-&CAPS => subdir_CAPS or TOP_CAPS
-&lc => subdir/lc or lc
+subdirmk - assistance for non-recursive use of make
+===================================================
-&_ => subdir_ or TOP_
-&/ => subdir/ or nothing
-&=_ => subdir or TOP
-&=/ => subdir or .
-&^ => $(top_srcdir)/subdir or $(top_srcdir)
-&~ => $(abs_top_srcdir)/subdir or $(abs_top_srcdir)
+Introduction
+------------
-&& => &&
-\& => &
+Peter Miller's 1997 essay _Recursive Make Considered Harmful_
+persuasively argues that it is better to arrannge to have a single
+make invocation with the project's complete dependency tree, rather
+than the currently conventional `$(MAKE) -C subdirectory' approach.
-& thing thing... & => each thing prefixed by &/ &^/ &~/ resp
-& ^ thing thing... & each thing is any non-ws
-& ~ thing thing... & & may be omitted before EOL or before \EOL
- other &'s not recognised
+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)
-CAPS is [A-Z][0-9_A-Z]*(?!\w)
-lc is [a-z][-+,0-9_a-z]*(?!\w)
+`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).
-&!<spaces or tabs> disables & *until* EOL (and disappears)
+Basic approach
+--------------
-&!STUFF<lwsp> STUFF is recognised instead of &
- the terminating lwsp is discarded too
- may also occur at eol
+The developer is expected to write a makefile fragment, in each
+relevant subdirectory, called `Subdir.sd.mk'.
-eg notably
- STUFF!& now & is recognised instead (ie back to normal)
- STUFFSTUFF STUFF
+These fragments may contain ordinary make language.
-eg
- &!@@@ @@@ is recognised instead of &
- @@@!& go back to &
+However, the sigil & is treated specially. By and large, it refers to
+`the current directory'. There are a variety of convenient
+constructions.
-&TARGETS[_things] is handled specially
- must be spelled precisely this way
- if no _things, means _all
+The result is that to a large extent, the Subdir.sd.mk has an easy way
+to namespace its "local" make variables, and an easy way to refer to
+its "local" filenames.
-Also, `all' is weird in that it is present even if not specified
+The Subdir.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 Subdir.sd.mk
+can simply refer to files in other subdirectories directly.
+
+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/Subdir.sd.mk, &TARGES 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.
+
+You can define other per-directory recursive targets too: simply
+mention (usually, by setting) the variable &TARGETS_zonk, or whatever.
+This will create a src/zonk target.
+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 mentioned in src but not lib, `make zonk' in
+lib will fail. If you want to make a target exist everywhere,
+mention its name in Perdir.sd.mk (see below).
+
+Perdir.sd.mk, inclusion
+-----------------------
+
+The file Perdir.sd.mk in the toplevel of fthe source is automatically
+processed after each individual directory's Subdir.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 Perdir.sd.mk.
+
+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 Subdir.sd.mk, or a separate
+file you `include' and declare using SUBDIRMK_MAKEFILES.
+
+Subdirectory 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 Subdir.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 Subdir.sd.mk
+ Subdir and Perdir)
+
+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.
+
+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.
+
+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 TOP_
+&/ => sub/dir/ or nothing
+&=_ => sub_dir or TOP
+&=/ => sub/dir or .
+&^ => $(top_srcdir)/sub/dir or $(top_srcdir)
+&~ => $(abs_top_srcdir)/sub/dir or $(abs_top_srcdir)
+
+&& => && for convenience in shell runes
+\& => & general escaping mechanism
+
+& thing thing... &
+& ^ thing thing... &
+& ~ thing thing... &
+ Convenience syntax for prefixing multiple filenames.
+ Introduced by & followed by lwsp (space or tab).
+ Each lwsp-separated non-ws word is prefixed by &/ &^/ &~/
+ respectively. No other & escapes are recognised.
+ This processing continues until a & 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)
+ args are processed for &
+
+&:include filename filename should usually be foo.sd.mk
+&:-include filename tolerate nonexistent file
+
+&!<lwsp> disables & until EOL (and then disappears)
+
+&!STUFF
+ changes the escape sequence from & to literally STUFF
+ STUFF may be any series of of non-whitespace characters,
+ and is terminated by EOL or lwsp. &!STUFF and the lwsp
+ are discarded.
+
+ After this, write STUFF instead of &, everywhere.
+ The effect is global and lasts until the next setting.
+ It takes effect on &:include'd files too, so maybe set
+ it back before using &:include.
+
+ Notably
+ STUFFSTUFF => STUFFSTUFF
+ \STUFF => STUFF
+ STUFF!& set escape back to &
+
+&TARGETS_things
+ Handled specially. If mentioned, 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. But mentioning &TARGETS_things in
+ a #-comment *does* work because the & filter does not care
+ about comments.
+
+ `all' is extra special: every directory has an `all'
+ target, which corresponds to &TARGETS.
+
+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.
+
+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.
+
+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 Subdir.sd.mk in each directory. The toplevel one should
+probably contain:
+
+ include subdirmk/usual.mk
+ include subdirmk/regen.mk
+
+Write a Perdir.sd.mk in the toplevel, if you want. It should probably
+have:
+
+ &:include subdirmk/cdeps.sd.mk
+ &:include subdirmk/clean.sd.mk
+
+
+Legal information
+-----------------
+
+subdirmk is
+ Copyright 2019 Mark Wooding
+ Copyright 2019 Ian Jackson
+
+ 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+.)
~ian / subdirmk.git / blobdiff