subdirmk - assistance for non-recursive use of make
===================================================

Introduction
------------

Peter Miller's 1997 essay _Recursive Make Considered Harmful_
persuasively argues that it is better to arrange to have a single
make invocation with the project's complete dependency tree, rather
than the currently conventional `$(MAKE) -C subdirectory' approach.

However, actually writing a project's build system in a non-recursive
style is not very ergonomic.  The main difficulties are:
  - constantly having to write out long file and directory names
  - the lack of a per-directory make variable namespace means
    long make variables (or namespace clashes)
  - it is difficult to arrange that one can cd to a subdirectory
    and say `make all' and have something reasonable happen
    (to wit, build an appropriate subset)

`subdirmk' is an attempt to solve these problems (and it also slightly
alleviates some of the boilerplate needed to support out-of-tree
builds well, and helps a bit with metaprogramming and rule writing).

Basic approach
--------------

The developer is expected to write a makefile fragment, in each
relevant subdirectory, called `Dir.sd.mk'.

These fragments may contain ordinary make language.  Unqualified
filenames are relative to the build toplevel, and all commands all run
there.

However, the sigil & is treated specially.  By and large, it refers to
`the build directory corresponding to this .sd.mk file', etc.
There are a variety of convenient constructions.

The result is that to a large extent, the Dir.sd.mk has an easy way
to namespace its "local" make variables, and an easy way to refer to
its "local" filenames (and filenames in general).

The Dir.sd.mk's are filtered, fed through autoconf in the usual way
(for @..@-substitutions) and included by one autogenerated toplevel
makefile.

So all of the input is combined and passed to one make invocation.
(A corollary is that there is no enforcement of the namespacing:
discipline is required to prefix relevant variable names with &, etc.)

Each subdirectory is also provided with an autogenerated `Makefile'
which exists purely to capture ordinary make invocations and arrange
for something suitable to happen.

Where there are dependencies between subdirectories, each Dir.sd.mk
can simply refer to files in other subdirectories directly.

Substitution syntax
-------------------

In general & expands to the subdirectory name when used for a
filename, and to the subdirectory name with / replaced with _ for
variable names.  (If your variables start with capital letters and
your filenames with lowercase.  Otherwise, use &/ or &_.)

Note that & is processed *even in makefile comments*.  The substitutor
does not understand make syntax, or shell syntax, at all.  However,
the substitution rules are chosen to work well with constructs which
are common in makefiles.

In the notation below, we suppose that the substitution is being in
done in a subdirectory sub/dir of the source tree.  In the RH column
we describe the expansion at the top level, which is often a special
case (in general in variable names we call that TOP rather than the
empty string).

&CAPS		=>	sub_dir_CAPS			or TOP_CAPS
&lc		=>	sub/dir/lc			or lc
	Here CAPS is any ASCII letter A-Z and lc is a-z.
	The assumption is that filenames are usually lowercase and
	variables usually uppercase.  Otherwise, use another syntax:

&/		=>	sub/dir/			or nothing
&_		=>	sub_dir_			or TOP_
&.		=>	sub/dir				or .
	(This implies that `&./' works roughly like `&/', although
	it can produce a needless `./')

&=		=>	sub_dir				or TOP

&^lc		=>	${top_srcdir}/sub/dir/lc
&^/		=>	${top_srcdir}/sub/dir/
&^.		=>	${top_srcdir}/sub/dir

&~lc		=>	${top_srcdir}/lc
&~/		=>	${top_srcdir}/
&~.		=>	${top_srcdir}

In general:
    ^   pathname of this subdirectory in source tree
    ~   pathname of top level of source tree
    /	terminates the path escape } needed if next is
    _   terminates the var escape  } not letter or space)
    .   terminates path escape giving dir name (excluding /)
    =	terminates var escape giving only prefix part (rarely needed)
  lwsp  starts multi-word processing (see below)

So pathname syntax is a subset of:
    '&' [ '^' | '~' ] [ lc | '/' | '.' ]

