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

The code fragment uses an historical oddity called multi-character character constant, also referred to as multi-chars.

'eax' is an integer constant whose value is implementation defined.

Here is an interesting page on multi-chars and how they can be used but should not:

http://www.zipcon.net/~swhite/docs/computers/languages/c_multi-char_const.html

---

Looking back further away into the rearview mirror, here is how the original C manual by Dennis Ritchie from the good old days ( https://www.bell-labs.com/usr/dmr/www/cman.pdf ) specified character constants.

2.3.2 Character constants

A character constant is 1 or 2 characters enclosed in single quotes ‘‘ ' ’’. Within a character constant a single quote must be preceded by a back-slash ‘‘\’’. Certain non-graphic characters, and ‘‘\’’ itself, may be escaped according to the following table:

    BS \b
    NL \n
    CR \r
    HT \t
    ffffd \ffffd
    \ \\

The escape ‘‘\ffffd’’ consists of the backslash followed by 1, 2, or 3 octal digits which are taken to specify the value of the desired character. A special case of this construction is ‘‘\0’’ (not followed by a digit) which indicates a null character.

Character constants behave exactly like integers (not, in particular, like objects of character type). In conformity with the addressing structure of the PDP-11, a character constant of length 1 has the code for the given character in the low-order byte and 0 in the high-order byte; a character constant of length 2 has the code for the first character in the low byte and that for the second character in the high-order byte. Character constants with more than one character are inherently machine-dependent and should be avoided.

The last phrase is all you need to remember about this curious construction: Character constants with more than one character are inherently machine-dependent and should be avoided.