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In Turbo C , void main () will be accepted, whereas in dev-cpp main () should return a value. 3.return 0 simply exits the program with exit status 0 , in other words the return value determines the exit status of the main thread. _tmain does not exist in C++. main does. _tmain is a Microsoft extension. main is, according to the C++ standard, the program's entry point. It has one of these two signatures: int main (); int main (int argc, char" argv[]); Microsoft has added a wmain which replaces the second signature with this: int wmain(int argc, wchar_t" argv[]); And then, to make it easier to switch between Unicode (UTF-16) and their multibyte character set, they've defined _tmain which, if Unicode is enabled, is ... The interesting part is "with no parameters". int main () and int main (void) are currently equivalent, since they are both function declarators and have no parameters. The following applies (6.7.6.3 ): 10 The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters. /--/ If you are trying to close a question where someone should be using this idiom and isn't, consider closing as a duplicate of Why is Python running my module when I import it, and how do I stop it? instead. For questions where someone simply hasn't called any functions, or incorrectly expects a function named main to be used as an entry point automatically, use Why doesn't the main () function run when I start a Python script? Where does the script start running?.
