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(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.