This source code is switching on a string in C. How does it do that?

Question

I\'m reading through some emulator code and I\'ve countered something truly odd:

switch (reg){
    case \'eax\':
    /* and so on*/
}

How i

Answer 1

According to the C Standard (6.8.4.2 The switch statement)

3 The expression of each case label shall be an integer constant expression...

and (6.6 Constant expressions)

6 An integer constant expression shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof operator.

Now what is 'eax'?

The C Standard (6.4.4.4 Character constants)

2 An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x'...

So 'eax' is an integer character constant according to the paragraph 10 of the same section

10. ...The value of an integer character constant containing more than one character (e.g., 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined.

So according to the first mentioned quote it can be an operand of an integer constant expression that may be used as a case label.

Pay attention to that a character constant (enclosed in single quotes) has type int and is not the same as a string literal (a sequence of characters enclosed in double quotes) that has a type of a character array.