&&		=>	&&		for convenience in shell runes

&\&		=>	&		general escaping mechanism
&\$		=>	$		provided for $-doubling regimes
&\NEWLINE				eats the newline and vanishes

&$VARIABLE	=>	${sub_dir_VARIABLE}	or ${TOP_VARIABLE}
	VARIABLE is ASCII starting with a letter and matching \w+

& thing thing... &
&^ thing thing... &
&~ thing thing... &
	Convenience syntax for prefixing multiple filenames.
	Introduced by & followed by lwsp where lc could go.
	Each lwsp-separated non-ws word is prefixed by &/ etc.
        etc. respectively.  No other & escapes are recognised.
	This processing continues until & preceded by lwsp,
	or until EOL (the end of the line), or \ then EOL.

&:<directive> <args>....
	recognised at start of line only (possibly after lwsp)

&:		=>	&:
	for make multiple targets syntax
	recognised anywhere *except* start of line

&:include filename		filename should usually be [&]foo.sd.mk
&:-include filename		tolerate nonexistent file
	RHS is &-expanded but filenames are relative to the top
	srcdir.  This implies that unqualified names are like &~/
	whereas &/ is like &^/.  &^ and &~ do not work here because
	they expand to constructions involving literally
	`$(top_srcdir)', but the RHS is not make-expanded.

&!<lwsp>	disables & until EOL (and then disappears)

&#	delete everything to end of line
	(useful if the RHS contains unrecognised & constructions)

&TARGETS_things
	Handled specially.  If mentioned at the start of a line
	(possibly following whitespace), declares that this
	subdir ought to have a target `things'.  The rule will be
		&/things:: $(&TARGETS_things)

	You may extend it by adding more :: rules for the target,
	but the preferred style is to do things like this:
		&TARGETS_check += & test-passed.stamp

	It is important to mention &TARGETS_things at least once in
	the context of each applicable directory, because doing so
	arranges that the *parent* will also have a `things' target
	which recursively implies this directory's `things'.

	Must be spelled exactly &TARGETS_things.  &_TARGETS_things,
	for example, is not magic.  To make the target exist
	without providing any prerequisites for it, write a line
	containing just `&TARGETS_things +='.

	`all' is extra special: every directory has an `all'
	target, which corresponds to &TARGETS.

Directives
- - - - -

&:warn [!]WARNTAG ...
	Suppress (with !) or re-enable (without !) warnings tagged
	WARNTAG (see section `Warnings', below).  The suppression list
	is reset at the start of processing in each subdirectory.
	Warnings that appear at the end of processing are controlled
	by the final warning state after processing all the toplevel
	input files (including Final.sd.mk).

&:local+global [!][&]VARIABLE ...
	Suppresses any warnings relating to forthcoming mentions
	to VARIABLE or &VARIABLE, as applicable.  Scope ends at
	the end of the current directory's Suffix.sd.mk.
	Prefixing with ! removes [&]VARIABLE from the suppresion list.

&:changequote NEWQUOTE
	changes the escape sequence from & to literally NEWQUOTE
	NEWQUOTE may be any series of of non-whitespace characters,
	and is terminated by EOL or lwsp.  The whole line is
	discarded.

	After this, write NEWQUOTE instead of &, everywhere.
	The effect is unscoped and lasts until the next setting,
	or until the end of the current directory's Suffix.sd.mk.
	It takes effect on &:include'd files too, so maybe set
	it back before using &:include.

	Notably
		NEWQUOTENEWQUOTE	=> NEWQUOTENEWQUOTE
		NEWQUOTE\NEWQUOTE	=> NEWQUOTE
		NEWQUOTE\$		=> $
		NEWQUOTE:changequote &	set escape back to &


Dollar doubling and macro assistance
------------------------------------

&$+		Starts dollar-doubling
&$-		Stops dollar-doubling
	Both are idempotent and local to the file or context.

This is useful both for make macrology involving $(eval ...), and
possibly for helping write complicated recipes involving shell
variables, inline Perl code, etc.

