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

As other have said, this is an int constant and its actual value is implementation-defined.

I assume the rest of the code looks something like

if (SOMETHING)
    reg='eax';
...
switch (reg){
    case 'eax':
    /* and so on*/
}

You can be sure that 'eax' in the first part has the same value as 'eax' in the second part, so it all works out, right? ... wrong.

In a comment @Davislor lists some possible values for 'eax':

... 0x65, 0x656178, 0x65617800, 0x786165, 0x6165, or something else

Notice the first potential value? That is just 'e', ignoring the other two characters. The problem is the program probably uses 'eax', 'ebx', and so on. If all these constants have the same value as 'e' you end up with

switch (reg){
    case 'e':
       ...
    case 'e':
       ...
    ...
}

This doesn't look too good, does it?

The good part about "implementation-defined" is that the programmer can check the documentation of their compiler and see if it does something sensible with these constants. If it does, home free.

The bad part is that some other poor fellow can take the code and try to compile it using some other compiler. Instant compile error. The program is not portable.

As @zwol pointed out in the comments, the situation is not quite as bad as I thought, in the bad case the code doesn't compile. This will at least give you an exact file name and line number for the problem. Still, you will not have a working program.