SubC Compiler
	By Nils M Holm, 2011--2022
	By DosWorld, 2022--
	Placed in the public domain
	In places where the concept of the public domain does not exist,
	the Creative Commons Zero license applies. See the file CC0.


	SUMMARY

	SubC is a compiler for a (mostly) strict and sane subset of
	C as described in "The C Programming Language", 2nd Ed (also
	known informally as "ANSI C" or "C89").

	A previous version of the compiler is described in great detail
	in the book "Practical Compiler Construction", which can be
	purchased at Lulu.com. See  http://www.t3x.org/reload/  for
	ordering information.

	The SubC compiler can compile itself. Unlike many other small C
	compilers, it does not bend the rules, though. Its code passes
	"gcc -Wall -pedantic" with little or no warnings (depending on
	the gcc version used). Of course, you can also bootstrap it with
	other C compilers, such as Clang or PCC.

	SubC is fast and simple. Its output is statically linked (where
	available) and typically small due to a non-bloated library). It
	uses a simple optimizer on per-expression basis.


	SUPPORTED SYSTEMS

	SubC generates code for GAS, the GNU assembler (except for the
	DOS version, which emits TASM-style syntax). It targets the
	following processors and operating systems:

		FreeBSD		386*	armv6*	x86-64
		Linux		386*	-	x86-64*
		NetBSD		386*	-	x86-64*
		OpenBSD		386%	-	-
		Windows/MinGW	386*	-	-
		Darwin		-	-	x86-64%(/)
		DOS		8086(/)

		  %  uses the syscall layer of the host libc
		  *  untested
		  !  experimental
		 (/) broken

	Platforms tagged "untested" are not regularly tested by myself
	and are therefore subject to potential bit rot. You can help
	me improve SubC by running "make tests" on an "untested"
	platform and let me know about the results.

	Platforms using the system's libc as a thin system call layer
	often cause build/stability problems due to the omnipresence of
	the GNU libc, which is not "thin" at all. Expect trouble on
	those systems!

	Platforms tagged "broken" currently will not compile or run
	properly for some reason. See the Todo file for details.

	The DOS version brings its own toolchain, which can be found in
	the s86/ directory, so no pre-existing DOS assembler or linker
	is required to compile SubC programs on DOS.

	Porting SubC to other 32-bit or 64-bit platforms should be
	quite straight-forward. See the file "Porting" and/or the book
	for a general road map.


	CHANGES TO THE BOOK VERSION

	Note: The book version runs on FreeBSD/386 exclusively.

	The current version uses an improved code generator, which
	emits much smaller and faster code than the book compiler.
	The techniques are described in the book, though.

	The current version of the SubC compiler adds support for
	the following parts of C language to the version described
	in "Practical Compiler Construction":

	o  There is some support for structs and unions.

	o  There is some support for typedefs.

	o  &array is now valid syntax (you no longer have to write
	   &array[0]).

	o  the auto, register, and volatile keywords are recognized
	   (as no-ops). Yes, volatile is safe, because SubC does not
	   have register variables.

	o  enums may now be local.

	o  extern identifiers may now be declared locally.

	o  Prototypes may have the static storage class.

	o  FILEs are now structs and can no longer be mistaken for
	   ints by the type checker.

	o  The #error, #line, and #pragma commands have been added.

	o  There is a (non-standard) kprintf() function, which is
	   like fprintf(), but uses a file descriptor.

	o  There is now a (slightly incompatible) varargs mechanism.
	   Here is how it works:

		#include <varargs.h>

		void p(int a, int b, ...) {
			int	first;
			void	*ap;

			ap = _va_start(&b);
			first = (int) _va_arg(&ap);
			vprintf("other args: %d %d %d\n", ap);
			_va_end(&ap);
		}

	o  The vprintf(), vfprintf(), and vsprintf() functions have
	   been added to the runtime library.

	o  A broader subset of C expression syntax is accepted
	   in constant expression contexts. For example, pointer
	   variables can be initialized with NULL.


	DIFFERENCES BETWEEN SUBC (THIS VERSION) AND FULL C89

	o  The following keywords are not recognized:
	   const, double, float, goto, long, short, signed,
	   unsigned.

	o  There are only two primitive data types: the signed int and
	   the unsigned char; there are also void pointers, and there
	   is limited support for int(*)() (pointers to functions
	   of type int).

	o  No more than two levels of indirection are supported, and
	   arrays are limited to one dimension, i.e. valid declarators
	   are limited to x, x[], *x, *x[], **x (and (*x)()).

	o  K&R-style function declarations (with parameter declarations
	   between the parameter list and function body) are not
	   accepted.

	o  There are no ``const'' variables.

	o  There are no unsigned integers, long integers, or signed
	   chars.

	o  Struct/union declarations must be global (struct and union
	   objects may be declared locally, though).

	o  There is no support for bit fields.

	o  Only ints, chars, and arrays of int and char can be
	   initialized in their declarations; pointers can be
	   initialized with 0 or NULL.

	o  Local arrays cannot have initializers.

	o  Local declarations are limited to the beginnings of function
	   bodies (they do not work in other compound statements).

	o  Arguments of prototypes must be named.

	o  There is no goto.

	o  There are no parameterized macros.

	o  The #if and #elif preprocessor commands are not recognized.

	o  The preprocessor does not accept multi-line commands.