Sometimes we will show $'s being doubled inside another construct.
This means the content of the construct is $-doubled: $-doubling is
locally enabled, and restored afterwards.

&:macro NAME	=>	define NAME
STUFF $ THINGS	..	STUFF $$ THINGS
&:endm		..	endef
	NAME is processed for &

&{..$..}	=>	${eval ${call ..$$..}}
	(matches { } pairs to find the end)
	content is $-doubled (unless it contains &$- to turn that off)
	contrast &(...), see "Convenience syntax for call", below.

Together &:macro and &{...} provide a more reasonable macro facility
than raw make.  They solve the problem that make expansions cannot
directly generate multiple rules, variables, etc.; instead, `$(eval )'
must be used, but that re-expands the argument, meaning that all the
literal text must be $-doubled.  This applies to the macro text and to
the arguments.  Also `$(eval $(call ...))' is an unfortunate syntax.
Hence &:macro and &{...}.

While dollar-doubling:
- - - - - - - - - - -

$	=>	$$	including $'s produced by other
			 &-expansions not mentioned here

&\$	=>	$
&$(	=>	$(
&$NN	=>	${NN}	where N are digits

A few contexts do not support $-doubling, such as directive arguments
or places where this might imply $-quadrupling.  (There is no way to
get $-quadrupling.)

Convenience syntax for call
- - - - - - - - - - - - - -

&(thing		=>	$(call thing
&( thing	=>	$(call thing
	and specially:
&(&lc		=>	$(call sub_dir_lc	or $(call TOP_lc
&( &lc		=>	$(call sub_dir_lc	or $(call TOP_lc
	even though lc would normally be thought a filename

Unlike &{...}, this does not involve any dollar-doubling.

Use this when the expansion is going to be a piece of text to be used
as part of a rule, filename, etc.  When the expansion is top-level
make text (eg, rules), use &:macro and &{...}.


Invocation, "recursive" per-directory targets
---------------------------------------------

Arrangements are made so that when you run `make foo' in a
subdirectory, it is like running the whole toplevel makefile, from the
toplevel, as `make subdir/foo'.  If `subdir/foo' is a file that might
be built, that builds it.

But `foo' can also be a conventional target like `all'.

Each subdirectory has its own `all' target.  For example a
subdirectory `src' has a target `src/all'.  The rules for these are
automatically generated from the settings of the per-directory
&TARGETS variables.  &TARGETS is magic in this way.  (In
src/Dir.sd.mk, &TARGETS of course refers to a make variable called
src_TARGETS.)

The `all' target in a parent directory is taken to imply the `all'
targets in all of its subdirectories, recursively.  And in the
autogenerated stub Makefiles, `all' is the default target.  So if you
just type `make' in the toplevel, you are asking for `&all'
(<subdir>/all) for every directory in the project.

In a parallel build, the rules for all these various subdirectory
targets may be in run in parallel: there is only one `make' invocation
at a time.  There is no sequencing between subdirectories, only been
individual targets (as specified according to their dependencies).

You can define other per-directory recursive targets too: set the
variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
at the start of a line).  This will create a src/zonk target (for
appropriate value of src/).  Unlike `all', these other targets only
exist in areas of the project where at least something mentions them.
So for example, if &TARGETS_zonk is set in src but not lib, `make
zonk' in lib will fail.  If you want to make a target exist
everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
below).

Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
--------------------------------------------------

The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
are automatically processed before and after each individual
directory's Dir.sd.mk, and the &-substituted contents therefore
appear once for each subdirectory.

This lets you do per-directory boilerplate.  Some useful boilerplate
is already provided in subdirmk, for you to reference like this:
  &:include subdirmk/cdeps.sd.mk
  &:include subdirmk/clean.sd.mk
For example you could put that in Suffix.sd.mk.

The top-level Dir.sd.mk is the first makefile included after the
autogenerated `main.mk' which merely has some basic settings and
includes.  So if you want to get in early and set global variables,
put them near the top of Dir.sd.mk.

