Related
This question already has answers here:
What should main() return in C and C++?
(19 answers)
Why is the type of the main function in C and c++ left to the user to define? [duplicate]
(6 answers)
Closed 9 years ago.
I am currently learning C and I have written many small programs. However, I have noticed that the main function could start as
main()
{
//code
}
or
int main()
{
//code
return 0;
}
or
int main(void)
{
//code
return 0;
}
Which option should I use? Thanks!
For 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.
This (is valid in C89) main() implicitly meant (previously) int main(void). However the default return type rule has been abandoned in C99. Also:
main() means - a function main taking an unspecified number of arguments of.
main(void) means "a function main taking no arguments.
Your first example uses a feature inherited from the outdated dialect of C which predated the first ANSI(1989) and ISO(1990) standard: namely, that you can write a function which doesn't specify its return type, and in that case the type defaults to int.
In early C, the void keyword and associated type did not exist. When programmers wanted to write procedures ("functions that have a side effect, but do not return anything"), they simulated it using this feature. They wrote a function without any keyword specifying the return type. They allowed the function to execute to it last statement without returning a value (or alternatively, they used return; to exit from the middle without supplying a value), and they wrote the calls to the function such that those calls did not try to use the return value:
parse_input() /* similar to a procedure in Pascal, but fake! */
{
/* ... */
if (condition())
return; /* no value */
/* ... */
/* fall off end here */
}
int main()
{
parse_input(); /* no return value extracted, everything cool! */
return 0;
}
Unfortunately, some programmers also started not caring about the termination status of a program and writing main itself in this procedure style:
main()
{
/* do something */
/* fall off the end without returning a value */
}
(A mixed style also existed: omitting the int declarator but returning an integer value.)
These programs failing to return a value had an indeterminate termination status. To the operating system, their execution could look successful or failed. Woe to the script writer who tried to depend on the termination status of such a program!
Then things took a turn for the worse. C++ came along and introduced void, and it was adopted into C. With the void keyword in C++, one could declare a function that actually returns nothing (and make it an error to have a return; statement in any other kind of function). The dummy programmers who used to write main with no return type got dumber, and started sticking this new-fangled, fresh-out-of-C++ void in front:
void main() /* yikes! */
{
/* do something */
/* fall off the end without returning a value */
}
By this time they had forgotten that when they wrote main(), it actually meant int main(), which made the function have a compatible type with the startup call invoked by the environment (except for the matter of neglecting to return a value). Now they actually had a different function type from the expected one, which might not even be successfully called!
Where things stand now is that in C++ and in the latest C++ standard, main is still required to return an int. But both languages make a concession for the original dummy programmers: you can let execution "fall off" the end of main and the behavior is as if return 0; had been executed there. So this trivial program now has a successful termination status as of C99 and, I think, C++98 (or possibly earlier):
int main()
{
}
But neither language makes a concession for the second-generation dumber programmers (and everyone else who read the C books that those programmers wrote in the 1980's and since). That is, void is not a valid return declarator for main (except where it is documented by platforms as being accepted, and that applies to those platforms only, not to the portable language).
Oh, and allowance for the missing declarator was removed from C in C99, so main() { } is no longer correct in new dialects of C, and isn't valid C++. Incidentally, C++ does have such a syntax elsewhere: namely, class constructors and destructors are required not to have a return type specifier.
Okay, now about () versus (void). Recall that C++ introduced void. Furthermore, though C++ introduced void, it did not introduce the (void) argument syntax. C++ being more rigidly typed introduced prototype declarations, and banished the concept of an unprototyped function. C++ changed the meaning of the () C syntax to give it the power to declare. In C++, int func(); declares a function with no arguments, whereas in C, int func(); doesn't do such a thing: it declares a function about which we do not know the argument information. When C adopted void, the committee had an ugly idea: why don't we use the syntax (void) to declare a function with no arguments and then the () syntax can stay backward compatible with the loosey-goosey legacy behavior pandering to typeless programming.
