Using a define that hasn't been #defined

[Tony Burden]

Recently I've run into a couple of hard-to-track bugs because I have
a section of conditionally compiled code using a symbol which hasn't
been defined. For example, take the trivial case of this "program"

-------

#define XYZZY 1

main() {
#if XYZZU
printf("xyzzy\n");
#else
printf("plugh\n");
#endif

}

------

Now, obviously that XYZZU was supposed to be an XYZZY, and I wanted to
conditionally compile the first piece of code, not the second. But when
I compile this, it doesn't even give me a friendly warning, and just
assumes XYZZU is 0. (Which, I understand, is in accordance with ANSI-
defined behavior.)

Now, my question is how other people avoid these same kinds of idiotic
mistakes? Even if I put in "#if !defined(XYZZY)..." type things for
every define, it still wouldn't catch all typos. There should be some
way to avoid this idiotic kind of error...

 

[Ian Collins]

Recently I've run into a couple of hard-to-track bugs because I have
a section of conditionally compiled code using a symbol which hasn't
been defined. For example, take the trivial case of this "program"

-------

#define XYZZY 1

main() {
#if XYZZU
printf("xyzzy\n");
#else
printf("plugh\n");
#endif

}

------

Now, obviously that XYZZU was supposed to be an XYZZY, and I wanted to
conditionally compile the first piece of code, not the second. But when
I compile this, it doesn't even give me a friendly warning, and just
assumes XYZZU is 0. (Which, I understand, is in accordance with ANSI-
defined behavior.)

Now, my question is how other people avoid these same kinds of idiotic
mistakes? Even if I put in "#if !defined(XYZZY)..." type things for
every define, it still wouldn't catch all typos. There should be some
way to avoid this idiotic kind of error...

Unit tests.

 

[Ben Pfaff]

Now, obviously that XYZZU was supposed to be an XYZZY, and I wanted to
conditionally compile the first piece of code, not the second. But when
I compile this, it doesn't even give me a friendly warning, and just
assumes XYZZU is 0. (Which, I understand, is in accordance with ANSI-
defined behavior.)

Now, my question is how other people avoid these same kinds of idiotic
mistakes? Even if I put in "#if !defined(XYZZY)..." type things for
every define, it still wouldn't catch all typos. There should be some
way to avoid this idiotic kind of error...

If you write your code carefully, so as never to use a macro that
is not defined, then you could turn on a compiler warning about
an undefined macro in an #if directive. GCC has -Wundef, for
example.

 

[Thad Smith]

#define XYZZY 1

main() {
#if XYZZU
printf("xyzzy\n");
#else
printf("plugh\n");
#endif

}

------

Now, my question is how other people avoid these same kinds of idiotic
mistakes?

main() {
#if XYZZU == 1
printf("xyzzy\n");
#elif XYZZU == 2
printf("plugh\n");
/* etc */
#else
#error no valid definition for XYZZU
#endif

}

 

[Jorgen Grahn]

Recently I've run into a couple of hard-to-track bugs because I have
a section of conditionally compiled code using a symbol which hasn't
been defined. For example, take the trivial case of this "program"

-------

#define XYZZY 1

main() {
#if XYZZU
printf("xyzzy\n");
#else
printf("plugh\n");
#endif

}

------

Now, obviously that XYZZU was supposed to be an XYZZY, and I wanted to
conditionally compile the first piece of code, not the second. But when
I compile this, it doesn't even give me a friendly warning, and just
assumes XYZZU is 0. (Which, I understand, is in accordance with ANSI-
defined behavior.)

Yes, I have been bitten by that one too.

Now, my question is how other people avoid these same kinds of idiotic
mistakes? Even if I put in "#if !defined(XYZZY)..." type things for
every define, it still wouldn't catch all typos. There should be some
way to avoid this idiotic kind of error...

(a) Macros suck, so don't expect it to be painless or safe. Google
"ifdef considered harmful" by Henry Spencer et al.

(b) If you can keep it a "boolean", use #ifdef FOO and #define FOO, and
try to stay away from #if and #define FOO 1.

/Jorgen

 

[Tom St Denis]

main() {
#if   XYZZU == 1
          printf("xyzzy\n");
#elif XYZZU == 2
          printf("plugh\n");
/* etc */
#else
        #error no valid definition for XYZZU
#endif

DING DING DING DING, winner.

Though if it's a binary flag I'd just use "ifdef" or "if defined(...)"
since it's less error prone.

Tom