Word For Today: “Uglification”

From /usr/include/«arch»/bits/select.h on my Debian system:

    #define __FD_ZERO(s) \
      do {       \
        unsigned int __i;       \
        fd_set *__arr = (s);       \
        for (__i = 0; __i < sizeof (fd_set) / sizeof (__fd_mask); ++__i)       \
          __FDS_BITS (__arr)[__i] = 0;       \
      } while (0)

Note how this macro brings the entire expression for “s” into the
scope containing those temporary “__i” and “__arr” variables. You just
better hope they won’t clash.

I think there is a clause in the C spec that says names beginning with
underscores (“uglified” names, I think they’re called) are reserved
for library implementors or something. But what happens if one library
implementation depends on another? What keeps the choices of names
from clashing in that situation? Just luck, I guess.

Basically, string-substitution macros are fiddly, fragile, and prone
to mysterious syntax errors in the target language if you’re not
careful. I thought initially that this was down to limitations in the
the C/C++ preprocessor; maybe a more powerful one, like m4, would
help. But it turns that is even more fiddly and fragile, and prone to
mysterious syntax errors if you’re not careful.