1 subdirmk - assistance for non-recursive use of make
2 ===================================================
7 Peter Miller's 1997 essay _Recursive Make Considered Harmful_
8 persuasively argues that it is better to arrange to have a single
9 make invocation with the project's complete dependency tree, rather
10 than the currently conventional `$(MAKE) -C subdirectory' approach.
12 However, actually writing a project's build system in a non-recursive
13 style is not very ergonomic. The main difficulties are:
14 - constantly having to write out long file and directory names
15 - the lack of a per-directory make variable namespace means
16 long make variables (or namespace clashes)
17 - it is difficult to arrange that one can cd to a subdirectory
18 and say `make all' and have something reasonable happen
19 (to wit, build an appropriate subset)
21 `subdirmk' is an attempt to solve these problems (and it also slightly
22 alleviates some of the boilerplate needed to support out-of-tree
28 The developer is expected to write a makefile fragment, in each
29 relevant subdirectory, called `Subdir.sd.mk'.
31 These fragments may contain ordinary make language.
33 However, the sigil & is treated specially. By and large, it refers to
34 `the current directory'. There are a variety of convenient
37 The result is that to a large extent, the Subdir.sd.mk has an easy way
38 to namespace its "local" make variables, and an easy way to refer to
39 its "local" filenames.
41 The Subdir.sd.mk's are filtered, fed through autoconf in the usual way
42 (for @..@-substitutions) and included by one autogenerated toplevel
45 So all of the input is combined and passed to one make invocation.
46 (A corollary is that there is no enforcement of the namespacing:
47 discipline is required to prefix relevant variable names with &, etc.)
49 Each subdirectory is also provided with an autogenerated `Makefile'
50 which exists purely to capture ordinary make invocations and arrange
51 for something suitable to happen.
53 Where there are dependencies between subdirectories, each Subdir.sd.mk
54 can simply refer to files in other subdirectories directly.
56 Invocation, "recursive" per-directory targets
57 ---------------------------------------------
59 Arrangements are made so that when you run `make foo' in a
60 subdirectory, it is like running the whole toplevel makefile, from the
61 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
62 be built, that builds it.
64 But `foo' can also be a conventional target like `all'.
66 Each subdirectory has its own `all' target. For example a
67 subdirectory `src' has a target `src/all'. The rules for these are
68 automatically generated from the settings of the per-directory
69 &TARGETS variables. &TARGETS is magic in this way. (In
70 src/Subdir.sd.mk, &TARGETS of course refers to a make variable called
73 The `all' target in a parent directory is taken to imply the `all'
74 targets in all of its subdirectories, recursively. And in the
75 autogenerated stub Makefiles, `all' is the default target. So if you
76 just type `make' in the toplevel, you are asking for `&all'
77 (<subdir>/all) for every directory in the project.
79 In a parallel build, the rules for all these various subdirectory
80 targets may be in run in parallel: there is only one `make' invocation
81 at a time. There is no sequencing between subdirectories, only been
82 individual targets (as specified according to their dependencies).
84 You can define other per-directory recursive targets too: simply
85 mention (usually, by setting) the variable &TARGETS_zonk, or whatever.
86 This will create a src/zonk target (for appropriate value of src/).
87 Unlike `all', these other targets only exist in areas of the project
88 where at least something mentions them. So for example, if
89 &TARGETS_zonk is mentioned in src but not lib, `make zonk' in
90 lib will fail. If you want to make a target exist everywhere,
91 mention its name in Perdir.sd.mk (see below).
93 Perdir.sd.mk, inclusion
94 -----------------------
96 The file Perdir.sd.mk in the toplevel of the source is automatically
97 processed after each individual directory's Subdir.sd.mk, and the
98 &-substituted contents therefore appear once for each subdirectory.
100 This lets you do per-directory boilerplate. Some useful boilerplate
101 is already provided in subdirmk, for you to reference like this:
102 &:include subdirmk/cdeps.sd.mk
103 &:include subdirmk/clean.sd.mk
104 For example you could put that in Perdir.sd.mk.
109 If want to set global variables, such as CC, that should only be done
110 once. You can put them in your top-level Subdir.sd.mk, or a separate
111 file you `include' and declare using SUBDIRMK_MAKEFILES.
113 If you need different settings of variables like CC for different
114 subdirectories, you should probably do that with target-specific
115 variable settings. See the info node `(make) Target-specific'.
117 Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in'
118 --------------------------------------------------------------------
120 There are two kinds of template files.
122 Filename .sd.mk .mk.in
124 Processed by &-substitution, autoconf only
127 Instantiated Usu. once per subdir Once only
129 Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/
130 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
132 How to include `&:include foo.sd.mk' `include foo.mk'
133 in all relevant .sd.mk in only one
134 (but not needed for Subdir.sd.mk
137 If you `include subdirmk/regen.mk', dependency management and
138 automatic regeneration for all of this template substitution, and for
139 config.status etc. is done for you.
141 Summary of recommended directory reference syntaxes
142 ---------------------------------------------------
144 Path construction &-expansions, meanings summary:
146 In build tree In source tree
147 This directory just & &,
148 Top level &. implies absolute &;
150 Adding `@' means "absolute path". This is not needed with &. because
151 there is never any need to use &. since it would expand to nothing.
