C void function with return value

Question

As far as I know, return statement in void function will throw an error.

But in the below program that is not the case.

Here the displayed output

Answer 1

The code you've presented is actually invalid in C99 for a variety of reasons, but the most painful one is below:

foo.c:5:17: warning: implicit declaration of function 'fun' is invalid in C99 [-Wimplicit-function-declaration]
    printf("%d",fun(fun(fun(i))));
                ^
foo.c:8:6: error: conflicting types for 'fun'
void fun(int i)
     ^
foo.c:5:17: note: previous implicit declaration is here
    printf("%d",fun(fun(fun(i))));
                ^

Note that if you gave a function prototype for fun(), which you really should, then you would get a different set of errors:

foo.c:7:25: error: passing 'void' to parameter of incompatible type 'int'
    printf("%d",fun(fun(fun(i))));
                        ^~~~~~