c preprocessor - fail if compiling after certain date

Question

I would like to make the compilation of some files to fail if attempted after a certain date. The reason for this: I found a couple of Y2K38 bugs which I don't have time to fix right now, but would like to make a note of them and I think it would be nice if compilation of the module would just fail after, say, 2020. (I might be insane, but this code is 20 years old I suspect it might survive another 30)

Answer 1

With GCC, you can do something like the following:

void __attribute__((error("Whoa. It's the future"))) whoa_the_future();

void check_for_the_future() {
    // "Feb  1 2011"
    const char *now = __DATE__;
    if (now[9] >= '2')
        whoa_the_future();
}

The way this works is that the error attribute tells GCC to generate a compile-time error if any calls to that function are left in the code after all of GCC's constant-folding, dead-code elimination, and similar passes have run. Since DATE is a compile-time constant, GCC can evaluate the if statement at compile time and remove the call.

At least one downside is that this depends on GCC's optimization passes, and so it won't work at gcc -O0

Honestly, you might be better off just adding a runtime check somewhere and failing fast.

Answer 2

Instead of dealing with the awkward format of the __DATE__ macro, why not roll your own?

gcc -DTHIS_YEAR=`/bin/date +%Y` yourprogram.c

Then your code can use expressions like #if THIS_YEAR >= 2020.