This here is the epitome of craziness. I could very well give up my search for more scary things in Emacs because I very much doubt I’ll find anything more fear-inducing, but first of all, let’s explain what’s wrong with it. Unlike my previous examples it’s not something I can show you by just pointing at code snippets, no, the problem is of a much more fundamental nature than that.
The exact details of Emacs’ build process have always been a bit of mystery to me. Sure, compiling the C sources to obtain an Emacs Lisp interpreter and using it to byte-compile all Emacs Lisp files makes sense. But loading these files with it to “dump” the Emacs binary? The only time I’ve heard of that was when reading a discussion about creating “executables” with Common Lisp which is apparently achieved by serializing the program’s current state to disk.
However, it works a bit differently for Emacs. After loading up the bare minimum temacs executable with all necessary Emacs Lisp files, the so-called unexec function reads in it own executable it’s originating from, scans over the entire process memory, then modifies the file to contain the state obtained and saves it into the emacs executable. In other words, this file contains a partial and very platform-specific reimplementation of a linker. You can take a look at other existing ones by searching for files names unex*.c, this will turn up support for Linux, BSD, Windows, DOS, Cygwin, the HP 9000 Series 800, OS X, AIX, IRIX, SGI and Solaris.
Not only is this a pretty gross and unportable hack which will probably fail on systems using memory security measures, no, it is something only a few people will fully understand, let alone be able to fix when faced with problems at this stage of the build process.
Personally I can only see its value for obtaining Emacs executables containing a desired state, something people ask for who have to deal with long startup times. Too bad that cannot be done anymore in sufficiently recent versions of Emacs where dumping an already dumped emacs executable is strictly forbidden.
I hope at least Guile Emacs will try getting rid of it. Either making startup fast enough or dumping to a platform-independent file that’s loaded by the emacs executable could work. Please nuke this nightmare from orbit as fast as possible.
edit: Guile Emacs got rid of it, the code is still included though.