	o  The preprocessor does not accept comments in (some) commands.

	o  The preprocessor does not recognize the # and ## operators.

	o  There may not be any blanks between the # that introduces
	   a preprocessor command and the subsequent command (e.g.:
	   "# define" would not be recognized as a valid command).

	o  The sizeof operator requires parentheses.

	o  Subscripting an integer with a pointer (e.g. 1["foo"]) is
	   not supported.

	o  Function pointers are limited to one single type, int(*)(),
	   and they have no argument types. Note that this declaration
	   will in fact generate a pointer to int(*)(void).

	o  There is no assert() due to the lack of parameterized macros.

	o  The atexit() mechanism is limited to one function (this may
	   even be covered by TCPL2).

	o  The signal() function returns int due to the lack of a more
	   sophisticated type system; the return value must be casted to
	   int(*)() manually.

	o  Most of the time-related functions are missing, in particular:
	   asctime(), gmtime(), localtime(), mktime(), and strftime().

	o  The clock() function is missing, because CLOCKS_PER_SEC
	   varies among systems.

	o  The ctime() function ignores the time zone.

	o  The varargs mechanism is slightly incompatible.

	o  The SubC compiler accepts // comments in addition to /* */
	   (but not in macros).


	SELECTING A TARGET PLATFORM

	The easiest way to prepare a build is to run the configure
	script in this directory. Don't worry, it is just a simple
	script that will figure out the host platform via uname and
	link a few machine-dependent files into place.

	If you want to configure the compiler manually: select one of
	the target descriptions (cg*.c) files in src/targets/cg and
	symlink it to src/cg.c. Also link the corresponding header
	file into place:

		(cd src && ln -fs targets/cg/cg386.c cg.c)
		(cd src && ln -fs targets/cg/cg386.h cg.h)

	Next select the C startup (crt0) file for your OS and CPU type
	from src/targets/OS-CPU/ and link it to src/lib/crt0.s, e.g.:

		(cd src/lib && \
		 ln -fs ../targets/freebsd-386/crt0-freebsd-386.s \
		 	crt0.s)

	If your OS/CPU combination is not supported, you might try
	to port the compiler. See the file "Porting" for details.

	You will also need some operating system-dependent
	definitions, which are kept in files named "sys-OS-CPU.h"
	in src/targets/OS-CPU/. Just symlink the appropriate file
	to src/sys.h:

		(cd src && \
		 ln -fs targets/freebsd-386/sys-freebsd-386.h sys.h)

	Finally, select a limits-*.h file from targets/include/ that
	reflects the machine word size of your target and link it to
	src/include/limits.h:

		(cd src/include && \
		 ln -fs ../targets/include/limits-32.h limits.h)


	COMPILING THE COMPILER

	The compiler sources are contained in the "src" directory,
	so all the subsequent steps assume that this is your current
	working directory. (I.e. do a "cd src" now.)

	On a supported system, just type "make".

	Without "make" the compiler can be bootstrapped by running:

		cc -o scc0 *.c

	To compile and package the runtime library:

		./scc0 -c lib/*.c
		ar -rc lib/libscc.a lib/*.o
		ranlib lib/libscc.a

	To compile the startup module:

		as -o lib/crt0.o lib/crt0.s

	To test the compiler, either run "make test" or perform the
	following steps:

		./scc0 -o scc1 *.c
		./scc1 -o scc *.c
		cmp scc1 scc

	There should not be any differences between the scc1 and scc
	executables.


	INSTALLING THE COMPILER

	The easy way would be to set up the PREFIX (and optionally
	SCCDIR and BINDIR) variables in src/Makefile to suit your
	taste and then run

		make dirs	# to create the directories
		make install

	If you want to install the SubC compiler manually, you will
	have to change the SCCDIR variable in the compiler itself.
	It points to the base directory which will contain the SubC
	headers and runtime library. SCCDIR defaults to ".", but can
	be overridden on the command line:

		./scc1 -o scc -D 'SCCDIR="INSTALLDIR"' *.c

	(where INSTALLDIR is where the compiler will be installed.)

	You can place the 'scc' executable wherever you want, as long
	as its location is covered by the PATH environment variable.
	The headers (include/*) go to INSTALLDIR/include, the library
	'lib/libscc.a' and the startup module 'lib/crt0.o' go to
	INSTALLDIR/lib.

	To test the installation just re-compile the compiler:

		rm scc && scc -o scc *.c


	DOS SUPPORT

	Please see the NOTES-DOS file!


	WINDOWS SUPPORT

	Please see the NOTES-WINDOWS file!


	THANKS

	To the Super Dimension Fortress (SDF.ORG) for providing
	free shell accounts on 64-bit NetBSD machines.

	To Bakul Shah for granting me remote access to a 64-bit
	FreeBSD system and a Linux VM.

	To "minux" for porting the runtime module to Linux/x86-64.

	To Jean-Marc Lienher (cod5.org) for porting the runtime module
	to MinGW Windows/386.

	To Romain LWPB for porting the runtime module to OpenBSD/386
	and Darwin/x86-64 as well as for modifying the x86-64 code
	generator to emit proper code for Darwin.

	To Paul Edwards for testing typedef.

	To everybody who test-drove SubC and submitted bug reports.

	To the Unknown Hacker for various minor and not so minor
	contributions.