dgsmith_50:

The following definitely doesn't do everything you want, as it's a quick knockup. It'll need some tuning, but I think you can tweak it into something useful for you:

$ cat C_preprocessor_light.pl
#!/usr/bin/perl
#
#   see perlmonks 951682
#
use strict;
use warnings;

my %DEF;
my @fl_print = (1);
while (<DATA>) {
    if (/^\s*#\s*define\s+(\w+)/)   { $DEF{$1}=1; }
    elsif (/^\s*#\s*undef\s+(\w+)/) { delete $DEF{$1}; }
    elsif (/^\s*#\s*ifdef\s+(\w+)/) {
        push @fl_print, (exists($DEF{$1}) and $fl_print[-1]);
    }
    elsif (/^\s*#\s*ifndef\s+(\w+)/){
        push @fl_print, (!exists($DEF{$1}) and $fl_print[-1]);
    }
    elsif (/^\s*#\s*endif/){
        pop @fl_print if @fl_print > 1;
    }
    print if ($fl_print[-1]);
}
__DATA__
/* Foo */
#ifndef BLARG2
BAR
#define BLARG
#define GRAK
zippy
#ifdef BLARG
happy
#ifndef GRAK
lucky
#endif
puppy
#endif
#foo
#endif
bloink
[download]

It doesn't hide exactly the lines I would want to hide, but it's close...

...roboticus

When your only tool is a hammer, all problems look like your thumb.