The file Final.sd.mk in the toplevel directory is processed and
the result included after all the other files.  Its subdirmk
filtering context inherits warning suppressions from the toplevel's
Dir.sd.mk etc., but not anything else.

subdirmk's filter script itself sets (only) these variables:
  top_srcdir
  abs_top_srcdir
  SUBDIRMK_MAKEFILES
  MAKEFILE_TEMPLATES
You are likely to want to define $(PWD), and shorter names for
top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).

Warnings
--------

subdirmk's `generate' program, which does the acual &-substitution,
can produce some warnings about your .sd.mk files.  These can be
suppressed with the &:warn directive.  The warning tags are:

    local+global
	The same VARNAME was used both with and without an & prefix.
	This can be confusing.  Also, if you avoid this then you will
	get a warning iff you accidentally leave off a needed &.
	The generation of this warning depends on scanning your
	makefile for things that look like variable references, which
	subdirmk does not do completely perfectly.  Exciting make
	syntax may evade this warning, or require suppressions.
	(You can suppress this warning for a particular VARNAME with
	the &:local+global directive.)

    single-char-var
	A variable expansion like $FBAR.  make's expansion rules
	interpret this as $(F)BAR.  It's normally better to write
	it this way, at least if the variable expansion is followed
	by more letters.  Note that &$FOO works differently to
	raw make: it expands to ${sub_dir_FOO}.

    broken-var-ref
        An attempt at variable expansion looking like $&...
	You probably expected this to mean $(TOP_F)BAR but it
	expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.

    unknown-warning
	&:warn was used to try to enable a warning that this version
	of subdirmk does not understand.  (Note that an attempt to
	*dis*able an unknown warning is only reported if some other
	warning was issued which might have been disabled.)


Guides, hints, and further explanations
=======================================

Incorporating this into your project
------------------------------------

Use `git-subtree' to merge the subdirmk/ directory.  You may find it
useful to symlink the DEVELOPER-CERTIFICATE file (git can store
symlinks as symlinks - just `git add' the link).  And you probably
want to mention the situation in your top-level COPYING and HACKING.

Symlink autogen.sh into your project toplevel.

In your configure.ac, say

  m4_include([subdirmk/subdirmk.ac])
  SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])

Write a Dir.sd.mk in each directory.  See the substitution syntax
reference, above, and the example/ directory here.  The toplevel
Dir.sd.mk should probably contain:

  include subdirmk/usual.mk
  include subdirmk/regen.mk

Write a Suffix.sd.mk in the toplevel, if you want.  It should probably
have:

  &:include subdirmk/cdeps.sd.mk
  &:include subdirmk/clean.sd.mk


Hints
-----

You can convert your project incrementally.  Start with the top-level
Makefile.in and rename it to Dir.sd.mk, and add the appropriate
stuff to configure.ac, and fix everything up.  Leave the existing
$(MAKE) -C for your existing subdirectories alone.  Then you can
convert individual subdirectories, or classes of subdirectories, at
your leisure.  (You must be /sure/ that each recursive (non-subdirmk)
subdirectory will be entered only once at a time, but your existing
recursive make descent system should already do that or you already
have concurrency bugs.)

Aside from this, be very wary of any invocation of $(MAKE) anywhere.
This is a frequent source of concurrency bugs in recursive make build
systems.  When combined with nonrecursive make it's all in the same
directory and there is nothing stopping the different invocations
ending up trying to make the same targets at the same time. That
causes hideous racy lossage.  There are ways to get this to work
reliably but it is advanced stuff.

If you make syntax errors, or certain kinds of other errors, in your
makefiles, you may find that just `make' is broken now and cannot get
far enough to regenerate a working set of makefiles.  If this happens
just rerun ./config.status by hand.

If you go back and forth between different versions of your code you
can sometimes find that `make' complains that one of your Dir.sd.mk
files is missing: typically, if iot was used and therefore a
dependency in some other version of your code.  If you run `make
clean' (or `make realclean') these dependencies are suppressed, which
will clear up the problem.


