main function return in a C/C++ program?Q: Why does everybody tell me that void main is bad?
Q: If void main is incorrect, how come the compiler lets it
compile?
A: The ANSI/ISO C Standard specifies that the main
function be declared in one of the following two ways:
int main (void);
or
int main (int argc, char **argv);
In both cases the return type is an int, and your main
function should therefore either return an int or call the
library function exit, when the program ends. The C++
standard includes a similar requirements for C++ programs.
Since the runtime environment assumes that main returns an
int, declaring main with any other return type, including
void, invites trouble. The compiler might compile such a
program, since the ANSI Standard doesn't require it to fail, but
the behavior of such a program is, in the Standard's parlance,
"undefined" (read: anything can happen). That is why GCC will print a
warning in these cases if you use the -Wall switch.
To summarize, using void main is unsafe and can
potentially do evil things to your program. It is best to avoid it.
Note that the C++ standard, in contrast to the C standard,
explicitly prohibits void main(), and explicitly says that if the
controls reaches the end of main without encountering a
return statement, the effect is that of executing return 0;. When compiling a C++ program, GCC automatically generates the
code to return zero from the main function, in case the
programmer leaves that out.