A commonly asked question:
Why do people say that main() has to return an int and void main(void) is wrong? I think it is all right because:
I have a book by /* somebody */ and all of the example programs are written this way.
My instructor tells us to write it that way.
The examples in my online help show it that way.
It works on my compiler.
First let's talk about the items above.
Many books are written by people who don't even bother to actually learn their subject. They are good at writing books and think they know everything. Some of them test their code by compiling and running it with their favorite compiler, and never notice that they are using compiler specific extensions. Some authors have written books containing code which they just wrote off the top of their head. They never compiled and ran it because it would not compile with ANY compiler.
Sadly, many instructors of computer programming classes do not know the actual standard for the language themselves. What they know they learned from reading the same books written by careless authors described above. This kind of ignorance of their subject matter would not be allowed in a subject like mathematics, chemistry, or physics, but it is all too common in computer programming.
Microsoft is particularly bad at this. Almost all of the examples in their online help start with void main(void). Yet if you look up main in the online help, it displays a proper definition. The example programs in the online help for Borland compilers always show main() properly returning an int.
The standards for the C and C++ programming languages specifically allow compiler writers to include extensions. In addition, the usual tradition is for a compiler to process your program and generate an executable if at all possible, even if your source code is questionable. The International Standard for each programming language defines what the language is, not whatever any particular compiler happens to accept. Most C compilers have some type of option to compile in strict standard mode. Again Microsoft is one of the worst offenders here. There is no way that I know of to get their compilers to issue an error or even a warning message if you define a void main() function in your code.
Some computer languages are proprietary, that is they belong to a particular company, and that company has final authority to determine what the language is and isn't.
One example is Java. This language is a product of Sun Microsystems, and it defined by them. Any other company which uses the Java language signs a contract with Sun and agrees that their version will be 100% compatible with Sun's standard.
Another language is Microsoft's Visual Basic. This is a programming package that only they sell, and only for their Windows operating systems. Since this language is completely theirs, the definition of what the language is is completely theirs.
This is not the case with most general purpose programming languages, and this includes C and C++. Nobody owns these languages or their definitions. Instead the languages are defined by International Standards, which are generated and maintained by ISO, the International Standards Organization.
ANSI, which is the American National Standards Institute, is the American national standards body and is one of the member bodies of ISO. You may have heard the term ANSI C or ANSI C++. These terms are in common use because it was ANSI that issued the first standard for C. When the ISO International Standard was adopted, ANSI adopted the ISO version and it became the ANSI version as well. The current ANSI standards for C and C++ are the ISO International standards.
The ANSI/ISO standards for C and C++ define the languages, not Microsoft or Borland or any other compiler vendor. Here is what the standards have to say:
ANSI/ISO/IEC 9899:1990 International Standard For C
The function called at program startup is named main. The implementation declares no prototype for this function. It can be defined with no parameters:
int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):
int main(int argc, char *argv[ ]) { /* ... */ } The newly ratified update to the C standard in 1999 will make this even clearer, perhaps because of all the clueless who could not understand that only the two formats above, both of which define main to return an int, are valid.
The draft of the new standard expands on the two definitions above by modifying the wording "It can be defined" to "It shall be defined with a return type of int".
ANSI/ISO/IEC 14882:1998 International Standard For C++
An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a return type of type int but otherwise its type is implementation defined. All implementations shall allow both of the following definitions of main: The two definitions which follow are identical to those in the C standard.
Practical Reasons To Return An int from main()
On many operating systems, the value returned by main() is used to return an exit status to the environment. On Unix, MS-DOS, and Windows systems, the low eight bits of the value returned by main() is passed to the command shell or calling program. This is often used to change the course of a program, batch file, or shell script.
Many compilers will refuse to compile a source code file containing a definition of main() which does not return an int.
On some platforms a program starting with void main() may crash on startup, or when it ends.
A program which contains a main() function that is not defined to return an int is just plain not real C or C++.
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