Global definitions
------------------

If want to set global variables, such as CC, that should only be done
once.  You can put them in your top-level Dir.sd.mk, or a separate
file you `include' and declare using SUBDIRMK_MAKEFILES.

If you need different settings of variables like CC for different
subdirectories, you should probably do that with target-specific
variable settings.  See the info node `(make) Target-specific'.


Directory templates `.sd.mk' vs plain autoconf templates `.mk.in'
--------------------------------------------------------------------

There are two kinds of template files.

 Filename                 .sd.mk                  .mk.in

 Processed by             &-substitution,         autoconf only
                          then autoconf

 Instantiated             Usu. once per subdir    Once only

 Need to be mentioned     No, but Dir.sd.mk       All not in subdirmk/
 in configure.ac?         via SUBDIRMK_SUBDIRS    via SUBDIRMK_MAKEFILES

 How to include           `&:include foo.sd.mk'   `include foo.mk'
			  in all relevant .sd.mk  in only one
                          (but not needed for     Dir.sd.mk
                           Prefix, Suffix, Final)

If you `include subdirmk/regen.mk', dependency management and
automatic regeneration for all of this template substitution, and for
config.status etc. is done for you.


Tables of file reference syntaxes
---------------------------------

In a nonrecursive makefile supporting out of tree builds there are
three separate important distinctions between different file
locations:

 (i) In the build tree, or in the source tree ?

 (ii) In (or relative to) the subdirectory to which this Dir.sd.mk
     relates, or relative to the project's top level ?

 (iii) Absolute or relative pathname ?  Usually relative pathnames
     suffice.  Where an absolute pathname is needed, it can be built
     out of &/ and an appropriate make variable such as $(PWD).

Path construction &-expansions are built from the following:

                      Relative paths in...
                      build     source
                                                       
  This directory      &         &^
  Top level           .         &~

In more detail, with all the various options laid out:

      Recommended     Relative paths in...   Absolute paths in...
             for      build     source       build         source
                                                       
  This       lc       &file     &^file       $(PWD)/&file  $(abs_src)/&file
  directory  any      &/file    &^/file      $(PWD)/&/file $(abs_src)/&/file
             several  & f g h   &^ f g h     $(addprefix...)
                                             
  Top        lc       file      &~file
  level      any      file      &~/file      $(PWD)/file   $(abs_src)/file
             .mk.in   file      $(src)/file  $(PWD)/file   $(abs_src)/file
             several  f g h     &~ f g h     $(addprefix...)

(This assumes you have appropriate make variables src, PWD and
abs_src.)


Subdirectory and variable naming
--------------------------------

The simple variable decoration scheme does not enforce a strict
namespace distinction between parts of variable names which come from
subdirectory names, and parts that mean something else.

So it is a good idea to be a bit careful with your directory naming.
`TOP', names that contain `_', and names that are similar to parts of
make variables (whether conventional ones, or ones used in your
project) are best avoided.

If you name your variables in ALL CAPS and your subdirectories in
lower case with `-' rather than `_', there will be no confusion.


Legal information
=================

subdirmk is
 Copyright 2019-2020 Ian Jackson
 Copyright 2019 Mark Wooding

    subdirmk and its example is free software; you can redistribute it
    and/or modify it under the terms of the GNU Library General Public
    License as published by the Free Software Foundation; either
    version 2 of the License, or (at your option) any later version.

    This is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Library General Public License for more details.

    You should have received a copy of the GNU Library General Public
    License along with this library as the file LGPL-2.
    If not, see https://www.gnu.org/.

Individual files generally contain the following tag in the copyright
notice, instead of the full licence grant text:
  SPDX-License-Identifier: LGPL-2.0-or-later
As is conventional, this should be read as a licence grant.

Contributions are accepted based on the git commit Signed-off-by
convention, by which the contributors' certify their contributions
according to the Developer Certificate of Origin version 1.1 - see
the file DEVELOPER-CERTIFICATE.

Where subdirmk is used by and incorporated into another project (eg
via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
the rest of the project are under that other project's licence(s).
(The project's overall licence must be compatible with LGPL-2.0+.)