Idle Time

Consider this code:

enum { foo, bar, baz } my_var;
switch (my_var) {
    case foo:
        int foo_alpha; //Line 7
        int foo_beta;
        break;

    case bar:
        my_var = baz;
    case baz:
        printf("Bar or baz encountered\n");
        break;
}

Try to compile it as C99.

Here’s what GCC says:

test.c: In function ‘main’:
test.c:7: error: syntax error before ‘int’

What!

First off, why not? What is invalid about this declaration statement?

Second, why is foo_alpha invalid and not foo_beta?

There are several definitions in the C99 specification that come together to cause this problem.

The first is that there is no such thing as a declaration statement, because declarations are not statements. See §6.7 and §6.8; note that neither definition includes the other. In the language that is C99, declarations and statements are separate concepts.

The second is the definition of a compound statement. The definition of a switch statement (which is part of §6.8.4) is:

switch ( expression ) statement

If you go back up to §6.8, you’ll see that another possible kind of statement is a compound statement, for which §6.8.2 gives this definition:

compound-statement:

• {   block-item-list_opt   }

block-item-list:

• block-item
• block-item-list   block-item

block-item:

• declaration
• statement

So a declaration is not a statement, a compound statement can contain declarations and/or statements, and a switch statement is a prefix upon (usually) a compound statement.

Now, the kicker. Read the relevant definition of a labeled statement from §6.8.1:

case   constant-expression   :   statement

Statement. Not block-item. Not declaration. Statements only.

So this is what the compiler sees in valid code (with a declaration not following a case label):

• selection statement (switch)
  • compound statement
    • labeled statement
      • statement
    • statement
    • statement
    • declaration
    • statement
    • jump statement (break)
    • labeled statement
    • statement
    • jump statement (break)
    • labeled statement
    • statement
    • jump statement (break)

Now, consider how the compiler sees my code above:

• selection statement (switch)
  • compound statement
    • labeled statement
      • declaration — wait, this isn’t a statement! ERROR
    • declaration — also a kind of block-item, so it’s perfectly valid here
    • jump statement (break)
    • labeled statement
    • statement
    • labeled statement
    • statement
    • jump statement (break)

I hope this makes clear that this isn’t a compiler bug; the C99 language really does work this way.

(One possible solution would be to make a declaration a kind of statement, but I don’t know what other ramifications that might have. [UPDATE 11:36: Jeff Johnson tells us why not.])

Categories: C; Programming. | Comments: 5 (feed).