README - Current Version

This source file is part of the SubC compiler, which is described in the book

Practical Compiler Construction.

You might prefer to download the compiler source code. It is in the public domain.

        SubC Compiler
        By Nils M Holm, 2011--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.


        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 See  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.


        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.


        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

        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);

        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.


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

        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

        o  There are no ``const'' variables.

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

        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).


        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 \

        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

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


        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


        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

        To test the installation just re-compile the compiler:

                rm scc && scc -o scc *.c


        Please see the NOTES-DOS file!


        Please see the NOTES-WINDOWS file!


        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 ( 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


        Send feedback, suggestions, etc to:

        n m h @ t 3 x . o r g

        See for current ways through my
        spam filter.

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