Why do I see so much C code that omits a return type for main? Why does it work?

Question

I went to this website that compiles C online and saw the main function declared without a return type.

I'm aware of some questions concerning this topic here , but I didn't find any about omitting the return type. I tried to compile the code using gcc and it worked.

Does this mean that if we don't put a return type on main , it will assume it is int (or any other type)?

Answer 1

The C89 standard, to preserve compatibility with the original K&R version of C that did not have function prototypes as we now know them, allowed functions to implicitly return int . Any function that was declared without an explicit return type (ie, void , float , etc.) was assumed by the compiler to return int .

Thus, when the main function was declared without a return type, it was assumed to return type int . All was well and good, since main was supposed to return int , according to the standard.

However, this changed in C99. The default/implicit int rule was removed from the language specification. Functions without an explicit return type are no longer assumed to return int .

That means that for any modern compiler, adhering to the current version of the C language specification, a declaration of main without a return type is invalid.

As for why it works on GCC, this is because by default, GCC still adheres to the C89/C90 standard, unless you explicitly specify -std=c99 as a compiler flag. And for why you still see this online, well, there are two reasons. The first is the one I've already given: it was legal in older versions of the language specification, and lots of old code hasn't been updated. The second reason is that, unfortunately, there is lots of bad C code online and in books. You may have just found some.

Answer 2

Try to turn on warnings, GCC should tell you that this is normally forbidden:

test.c:1:1: warning: return type defaults to ‘int’ [-Wreturn-type]
 main()
 ^

This used to have a defined meaning in older C, where the default type of functions was int . Compilers still support it in order not to break code and because it is not a big issue, ie the code isn't suddenly ambiguous.

Answer 3

If the return type is omitted in C89, the function is presumed to return a value of type int . In C99/11, it is not legal to omit the return type of a function. The signature of main is defined as:

C11 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;10) or in some other implementation-defined manner.

Compile your code with -std=c99 flag and your compiler will raise a warning about the omitted return type.