look at this code in c++

int main()
printf(" ");
where does this 0 returns

plz elaborate it completly

The int returned by main() is a way for a program to return a value to "the system" that invokes it. On systems that doesn't provide such a facility the return value is ignored, but that doesn't make "void main()" legal C++ or legal C.

- stroustrup

on systems where it is supported, you could check the return value.
eg. return from a call to int system( const char* )
or in a shell eg. diff port-supfile standard-supfile > /dev/null || echo FILES ARE DIFFERENT