You can guess what happened next: the C++ people looked at this (void) hack, threw up their arms and copied it into C++ for the sake of cross-language compatibility. Which in hindsight is amazing when you look at how the languages have diverged today and basically no longer care about compatibility to that extent. So (void) unambiguosly means "declare as having no arguments", in both C and C++. But using it in C++ code that is obviously pure C++ never intended to be C is ugly, and poor style: for instance, on class member functions! It doesn't make much sense to write things like class Foo { public: Foo(void); virtual ~Foo(void) /*...*/ };
Of course, when you define a function like int main() { ... }, the function which is defined has no arguments, regardless of which language it is in. The difference is in what declaration info is introduced into the scope. In C we can have the absurd situation that a function can be fully defined, and yet not declared, in the same unit of program text!
When we write main, usually it is not called from within the program, and so it doesn't matter what the definition declares. (In C++, main must not be called from the program; in C it can be). So it is immaterial whether you write int main() or int main(void), regardless of whether you're using C or C++. The thing which calls main does not see any declaration of it (that you write in your program, anyway).
So just keep in mind that if you write:
int main() /* rather than main(void) */
{
}
then although it is perfect C++ and correct C, as C it has a slight stylistic blemish: you're writing an old-style pre-ANSI-C function that doesn't serve as a prototype. Though it doesn't functionally matter in the case of main, you may get a warning if you use some compilers in a certain way. For instance, GCC, with the -Wstrict-prototypes option:
test.c:1:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
Because -Wstrict-prototypes is a darn useful warning to turn on when programming in C, for improved type safety, (along with -Wmissing-prototypes), and we strive to eliminate warnings from our compile jobs, it behooves us to write:
int main(void) /* modern C definition which prototypes the function */
{
}
which will make that diagnostic go away.
If you want main to accept arguments, then it is int main(int argc, char **argv) where the parameter names are up to you.
In C++, you can omit parameter names, so this definition is possible, which serves nicely in the place of main().
int main(int, char **) // both arguments ignored: C++ only
{
}
Since the argument vector is null-pointer-terminated, you don't need argc, and C++ lets us express that without introducing an unused variable:
#include <cstdio>
int main(int, char **argv) // omitted param name: C++ only
{
// dump the arguments
while (*argv)
std::puts(*argv++);
}
first :
declares a function main - with no input parameters. Although main should have returns ( your compiler will take care of this )
2nd/3rd:
Declare a function main which returns an int and takes in no input parameters
You should use 3rd format. Rather this is the best way:
int main(int argc, char *argv[]){
return 0;
}
You should use 1 one of these 4 choices:
int main(void);
int main();
int main(int argc, char **argv);
int main(int argc, char *argv[]);
where it's conventional to use the names argc and argv; you can change them but don't.
Take care never to use void main(void); which is too-often seen in production code.
By default main function returns an integer type, hence its "int main()" or you can give simply "main()"
"main(void)" is same as "main()", it tells the compiler that main function has no arguments.
In case if you want to pass arguments via main function:
int main(int argc, char *argv[]){
return 0;
}
main(){}
The above line give you an error. The default return type of any function in c is int. As the above code return nothing it gives you an error.
int main(){
//body
return 0;
}
In above code it fulfill all requirement so the above code will run.In above code we pass no argument in the function. So this function can take global and local variables to process.
int main(void)
{
//code
return 0;
}
In above code we pass no argument in the function. But specifying void tells the compiler that it does not take any argument. void is the default datatype of argument that signifies no input.
This question already has answers here:
Return type of main function [duplicate]
(5 answers)
Closed 9 years ago.
In a book (don't remember which one) they used:
void main(void)
In school I learned:
int main(void)
Is there any case when void main(void) is actually correct? Or at least not explicitly wrong?
EDIT:
According to the proposed answers since C99 it is not correct. What about earlier versions? Is it explicitly wrong or just nothing said about it? Why do C compiler not complain about it?
Never, ever use void main(void). It is not standard.
Always use one of
int main(void);
int main();
int main(int argc, char **argv);
int main(int argc, char *argv[]);
And the best use of said book is to use it to light your first fire for the winter.
Per the C standard
C99 §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;10 or in some other implementation-defined manner.
