Question

main()
{
  printf("Hello World.");
}

Why does no warning is produced in gcc compiler even though we declare main() with return type ‘int’

Answer 1

Because you’re not using the -Wall flag. When you do, you should get:

foo.c:1: warning: return type defaults to ‘int’
foo.c: In function ‘main’:
foo.c:1: warning: implicit declaration of function ‘printf’
foo.c:1: warning: incompatible implicit declaration of built-in function ‘printf’
foo.c:1: warning: control reaches end of non-void function