an easy way to check for the existence of a symbol

[gvvaughan]

I'm writing some Lua code (GNU Zile) that would gain a huge speed boost by using memrchr to scan back through an alien.buffer -- but memrchr is a GNU extension not available on Mac OS X among others. I have a brute force reverse search implemented in Lua over an alien.array, but I'd like to have that as a fallback preferring memrchr from libc when available.

Unfortunately, alien throws an error during parsing:

  $ lua52 -l alien -e 'print (tostring (alien.default.memchr)))'
  alien function memchr, library default
  $ lua52 -l alien -e 'print (tostring (alien.default.memrchr)))'
  /usr/local/opt/lua52/bin/lua: dlsym(RTLD_DEFAULT, memrchr): symbol not found
  stack traceback:
          [C]: in function '__index'
          (command line):1: in main chunk
          [C]: in ?
  $ lua52 -l alien -e 'if false then print (tostring (alien.default.memrchr))) end'
  /usr/local/opt/lua52/bin/lua: dlsym(RTLD_DEFAULT, memrchr): symbol not found
  stack traceback:
          [C]: in function '__index'
          (command line):1: in main chunk
          [C]: in ?

The best workaround I have is to write a test function in a string, and call loadstring() inside pcall to protect from the parse-time error.

Much better would be simply return nil for missing symbols, and then I could write much cleaner code:

local def = alien.default
if def.memrchr ~= nil then
  def.memrchr:types ("pointer", "pointer", "int", "size_t")
else
  function memrchr (buf, ch, o) ... end
end

Thoughts? I'll try to write a patch if you agree this is a good way forward.