10) 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 closing clause grants implementations their own vices, which is to say, if a program does not follow this, it is no longer standard-compliant and instead reliant on the implementation for compatibility. If you want your main() to work everywhere, follow one of these and you'll be ok.
void main(void) is allowed by some(all?) C compilers. However, it should not be used anyway. Because at least since C99 is it not allowed. However, I did not find a C compiler which complains about it.
E.g. void.c:
#include <stdio.h>
void main(void)
{
printf("hello world");
}
gcc void.c
Compiles.
Also check http://www.compileonline.com/compile_c_online.php
So in conclusion (even though I do not find references): In earliest C versions void main(void) was probably not forbidden.
However:
Without specifying a return value, you just do not now what your program returns. So, standard or not, correct or false, do not use it, because it makes your program non-deterministic.
This is my program:
main()
{
printf("hello world\n");
}
I get this warning when compiling it:
function should return a value
When changing main() to void main(), the warning disappears.
Why is that so?
There are few things which you should take note of :
The int is the main() function's return type. That means that the kind of value main() can
return is an integer.
main( ) was tolerated by the C90 compilers but not by C99 compilers which means its not a part of C99 standard anymore , so don't do this.
void main() is not a standard form ,some compilers allow this, but none of the standards have ever listed it as an option. Therefore,
compilers don't have to accept this form, and several don't. Again, stick to the standard form,
and you won't run into problems if you move a program from one compiler to another.
And one last thing , instead of writing main like this :
int main() // here you are being silent about passing arguments to main , meaning it may or may not take arguments
write like this :
int main(void)// this specifies there are no arguments taken by main
You might wanna look at the C99 standard for further details.
Quick summary: If you don't want to use command-line arguments, you should write:
int main(void) {
/* body of main function */
}
If you do:
int main(int argc, char *argv[]) {
/* ... */
}
These are the only portable ways to define the main function.
You should probably have a return 0; at the end, though it's not strictly necessary. Returning 0 indicates successful execution. There are ways to indicate that execution failed; I won't get into that here.
There's some history behind this. The rules for a valid definition of the main function have changed a bit across different releases of the C standard.
Before the introduction of the first official standard for C in 1989, the most common form was:
main()
{
/* ... */
}
Or, if you wanted to use command-line arguments:
main(argc, argv)
/* argc is implicitly of type int */
char *argv[];
{
/* ... */
}
There was no way to define a function that didn't return a value. If you didn't specify a return type, it defaulted to int.
The 1989 ANSI C standard (which was republished with editorial changes as the 1990 ISO C standard) introduced prototypes, function declarations and definitions that specify the parameter types. There are two equally valid definitions for main. You can use one or the other depending on whether you need to use command line arguments:
int main(void) {
/* ... */
}
or
int main(int argc, char *argv[]) {
/* ... */
}
(char *argv[] can also be written as char **argv. This rule applies only to parameter definitions.)
A given compiler may or may not choose to permit other forms. For example, some compilers support a third parameter envp.
Somehow, some authors have gotten the idea that void main() or void main(void) is valid. It can be valid for some particular compiler, but only if that compiler explicitly supports it. It's not portable. The odd thing about this is that the same standard that first introduced the void keyword simultaneously established the rule that main's return type is int.
void main() is useful as an indicator that the author of the book you're reading doesn't know the C language very well, and that you should find another book.
The story is different for "freestanding" (embedded) systems. For such systems, the program's entry point is entirely implementation-defined, and might not even be called main. Defining it as void main(void) may well be valid for such systems.
The 1999 ISO C standard dropped the "implicit int" rule. Taking advantage of that rule was probably never a good idea in the first place. As of ISO C 1990, you could legally use:
main(void) { /* ... */ }
because it was equivalent to:
int main(void) { /* ... */ }
As of the 1999 standard, the int is mandatory.
The 1999 standard also added a special-case rule: reaching the closing } of the main function is equivalent to executing return 0;. It's still not a bad idea to add the explicit return 0;, especially if your code might be compiled with a pre-C99 compiler.
The 2011 ISO C standard didn't make any changes in this area.
The difference between int main() and int main(void) is that the latter explicitly says that main takes no arguments; the former doesn't specify how many arguments it takes. Use the int main(void) form. There have been debates about whether int main() is even legal.
