i to the function display( )? Will it not become automatically available.

to the function display( )? No. Because by default the scope of a variable is local to the function in which it is defined. The presence

of i is known only to the function main( ) and not to any other function. Similarly, the variable k is local to the function display( ) and hence it is not available to main( ). That is why to make the value of i available to display( ) we have to explicitly pass it to display( ). Likewise, if we want k to be available to main( ) we will have to return it to main( ) using the return statement. In general we can say that the scope of a variable is local to the function in which it is defined.

Calling Convention

Calling convention indicates the order in which arguments are passed to a function when a function call is encountered. There are two possibilities here:

(a)

(b)

Arguments might be passed from left to right.

Arguments might be passed from right to left.

C language follows the second order.

Consider the following function call:

fun (a, b, c, d ) ;

In this call it doesn’t matter whether the arguments are passed from left to right or from right to left. However, in some function call the order of passing arguments becomes an important consideration. For example:

int a = 1 ;

printf ( "%d %d %d", a, ++a, a++ ) ;

It appears that this printf( ) would output 1 2 3.

This however is not the case. Surprisingly, it outputs 3 3 1. This is because C’s calling convention is from right to left. That is, firstly

1 is passed through the expression a++ and then a is incremented to 2. Then result of ++a is passed. That is, a is incremented to 3 and then passed. Finally, latest value of a, i.e. 3, is passed. Thus in right to left order 1, 3, 3 get passed. Once printf( ) collects them it prints them in the order in which we have asked it to get them printed (and not the order in which they were passed). Thus 3 3 1 is printed.