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ebfc is a compiler for a tiny programming language called Brainfuck.
(If the name of the language offends you, then you might as well stop
reading now and just delete this entire subdirectory.) This language
was created by Urban Müller, apparently solely for the purpose of
being able to create a compiler for the Amiga under 256 bytes in size.
(My own efforts along these lines can be found at my website of tiny
ELF executables, by the way.) I chose to write a compiler for this
language so as to keep the details of the language translation to a
minimum, and focus on the process of building an ELF file completely
from scratch.
ebfc can compile Brainfuck programs into executables, shared
libraries, and/or object files. Refer to the man page for a
description of the command-line options and of the Brainfuck language
itself.
Most of the code that deals with creating the ELF file is built into a
separate library, called elfparts. This library is too primitive to be
a real general-purpose library (though it might serve as a starting
point for such a thing). The main justification for the separation is
to clearly distinguish the compilation activities from the more
general housekeeping involved in building a functional ELF file.
The file elfparts.txt contains a description of the library interface.
Most of the compiler's work is very straightforward. The Brainfuck
commands all translate directly to a few bytes of machine language.
The real work is mainly in creating the prolog code, which has
different requirements depending on the type of file being generated.
There are two "objects", the program and the 32k array. Compiling to
an executable merely requires that the address of the data segment be
properly inserted into the prolog code, and the address of the text
segment is placed in the ELF header's e_entry field. Compiling to an
object file instead requires that these addresses are added to a
relocation table, with the function being global and the array being
local. Finally, compiling to a shared library requires that the
function be added to the symbol table, and the array's address be
indirectly computed via the global offset table. And, of course, the
actual code to use for the prolog depends on how the function is being
invoked (i.e., from the OS, another program, or another part of the
same program).
The ebfc/bf/ subdirectory contains a few Brainfuck programs, which can
be used to test the compiler. prime.b was written by Urban Müller;
the others were written by myself.
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