You can likely get away with writing void main(), since it's an error that compilers are not actually required to diagnose (it's undefined behavior unless the implementation documents it).
The bottom line: The proper definition of main has a long and varied history, and there are a lot of variant forms you can probably get away with using. But unless you're programming for an embedded system, there is no point in using anything other than one of the two officially valid forms:
int main(void) { /* ... */ }
int main(int argc, char *argv[]) { /* ... */ }
c automatically implies the datatype int to functions with no declared datatype. So as far as the compiler is concerned the above is:
int main()
{
printf("hello world\n");
}
This expects that you would return an integer at the end of it with a return statement. If you explicitly specify it as void main() you are telling the compiler that the function does not have a return value, hence no warning.
The reason that this is not an error is that if not specified, main() will return 0; at the end of execution. However the compiler is still giving you a warning that this is happening.
Best practice is to use int main() and then return 0 at the end of your program execution like this.
int main()
{
printf("hello world\n");
return 0;
}
See: this question for more information.
You got the warning because you didn't specify the return type of main.
You should always use int main, and return an int number, usually 0 for success.
int main()
{
printf("hello world\n");
return 0; //you can omit this since C99
}
Using void main on a hosted environment(normally we are, if not, the following doesn't have to be true) leads to undefined behavior, even though it works in some compilers, never use it.
The standard says main has two kinds of prototype, both returns int:
C11 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;10) or in some other implementation-defined manner.
write
return 0 ;
at the last line.
What does the following mean :
int main(void) {...}
VS
int main() {...}
?
I think that int main() {...} means that main doesn't receive any parameters (from command line) , however:
int main(int argc, char *argv[])
does.
But, what does int main(void) {...} mean? And, what does void stand for ?
I've looked here but it's somehow a different question .
In C++, there is no difference.
In C, the difference is questionable. Some love to argue that the latter version (the one without void) is technically just a common implementation extension and not guaranteed to work by the standard because of the wording in the standard. However, the standard clearly states that in a function definition an empty set of parameters has a well-defined behaviour: that the function does not take any parameters. Thus such a definition for main matches the following description in the standard:
It [main] shall be defined with a return type of int and with no parameters.
There is, however, a noticeable difference between the two: namely, the version without void fails to provide a correct prototype for the function:
// this is OK.
int main()
{
if (0) main(42);
}
// this requires a diagnostic to be shown during compiling
int main(void)
{
if (0) main(42);
}
Oh, and just to be complete: the void has the following meaning in all function declarators:
(6.7.6.3p10) The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.
First of all, there is a difference of what is allowed for hosted systems and freestanding systems, as shown here.
For hosted systems, 5.1.2.2.1 Program startup applies:
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)
... (more text follows regarding argv/argc etc styles).
The interesting part is "with no parameters". int main() and int main (void) are currently equivalent, since they are both function declarators and have no parameters. The following applies (6.7.6.3 ):
10 The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.
/--/
14 An identifier list declares only the identifiers of the parameters of the function. An empty
list in a function declarator that is part of a definition of that function specifies that the
function has no parameters. The empty list in a function declarator that is not part of a
definition of that function specifies that no information about the number or types of the
parameters is supplied.145)
Emphasis mine, the bold text is what applies to int main(). There is also note 145) at the end of the text, which says "See ‘‘future language directions’’ (6.11.6)":
6.11.6 Function declarators
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.
And here is the difference. Being a function declarator, int main() is bad style because of the above, since it is not guaranteed to work in the next version of the C standard. It is flagged as an obsolescent feature in C11.
You should therefore always use int main (void) on a hosted system and never int main(), even if the two forms are, for now, equivalent.
In C++ both forms are completely equivalent, but there int main() is the preferred style for subjective, cosmetic reasons (Bjarne Stroustrup says so... which is probably quite a bad rationale for explaining why you do something in a particular way).
In C, in a prototype (not in C++ though) an empty argument list means that the function could take any arguments (in the definition of a function, it means no arguments). In C++, an empty parameter list means no arguments. In C, to get no arguments, you have to use void. See this question for a better explanation.
