correctly declaring the main() function in ANSI C

问题:

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• What should main() return in C and C++? 18 answers

The C standard say:

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.

However, Kernighan & Ritchie in their second edition (the canonical ANSI C) bible just use:

main()
{
  /* taram pampam ... */

  return 0;
}

Who is right? Does it have to do with function without return value automatic assume to be returning int in C?

回答1:

Well, if you want ANSI C, then by definition the standard is right.

In C89/C90 the int return type is implied, so the K&R definition would be acceptable.

In C99 this is no longer the case.

The C90 standard has the following wording (5.1.2.2.1 Program startup), which is very similar to the C99 wording (probably most significantly it uses the less strong 'can' instead of 'shall'):

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[]) { /* ... */ }

If they are defined, the parameters to the main function shall obey the following constraints:

[etc. ...]

There's nothing in that section directly about the fact that leaving off the return type will result in it defaulting to int.

Frankly, I have a hard time finding exactly where that behavior is specified by the standard. The closest I can come is in 6.7.1 (Functions definitions) where the grammar for function definitions indicates that the 'declaration-specifiers' are optional, and the examples say:

Examples:

1. In the following:

     extern int max(int a, int b)
     {
         return a > b ? a : b;
     }
   

   extern is the storage class specifier and int is the type specifier (each of which may be omitted as those are the defaults)...

回答2:

Yes, in C89 (the original C standard), if a function is declared without a return type, it is assumed to return int. C99 requires an explicit return type on all functions.

回答3:

Also, there's a subtle difference (at least in declarations) between main() and main(void) --

main()

is a function (implicitly) returning int and taking an unspecified number of arguments

main(void)

takes no arguments.

回答4:

The version of K&R I have was printed in 1988. The Standard wasn't out by then, so there are some inconsistencies. However, most of the 2nd edition complies with the C89 Standard.

I found a text version of the C89 Standard (YAY for Google); it says:

"Program startup"

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[]) { /*...*/ }