void main() is not legal in C++ but is legal in C.

You've come to this page because you've said something similar to

void main() is not legal in the C language. main() is required to return int.

This is the Frequently Given Answer to that false assertion.

The ISO C++ Standard (ISO/IEC 14882:1998) specifically requires main to return int. It has an explicit "shall" constraint upon well-formed programs. Here it is in § 3.6.1 ¶ 2:

It shall have a return type of int, but otherwise its type is implementation-defined. All implementations shall allow both of the following definitions of main:

• int main() { /* … */ }

  and

• int main(int argc, char* argv[]) { /* … */ }

But the ISO C Standard (ISO/IEC 9899:1999) actually does not mandate this as the C++ standard does. This comes as a surprise to many people. But despite what many documents say, including the Usenet comp.lang.c FAQ document (at great length), the actual text of the C Standard allows for main returning other types.

In my opinion, this is a defect in the C Standard that needs fixing with a corrigendum. (I've written a proposed revised wording that such a corrigendum could use.) It provides the authors of bad C programming books with the very loophole that they have been needing for the past decade or so.

Nonetheless, this is what the C Standard says right now, and has said for several years, and those sections of the Usenet comp.lang.c FAQ document that state that void main() is disallowed are all completely wrong. In the event of a conflict such as this between the C Standard and the Usenet comp.lang.c FAQ document, the C Standard wins.

The C Standard currently says that an implementation is free to define any additional forms for main that it cares to. § 5.1.2.2.1 ¶ 1 of the C Standard lists the allowable forms for the definition of main. Because of the semi-colon, its final sentence parses as a follows:

It shall be defined

• with a return type of int and
  • with no parameters […] or
  • with two parameters […] or equivalent;
  or
• in some other implementation-defined manner.

Furthermore, § 5.1.2.2.3 ¶ 1 says, of main,

If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

which indicates that allowing forms that do not return int is intentional.

So if one's compiler's documentation happens to say anywhere that main may have the return type void then main may indeed have the return type void and a program with void main() is a conforming program.

This is the case for at least the following compilers:

• Watcom C/C++. The C Library Reference for Watcom's C compiler says that "the main function can be declared to return void".

• IBM VisualAge C/C++. The Language Reference for IBM VisualAge C/C++ says that main "can also be declared to return void".

• Microsoft Visual C/C++. The MSDN documentation says that main "can be declared as returning void".

Some compilers do not provide this loophole:

• MetaWare High C/C++ does not list any additional ways of defining main.

• EMX C/C++ does not do so, either.

Other compilers are in between:

• The documentation for Borland C/C++ is littered with sample programs that define a void main() function, but it does not explicitly list this as a legal definition of main, so - somewhat ironically - most of the example code in Borland's documentation is non-conforming.

• The documentation for Comeau C/C++ implies that main may have a return type other than int where it discusses the semantics of falling off the end of main() without a return statement, but does not explicitly specify what additional definitions of main it allows.

  When this page was first published, Comeau C/C++ used void main() in its examples as well. However, Greg Comeau was shown this web page, and in response changed the examples to use int main(). He did not correct the aforementioned implication, however. (Nor did he inform me of the change. I found out only by accident.)

• The documentation for Digital Mars C/C++ also uses void main() in its examples.

The documentation for other vendors' C compilers has not yet been investigated at the time of writing. This list may grow.