In C++ having a function foo(void) and foo() is the same thing. However, in C it's different: foo(void) is a function that has no arguments, while foo() is a function with unspecified arguments.
In C++, there is no difference, both are same.
Both definitions work in C also, but the second definition with void is considered technically better as it clearly specifies that main can only be called without any parameter.
In C, if a function signature doesn’t specify any argument, it means that the function can be called with any number of parameters or without any parameters. For example, try to compile and run following two C programs (remember to save your files as .c).
In C++, there is no difference between the two, and int main() is a legal signature and return type for main.
I know the thread is old but this question was bothering me for a while a few years ago so I wanted to throw in my half a cent(if that).
I always treat C functions as if they have fixed amount of arguments regardless of context, unless they use va_args. That is, I trust main to ALWAYS have the prototype:
int main(int argc, char **argv).
even if no arguments are passed, the function has these arguments on the stack because the main function does not have function overloading.
C does have the ability to have primitive overloading through just pretending the argument is not there. In which case, the argument is still passed and is on the stack but you never access it, so it merely reduces size of the source code.
Saying int main() simply means that I know that the function may have parameters, but I am not using them, so I write int main().
Saying int main(void) says that main CERTAINLY has no arguments, and implies that there are two different function prototypes:
int main(void);
int main(int argc, char **argv);
Since C has no function overloading, this is somewhat misleading to me, and I distrust code that has main(void) in it. I would not if main NEVER took any parameters, in which case main(void) would be completely OK.
NOTE: In some implementations, there are more parameters in main than argc and argv, such as env, but this does not bother me because I know that I do not explicitly say that those are the only two parameters, but those are the minimal parameters and it's okay to have more, but not less. This is in contrast to downright saying int main(void) which yells at me as THIS FUNCTION HAS NO PARAMETERS, which isn't true, since it does, they are just omitted.
Here is my basis code:
/* sample.c - build into sample. */
#include <stdio.h>
int main(void)
{
int _argc = *((int *)2686800);
char ***_pargv = (char ***)2686804;
int i;
for (i = 1; i < _argc; ++i) {
printf("%s ", (*_pargv)[i]);
}
return 0;
}
./sample I clearly have arguments
The function clearly has arguments passed to it, despite going out of the way to explicitly say that it doesn't by typing void into the function prototype.
As eq- says above:
(6.7.6.3p10) The special case of an unnamed parameter of type void as the
only item in the list specifies that the function has no parameters.
Thus saying that the function has void as an argument but actually having arguments on the stack is a contradiction.
My point is that arguments are still there, so explicitly asserting that main is void of arguments is dishonest. The honest way would be to say int main(), which claims nothing about how many parameters it has, only how many parameters you are care about.
NOTE2: The _argc, _pargv are system dependent, to find your values you must find them out by running this program:
/* findargs.c */
#include <stdio.h>
int main(int argc, char **argv)
{
printf("address of argc is %u.\n", &argc);
printf("address of argv is %u.\n", &argv);
return 0;
}
These values should remain correct for your specific system.
In C++, there is no difference between int main() and int main(void).
But in C they are little bit different.
int main() indicates that the main function can be called with any number of parameters or without any parameter. On the other hand, int main(void) indicates that the main function will be called without any parameter
#include <stdio.h>
int main()
{
static int i = 5;
if (--i){
printf("%d ", i);
main(10);
}
}
Output: 4 3 2 1
#include <stdio.h>
int main(void)
{
static int i = 5;
if (--i){
printf("%d ", i);
main(10);
}
}
It will show error. Because, in int main(void) parameter is void but in the program we have taken main(10) (which defines some value, not void)
Technically, if your host is partially POSIX compliant, then you have
int main(); // this legacy is always used by the run time library
int main(int argc); // illegal by compiler
int main(int argc, char** argv); // required by C standards
int main(int argc, char** argv, char** envp); // required by POSIX standard
If you have a Mac, there is also this
int main(int argc, char** argv, char** envp, char** apple); // required by Macintosh standard
Your host will send all the arguments, so a host will always send argc, argv, and envp (and apple if you are using an Apple product), but the programmer could have their main declared as taking void. The implicit function pointer typecast is technically an undefined behavior.
