signature=0d52364cc40c642f1fcf9faf828c7582,language lawyer - What are the valid signatures for C's m...

Standard C

For a hosted environment (that's the normal one), the C99 standard says:

5.1.2.2.1 Program startup

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and 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[]) { /* ... */ }

or equivalent;9) or in some other implementation-defined manner.

9) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char **argv, and so on.

The C11 and C18 standards say essentially the same as the C99 standard.

Standard C++

The C++98 standard says:

3.6.1 Main function [basic.start.main]

1 A program shall contain a global function called main, which is the designated start of the program. [...]

2 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:

int main() { /* ... */ }

and

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

The C++ standard explicitly says "It [the main function] shall have a return type of type int, but otherwise its type is implementation defined", and requires the same two signatures as the C standard. So a 'void main()' is directly not allowed by the C++ standard, though there's nothing it can do to stop a non-standard conforming implementation from allowing alternatives (nor a standard conforming implementation from allowing alternatives as extensions to the standard).

The C++03, C++11, C++14, and C++17 standards say essentially the same as C++98.

Common Extension

Classically, Unix systems support a third variant:

int main(int argc, char **argv, char **envp) { ... }

The third argument is a null-terminated list of pointers to strings, each of which is an environment variable which has a name, an equals sign, and a value (possibly empty). If you do not use this, you can still get at the environment via 'extern char **environ;'. This variable is (still) not declared in any POSIX header (previous versions of this answer notwithstanding).

This is recognized by the C standard as a common extension, documented in Annex J:

###J.5.1 Environment arguments

¶1 In a hosted environment, the main function receives a third argument, char *envp[],

that points to a null-terminated array of pointers to char, each of which points to a string

that provides information about the environment for this execution of the program (5.1.2.2.1).

Microsoft C

The Microsoft VS 2010 compiler is interesting. The web site says:

The declaration syntax for main is

int main();

or, optionally,

int main(int argc, char *argv[], char *envp[]);

Alternatively, the main and wmain functions can be declared as returning void (no return value). If you declare main or wmain as returning void, you cannot return an exit code to the parent process or operating system by using a return statement. To return an exit code when main or wmain is declared as void, you must use the exit function.

It is not clear to me what happens (what exit code is returned to the parent or o/s) when a program with void main() does exit — and the MS web site is silent too.

Interestingly, MS does not prescribe the two-argument version of main() that the C and C++ standards require. It only prescribes a three-argument form where the third argument is char **envp, a pointer to a list of environment variables.

The Microsoft page also lists some other alternatives — wmain() which takes wide-character strings, and some more.

The Microsoft VS 2005 version of this page does not list void main() as an alternative. The versions from Microsoft VS 2008 onwards do.

Is int main() the same as int main(void)?

For a detailed analysis, see the end of my answer to What should main() return in C and C++. (It seems that I once considered that this question referred to C++, even though it doesn't and never did. In C++, there is no difference between int main() and int main(void) and int main() is idiomatic C++.)

In C, there is a difference between the two notations, but you only notice it in esoteric cases. Specifically, there's a difference if you call the main() function from your own code, which you're allowed to do in C and are not allowed to do in C++.

The int main() notation does not provide a prototype for main(), but that only matters if you call it recursively. With int main(), you might later (in the same function, or in another function) write int rc = main("absolute", "twaddle", 2): and formally the compiler shouldn't complain to the extent of refusing to compile the code, though it might legitimately complain (warn you) about it (and using -Werror with GCC would convert the warning into an error). If you use int main(void), the subsequent call to main() should generate an error — you said the function takes no arguments but tried to provide three. Of course, you can't legitimately call main() before you've declared or defined it (unless you are still using C90 semantics) — and the implementation does not declare a prototype for main(). NB: The C11 standard illustrates both int main() and int main(void) in different examples — both are valid in C, even though there's the subtle difference between them.