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                The Myrddin Compiler Build System
                            Aug 2014
                          Ori Bernstein

TABLE OF CONTENTS:

    0. DEPENDENCIES
    1. USAGE
    2. CREATING MAKEFILES
        2.1. Example
        2.2. Targets
        2.3. Variables
    3. CONFIGURE SCRIPTS
    4. INTERNALS
        4.1: Automatic dependency generation
        4.2: Automatic dependency generation
    4. BUGS

0. DEPENDENCIES:

    c.mk depends on GNU Make, using it's builtins for a
    number of variable expansions. On systems where the default make
    program is not GNU Make, please run 'gmake' or similar. Otherwise,
    you will end up with obscure failures. A method of making this
    script portable would be very welcome.

    Beyond that, all that is needed to run this is a portable POSIX
    system.

1. USAGE:

    run 'make' and the default targets will be built.  The usual targets
    are supported. That means:

        all:
            Builds all targets. This will build every target
            specified in the makefile, recurse into subdirectories,
            and do everything needed. If dependencies are in other
            directories, it will 'cd' into those directories and
            call make as needed.

        target-name:
            This builds the specified target and it's dependencies.

        install:
            This installs the software to the prefix specified in the
            configuration, or /usr/local if there is nothing more
            specific specified.

            If DESTDIR is set, it will install the files to the prefix
            $(DESTDIR)/$(INST_ROOT), although all paths will still be
            relative to $(INST_ROOT). This is for packaging, and matches
            the behavior of automake.

        uninstall:
            This uninstalls the software, removing all files installed
            by 'make install'.

        clean:
            This removes all temporary files created by the build
            process, allowing a fresh build to occur.

    When a target is not specified, 'all' is assumed.

2. CREATING MAKEFILES:

    c.mk is only a set of rules to make common actions simple. There is
    nothing preventing custom make from being added for 

    2.1: Example

        c.mk is designed around the convention of having
        one primary target per directory. This primary target can be
        either a binary or a library, for the most part, although
        there is nothing that prevents you from defining your own
        primary targets.

        So, let's dissect an example makefile:

            BIN = mybinary
            INSTBIN = $(BIN)
            OBJ = foo.o bar.o baz.o

            include config.mk
            include c.mk

            custom-rule: custom-dep
                    insult-your-momma

    2.2: Targets

        Currently, there are several targets supported by c.mk

            SUB
                This target will recurse into the subdirectories
                listed, building everything in them.

            BIN
                This target will build a single binary, and will
                not install it. It uses $(OBJ) as its input.

            LIB
                This target will build a single static library, and will
                not install it. It uses $(OBJ) as its input. It
                conflicts with $(BIN)

            INSTBIN
                Same as BIN, but installed to $(INST_ROOT)/bin

            INSTLIB
                Same as LIB, but installed to $(INST_ROOT)/lib

            INSTHDR
                Used for defining headers to install to system include
                directories. Does not generate headers. Takes multiple
                arguments and installs them to $(INST_ROOT)/include

            INSTPKG
                Used for defining pkgconfig files to install to
                system pkgconfig directories. Does not generate
                them from inputs. Takes multiple arguments and
                installs them to $(INST_ROOT)/lib/pkgconfig.

            INSTMAN
                Used for defining manpage files to install to
                system man directories. Does not generate
                them from inputs. Takes multiple arguments and
                installs them to $(INST_ROOT)/lib/pkgconfig.

    2.3: Variables:

        c.mk respects the usual environment variables. If you set
        CFLAGS, it will compile with those flags. If you set LDFLAGS,
        it will link with those flags. It also supports a number of
        extra configuration variables:
        
            EXTRA
                Dependencies that should be built when 'make all' is
                run. These can be, for example, additional binaries,
                rules, etc.

            EXTRADEP
                Dependencies that should be added when compiling a
                BIN or LIB target
            
            GENHDR
                Headers that are generated for consumption by C code.

            CLEAN
                Files that should be removed when runngng

            INCS
                Include paths. These are passed to the C compiler when
                building C code.

            DEPS
                Paths to local dependencies, such as convenience
                libraries. This is a relative path to a libfoo.a
                file. All libraries that are depended on in this
                manner will be automatically built before they
                are used.

                The path to them will also be added to the include
                path if needed, and they will be linked in to binaries
                that are built.

3. CONFIGURE SCRIPTS:

    This is not mandatory, although convention requests it. This is
    simply a shell script that will probe the system, and generate
    the 'config.mk' file, This can be generated a number of ways.

    Although Myrddin doesn't currently use it, autoconf is a perfectly
    viable method of generating a config.mk setup. Myrddin currently
    uses a simple script that mocks autotools, instead.

4. INTERNALS:

    c.mk is a relatively simple set of commands. It defines make targets
    for the various default rules, and uses the variables described
    above to substitute into them, with fairly traditional structure.

    It begins by setting the default goal (ie, the target that is built
    when 'make' is run with no arguments) to 'all', and proceeds to
    define a few internal variables.

    4.1: Automatic dependency generation

        _DEPSDIR and _DEPS are defined early on. _DEPSDIR is simply
        a directory named '.deps', which is created before any
        compilation occurs, and contains the dependency makefile
        fragments that GCC generates.

        _DEPS is the list of dependency filename fragments, generated
        by substituting '.deps/$FILEBASE.d' for every .o file define
        in $(OBJS)

        CFLAGS also unconditionally has the required flags for
        dependency generation added to the compiler command, and
        later on, the dependencies are included into the makefile
        if they exist.

    4.2:

        The general target rules are defined. There are many for
        loops and bits of shell scripting to support them, but the
        pattern is fairly basic.

            well-known-target: prerequisites
                    do-commands
    
        Some of the commands are fairly long. For example, the install
        and uninstall targets will loop through a list of files for each
        of the main target types.

    is simply '.deps', and is the location where GCC's generated
    dependency information goes. _DEPS
    
4. BUGS:

    - Currently, 'make uninstall' does not remove empty directories that
      it created.

    - You can define BIN/INSTBIN, LIB/INSTLIB in a way that does not
      match. We do not error on this.

    - Well known targets are defined relatively inflexibly. It should
      be easier to include modules and have them add on to the base
      rule set.