c - Why does omitting explicit 'int' type for a parameter fail to compile in gcc sometimes?

Question

When declaring variables in C you can omit the type sometimes if you want to declare an int.

Why does omitting explicit 'int' type for a parameter fail to compile in gcc with other non-int parameters, unless declared in the K&R style?

This code generates an error:

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

With the following output:

$gcc XXX.c -oXXX
XXX.c:X:X: error: expected ‘)’ before ‘char’
 main(argc, char *argv[])
            ^

However, if I write K&R style types for the parameters I can omit specifying an int type for the first parameter:

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

And that compiles fine.

I suspect the reason is that when making the first C standard they decided that the K&R automatic-int notation should be completely seperate from the newer syntax for function parameters, especially since by the time the standard was being pulled together the automatic-int notation was likely already considered poor notation.

My interest in these rules is academic, I don't generally write this old-style C.

Answer 1

The compiler is employing one of two grammars.

When applying the K&R grammar, undeclared parameters are allowed, and default to ints.

When applying the non-K&R grammar, all parameters must comply with the parameter declaration syntax i.e. declared with types and names.

You invoke one or the other by choosing the corresponding declaration style.