What really are the valid signatures for main function in C? I know:
int main(int argc, char *argv[])
Are there other valid ones?
The C11 standard explicitly mentions these two:
int main(void);
int main(int argc, char* argv[]);
although it does mention the phrase "or equivalent" with the following footnote:
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.
In addition, it also provides for more (implementation-defined) possibilities.
The relevant text (section 5.1.2.2.1, but this particular aspect is unchanged from C99) states:
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; or in some other implementation-defined manner.
If they are declared, the parameters to the main function shall obey the following constraints:
The value of argc shall be nonnegative.
argv[argc] shall be a null pointer.
If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.
If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters.
The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.
Note that this is for a hosted environment, the ones you normally see in C programs. A free-standing environment (such as an embedded system) is far less constrained, as stated in 5.1.2.1 of that same standard:
In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.
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.
POSIX supports execve(), which in turn supports
int main(int argc, char *argv[], char *envp[])
The added argument is the environment, i.e. an array of strings of the form NAME=VALUE.
http://en.wikipedia.org/wiki/Main_function_(programming)#C_and_C.2B.2B
Besides the usual int main(int argc, char *argv[]) and the POSIX int main(int argc, char **argv, char **envp), on Mac OS X also supports
int main(int argc, char* argv[], char* envp[], char* apple[]);
Of course it's Mac-only.
On Windows there's
int wmain(int argc, wchar_t* argv[], wchar_t* envp[]);
as the Unicode (actually, wide-character) variant. Of course there is WinMain too.
int main(void)
Under some OS (for example, Windows) also such is valid:
int main(int argc, char **argv, char **envp)
where envp gives an environment, otherwise accessible through getenv()
Related
What really are the valid signatures for main function in C? I know:
int main(int argc, char *argv[])
Are there other valid ones?
The C11 standard explicitly mentions these two:
int main(void);
int main(int argc, char* argv[]);
although it does mention the phrase "or equivalent" with the following footnote:
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.
In addition, it also provides for more (implementation-defined) possibilities.
The relevant text (section 5.1.2.2.1, but this particular aspect is unchanged from C99) states:
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; or in some other implementation-defined manner.
If they are declared, the parameters to the main function shall obey the following constraints:
The value of argc shall be nonnegative.
argv[argc] shall be a null pointer.
If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.
If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters.
The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.
Note that this is for a hosted environment, the ones you normally see in C programs. A free-standing environment (such as an embedded system) is far less constrained, as stated in 5.1.2.1 of that same standard:
In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.
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.
POSIX supports execve(), which in turn supports
int main(int argc, char *argv[], char *envp[])
The added argument is the environment, i.e. an array of strings of the form NAME=VALUE.
http://en.wikipedia.org/wiki/Main_function_(programming)#C_and_C.2B.2B
Besides the usual int main(int argc, char *argv[]) and the POSIX int main(int argc, char **argv, char **envp), on Mac OS X also supports
int main(int argc, char* argv[], char* envp[], char* apple[]);
Of course it's Mac-only.
On Windows there's
int wmain(int argc, wchar_t* argv[], wchar_t* envp[]);
as the Unicode (actually, wide-character) variant. Of course there is WinMain too.
int main(void)
Under some OS (for example, Windows) also such is valid:
int main(int argc, char **argv, char **envp)
where envp gives an environment, otherwise accessible through getenv()
So I'm learning about functions in a book.
It says we need to prototype or declare functions so the compiler can understand if they are correctly called or not.
But why does the main function works without a prototype?
I used to write main functions in my learning process like this:
int main(void)
So it will not get any argument because of (void)
I tried to run my program with argument for example > ./a.out 2
int main(int y){
printf("%s %d\n","y is",y);
}
When I run it normally y is 1, when run it with > ./a.out 1 y is 2, when there is more than one argument it increases by one. So it's not the right way but what causes this?
Declaring y as char says nothing so my guess is it works like the return value of scanf(). It returns number of successful inputs.
A function must be either declared (i.e. a prototype) or defined before it is called. The main function is different from other functions in that it's called by the program startup code and not some other function.
There are however restrictions on what the signature of main can be. On a regular hosted implementation, the C standard says it can be either:
int main(void)
Or:
int main(int argc, char **argv)
The latter case is used to read command line arguments. argc contains the number of arguments passes, including the program name. argv contains the actual arguments as an array of char *.
Many systems also support:
int main(int argc, char **argv, char **envp)
Where envp contains the environment variables known to the program.
The prototype you're using: int main(int y) is not supported in any implementation I'm aware of, so attempting to use such a prototype for main invokes undefined behavior.
When I run it normally y is 1, when run it with > ./a.out 1 y is 2, when there is more than one argument it increases by one. So it's not the right way but what causes this?
The standard entry for program startup kind of answers both your questions:
N1570 § 5.1.2.2.1 Program startup
1 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[]) { /* ... */ }
[...]
As dbush already stated in the accepted answer these are the only two main implementations allowed by the standard.
The standard leaves the responsability of dealing with undefined constructs opened and imposes no requirements for what the behavior should be, a given implementation may deal with the situation in any way it considers appropriate, this is known as undefined behavior.
