Thread: Why make a constant a constant again?

The following is from a FreeBSD modem dialup program.

in ward.h

Code:

#define SPEAKER_ON      "M1"
#define SPEAKER_OFF     "M0"
#define SPKR_VOL_0      "M1L0"
#define SPKR_VOL_1      "M1L1"
#define SPKR_VOL_2      "M1L2"
#define SPKR_VOL_3      "M1L3"

And in modem.c

Code:

const char *Vol [] = { 
        SPEAKER_OFF, SPKR_VOL_0, SPKR_VOL_1, SPKR_VOL_2, SPKR_VOL_3
};

[[email protected] award-1.0]# grep -iR SPEAKER_OFF /tmp/award-1.0
/tmp/award-1.0/modem.c: SPEAKER_OFF, SPKR_VOL_0, SPKR_VOL_1, SPKR_VOL_2, SPKR_VOL_3
/tmp/award-1.0/ward.h:#define SPEAKER_OFF "M0"

#define already makes stuff like SPEAKER_OFF constant. Why make it constant again in modem.c? Also, why not use these constants?

I think part of the confusion stems from what #define is really doing.
#define doesn't create constants as you might be thinking of them. #define will just create a macro, which is essentially text substitution. So if you have the following:

Code:

#define FOO "foo"
const char *a[] = { FOO };

You'll actually wind up with the following code:

Code:

const char *a[] = { "foo" };

FOO isn't a variable at all, but something that's simply replaced. That explains why it acts like a constant:

Code:

#define FOO "foo"
FOO = "bar";

This doesn't work because it'd expand to: "foo" = "bar"; nonsense, clearly. But macros don't have a type, so they're not flagged as "constant". They expand to something that might have a type; in the instant case you see that they simply expand to a string literal, which has type array-of-char. No const! See the above posts for why you'd then want to use const yourself.