The important thing to note is, there is no return type for fun(), the program still compiles and runs fine in most of the C compilers. In C, if we do not specify a return type, compiler assumes an implicit return type as int. However, C99 standard doesn’t allow return type to be omitted even if return type is int. This was allowed in older C standard C89.
In C++, the above program is not valid except few old C++ compilers like Turbo C++. Every function should specify the return type in C++.