What seems to be happening is that your compiler is assuming that y is argc, which is allowed (as you can see in the second snippet of highlighted citation above), and argc stores the number of arguments in the command line, which is consistent with the results you're having, but again, this behavior may differ in different compilers, systems or even versions of the same compiler.
I recently came across the following in my searches regarding environment variables in C:
int main (int argc, char *argv[], *char *envp[])
I have searched around and can't find anything conclusive regarding my question.
What are all of the available arguments that main() can accept?
The C99 and C11 draft standards allow for implementation defined set of parameters to main, these parameters are going to be specific to those systems(non-portable). From section 5.1.2.2.1:
[...]or in some other implementation-defined manner[...]
The only additional parameters I can find documented are envp and apple, we can find a good description in Wikipedia's C and C++ section on Entry Points:
Other platform-dependent formats are also allowed by the C and C++
standards, except that in C++ the return type must always be int;[6]
for example, Unix (though not POSIX.1) and Microsoft Windows have a
third argument giving the program's environment, otherwise accessible
through getenv in stdlib.h:
int main(int argc, char **argv, char **envp);
Mac OS X and Darwin have a fourth parameter containing arbitrary
OS-supplied information, such as the path to the executing binary:[7]
int main(int argc, char **argv, char **envp, char **apple);
It looks like Windows has a Microsoft specific wmain which takes wchar_t:
int wmain(int argc, wchar_t *argv[], wchar_t *envp[]);
The alternative is the wide-character version:
int main(int argc, wchar_t* argv[], wchar_t* envp[])
The main function is specified in the language specification as the following, no other function signature is provided besides a get-out clause for implementation-specific entrypoint functions (like Apple's 3rd apple parameter) or Microsoft's WinMain function.
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 or in some other implementation-defined manner.
If they are declared, the parameters to the main function shall obey the following constraints:
The value of argc shall be nonnegative.
argv[argc] shall be a null pointer.
If the value of argc is greater than zero, the array members argv[0] through
argv[argc-1] inclusive shall contain pointers to strings, which are given
implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.
If the value of argc is greater than zero, the string pointed to by argv[0]
represents the program name argv[0][0] shall be the null character if the
program name is not available from the host environment. If the value of argc is
greater than one, the strings pointed to by argv[1] through argv[argc-1]
represent the program parameters.
The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.
Does this code follow C standards (e.g. C89, C99, C10x)?
void
main(int a,int b, int c, int d,char *msg){
if(d==1){
printf("%s\n",msg);
}else{
main(1,2,3,1,&"Hello Stackoverflow");
}
}
If not, why?
There's one error: &"Hello Stackoverflow" does not have type char*, so you shouldn't pass that to a function expecting that type.
Apart from that, this program is allowed by the Standard as an implementation-specific extension, but a compiler has the freedom to decline it.
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; or in some other implementation-defined manner.
(2011 Standard, latest draft section 5.1.2.2.1, emphasis added.)
There is no ban on recursive calls to main in the C Standard. This is a difference with C++, which does outlaw that.
You mean beside it won't run? main is defined to take int, char** as arguments.
Depending on the compiler, this either will fail to start up as the run-time can't find main(int, char**), or on older compilers it'll just crash because it piddles on the stack.
It's only valid under C99 and later if the implementation explicitly documents that main may take 5 parameters (4 int and 1 char *) and return void (that's the "or in some other implementation-defined manner" clause that larsmans referenced in his now-un-deleted answer, and I don't think that clause was present in C89).
Otherwise the behavior is undefined, meaning the compiler may or may not choke on it.
What really are the valid signatures for main function in C? I know:
int main(int argc, char *argv[])
Are there other valid ones?
The C11 standard explicitly mentions these two:
int main(void);
int main(int argc, char* argv[]);
although it does mention the phrase "or equivalent" with the following footnote:
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.
In addition, it also provides for more (implementation-defined) possibilities.
The relevant text (section 5.1.2.2.1, but this particular aspect is unchanged from C99) states:
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; or in some other implementation-defined manner.
If they are declared, the parameters to the main function shall obey the following constraints:
The value of argc shall be nonnegative.
argv[argc] shall be a null pointer.
If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.
If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters.
The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.
Note that this is for a hosted environment, the ones you normally see in C programs. A free-standing environment (such as an embedded system) is far less constrained, as stated in 5.1.2.1 of that same standard:
In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.
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.
POSIX supports execve(), which in turn supports
int main(int argc, char *argv[], char *envp[])
The added argument is the environment, i.e. an array of strings of the form NAME=VALUE.
http://en.wikipedia.org/wiki/Main_function_(programming)#C_and_C.2B.2B
Besides the usual int main(int argc, char *argv[]) and the POSIX int main(int argc, char **argv, char **envp), on Mac OS X also supports
int main(int argc, char* argv[], char* envp[], char* apple[]);
Of course it's Mac-only.
On Windows there's
int wmain(int argc, wchar_t* argv[], wchar_t* envp[]);
as the Unicode (actually, wide-character) variant. Of course there is WinMain too.
int main(void)
Under some OS (for example, Windows) also such is valid:
int main(int argc, char **argv, char **envp)
where envp gives an environment, otherwise accessible through getenv()