The return value for main indicates how the program exited. Normal exit is
represented by a 0 return value from main. Abnormal exit is signaled by a
non-zero return, but there is no standard for how non-zero codes are
interpreted. As noted by others, void main() is prohibited by the C++
standard and should not be used. The valid C++ main signatures are:
int main()
and
int main(int argc, char* argv[])
which is equivalent to
int main(int argc, char** argv)
It is also worth noting that in C++, int main() can be left without a
return-statement, at which point it defaults to returning 0. This is also
true
with a C99 program. Whether return 0; should be omitted or not is open to
debate. The range of valid C program main signatures is much greater.
Efficiency is not an issue with the main function. It can only be entered and left once (marking the program's start and termination) according to the C++ standard. For C, re-entering main() is allowed, but should be avoided.