why it is necessary to use semicolon after functions prototype

Question

Why we use a semicolon after the int add(int,int) statement in second line.

#include<stdio.h>
int add(int,int);
int main()
{
int a,b,c;
scanf("%d %d",&a,&b);
c=add(a,b);
printf("The sum of the 2 numbers is %d",c);
return 0;
}
int add(int x,int y)
{
int sum;
sum=x+y;
return sum;
}

Answer 1

In the C grammar declarations are defined the following way

declaration:
    declaration-specifiers init-declarator-listopt ;
                                                  ^^^ 

As you can see the semicolon is required.

And this

int add(int,int);

is a function declaration. Thus you have to place a semicolon at the end of the declaration.

Compare two programs

int main( void )
{
    int add( int x, int y )

    {
        //...    
    }
}

and

int main( void )
{
    int add( int x, int y );

    {
        //...    
    }
}

The first program is invalid because the compiler will think that the function add is defined within function main.

The second program is valid. There is a function declaration and a code block within main.

So semicolons are needed that to distiguish declarations from other program constructions.