To prevent the typecast undefined behavior, int main() is a neutral form that means it could take any fixed number of arguments using canonical type promotion (int or larger, and double or larger) and int main(int argc, ...) means it could take any variable number of arguments also with canonical type promotion. In other words, the form return_type function_name() is an exception to undefined behavior.
In C++:
ㅤㅤint main() and int main(void) are the same in C++. They both take 0 and only 0 parameters.
In C:
ㅤㅤint main() takes as many arguments as you want. The function just won't use them. intㅤㅤㅤ main(void) makes it so passing an argument will create an error and make it impossible to ㅤㅤpass arguments.
ㅤ
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
What should main() return in C/C++?
Difference between void main and int main?
I have always been using the main method in C like
void main(){ // my code }
and it works pretty well for me.
I also know about the other int return type:
int main(void)
int main()
int main(int argc, char *argv[])
But I have not been able to find any resource that says that I can use void as a return type. Every book suggests that the return type must be int or else it be omitted. Then why does void main() work?
Is this dependent on the version of C that I am using? Or does it work because I use a C++ IDE? Please reply specific to C and not C++.
Only book authors seem to be privy to the place where a return type of void for main() is allowed. The C++ standard forbids it completely.
The C standard says that the standard forms are:
int main(void) { ... }
and
int main(int argc, char **argv) { ... }
allowing alternative but equivalent forms of declaration for the argument types (and the names are completely discretionary, of course, since they're local variables to the function).
The C standard does make small provision for 'in some other implementation defined manner'. The ISO/IEC 9899:2011 standard says:
5.1.2.2.3 Program termination
If the return type of the main function is a type compatible with int, a return from the
initial call to the main function is equivalent to calling the exit function with the value
returned by the main function as its argument;11) reaching the } that terminates the
main function returns a value of 0. If the return type is not compatible with int, the
termination status returned to the host environment is unspecified.
11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main
will have ended in the former case, even where they would not have in the latter.
This clearly allows for non-int returns, but makes it clear that it is not specified. So, void might be allowed as the return type of main() by some implementation, but you can only find that from the documentation.
(Although I'm quoting C2011 standard, essentially the same words were in C99, and I believe C89 though my text for that is at the office and I'm not.)
Incidentally, Appendix J of the standard mentions:
J.5 Common extensions
The following extensions are widely used in many systems, but are not portable to all
implementations. The inclusion of any extension that may cause a strictly conforming
program to become invalid renders an implementation nonconforming. Examples of such
extensions are new keywords, extra library functions declared in standard headers, or
predefined macros with names that do not begin with an underscore.
J.5.1 Environment arguments
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).
Why does void main() work?
The question observes that void main() works. It 'works' because the compiler does its best to generate code for programs. Compilers such as GCC will warn about non-standard forms for main(), but will process them. The linker isn't too worried about the return type; it simply needs a symbol main (or possibly _main, depending on the system) and when it finds it, links it into the executable. The start-up code assumes that main has been defined in the standard manner. If main() returns to the startup code, it collects the returned value as if the function returned an int, but that value is likely to be garbage. So, it sort of seems to work as long as you don't look for the exit status of your program.
From the horse's mouth:
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[]) { /* ... */ }
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 loophole is the "some other implementation-defined manner". An implementation may allow main to return void (or any other type), but it must explicitly document that such a signature is allowed. Otherwise the behavior is undefined, meaning the compiler can do anything it wants. The program may execute without any problems. It may execute, but leave the environment in a bad state. It may crash on exit. It may fail to load at all.
It is dependent on the compiler you are using, but void main is not compilable everywhere. I have seen compilers that won't compile a program with void main. I can not recall the particular case(for c), but I know for sure this happens in g++(yes I know this is c++).
The standard calls for main() to return int, but a lot of C compilers allow you to specify the return type of main() as void.
I recommend you get into the habit of returning int. Adding a
return 0;
to the end of your main() isn't too much effort.
I was acceptable in C89 but it is no longer considered "safe". Under C99, it is no longer acceptable.
http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?id=1043284376&answer=1044841143