C: wrongly nested switch/case blocks surprisingly works

Question

I just discovered the following section in some code I maintain:

switch (m) {
    case 62: { // opening
        // some declarations
        // do some stuff
        break;
    case 63:
        // do some other stuff
        break;
        }      // closing
    default:
        // default stuff
        break;
 }   

The block opening is meant to declaring some local variables, but the closing brace is wrongly placed and occurs after the case 63.

I have never noticed this for months as it compiles well in Visual Studio 2010. I've tried debugging it and both cases work fine.

How can it be ? Is this correct C syntax ?

Answer 1

The case statements are just like goto labels and so that is allowed syntax. Duff's device is a famous use-case.

Try to avoid doing it though.

Answer 2

6.8.1 Labeled statements, C99

Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them.

i.e. The curly braces have no effect on how the switch-case labels work but it merely creates a new scope.

This explains why the seemingly misplaced curly braces don't result in a syntax error.

Answer 3

Surprisingly, this is correct syntax per the language standard. You can even do this:

switch (m) break;

or this:

switch (m);

case const-expr: works pretty much like a regular label that you'd use with goto.