152 `/' terminates the escape (needed if the next thing is not a lowercase
153 character, or space). `=' means "just the value, no /". Space starts
154 multi-word processing.
156 Recommended In build tree In source tree
157 when Relative Absolute Relative Absolute
159 This lc &file &@file &,file &@,file
160 directory any &/file &@/file &,/file &@,/file
161 several & f g h &@ f g h &, f g h &@, f g h
163 Top lc &.file &;file &@;file
164 level any file &./file &;/file &@;/file
165 several f g h &. f g h &; f g h &@; f g h
166 .mk.in file $(abs)/file $(src)/file $(abs_src)/file
171 In general & expands to the subdirectory name when used for a
172 filename, and to the subdirectory name with / replaced with _ for
175 Note that & is processed *even in makefile comments*. The substitutor
176 does not understand make syntax, or shell syntax, at all. However,
177 the substitution rules are chosen to work well with constructs which
178 are common in makefiles.
180 In the notation below, we suppose that the substitution is being in
181 done in a subdirectory sub/dir of the source tree. In the RH column
182 we describe the expansion at the top level, which is often a special
183 case (in general in variable names we call that TOP rather than the
186 &CAPS => sub_dir_CAPS or TOP_CAPS
187 &lc => sub/dir/lc or lc
188 Here CAPS is any ASCII letter A-Z and lc is a-z.
189 The assumption is that filenames are usually lowercase and
190 variables usually uppercase. Otherwise, use another syntax:
192 &_ => sub_dir_ or TOP_
193 &=_ => sub_dir or TOP
195 &/ => sub/dir/ or nothing
198 &,lc => $(top_srcdir)/sub/dir/lc
199 &,/ => $(top_srcdir)/sub/dir/
201 &;lc => $(top_srcdir)/sub/dir/lc
202 &;/ => $(top_srcdir)/sub/dir/
204 &@lc => $(PWD)/sub/dir/lc
205 &@/ => $(PWD)/sub/dir/
210 &@,lc => $(abs_top_srcdir)/sub/dir/lc
211 &@,/ => $(abs_top_srcdir)/sub/dir/
213 &@;lc => $(abs_top_srcdir)/sub/dir/lc
214 &@;/ => $(abs_top_srcdir)/sub/dir/
217 = return subdir without delimiter (not allowed with `,' `;' `@')
218 , pathname of this subdirectory in source tree
219 ; pathname of top level of source tree
220 . pathname of this directory in build tree, implies absolute pathnames
221 @ absolute pathnames (forbidden with `.', must come first)
223 So pathname syntax is a subset of:
224 '&' [ '@' ] [ ',' | ';' | '.' ] [ lc | '/' ]
226 To avoid incomprehensible .sd.mk files, some combinations are not
227 allowed. For example `&@=./' would mean `$(PWD)/sub/dir' but can
228 be spelled `$(PWD)/&=/', but more normally the trailing / can be
229 tolerated, so use `&@/'.
231 && => && for convenience in shell runes
232 \& => & general escaping mechanism
235 &. thing thing... & &@. thing thing... &
236 &, thing thing... & &@, thing thing... &
237 &; thing thing... & &@; thing thing... &
238 Convenience syntax for prefixing multiple filenames.
239 Introduced by & followed by lwsp where lc could go.
240 Each lwsp-separated non-ws word is prefixed by &/ &./ &@./
241 etc. respectively. No other & escapes are recognised.
242 This processing continues until & preceded by lwsp,
243 or until EOL (the end of the line), or \ then EOL.
245 &:<directive> <args>....
246 recognised at start of line only (possibly after lwsp)
247 args are processed for &
249 &:include filename filename should usually be foo.sd.mk
250 &:-include filename tolerate nonexistent file
251 filenames are relative to $(top_srcdir)
253 &!<lwsp> disables & until EOL (and then disappears)
255 &# delete everything to end of line
256 (useful if the RHS contains unrecognised & constructions)
259 changes the escape sequence from & to literally STUFF
260 STUFF may be any series of of non-whitespace characters,
261 and is terminated by EOL or lwsp. &!STUFF and the lwsp
264 After this, write STUFF instead of &, everywhere.
265 The effect is global and lasts until the next setting.
266 It takes effect on &:include'd files too, so maybe set
267 it back before using &:include.
270 STUFFSTUFF => STUFFSTUFF
272 STUFF!& set escape back to &
275 Handled specially. If mentioned, declares that this
276 subdir ought to have a target `things'. The rule will be
277 &/things:: $(&TARGETS_things)
279 You may extend it by adding more :: rules for the target,
280 but the preferred style is to do things like this:
281 &TARGETS_check += & test-passed.stamp
283 It is important to mention &TARGETS_things at least once in
284 the context of each applicable directory, because doing so
285 arranges that the *parent* will also have a `things' target
286 which recursively implies this directory's `things'.
288 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
289 for example, is not magic. But mentioning &TARGETS_things in
290 a #-comment *does* work because the & filter does not care
293 `all' is extra special: every directory has an `all'
294 target, which corresponds to &TARGETS.
296 Subdirectory and variable naming
297 --------------------------------
299 The simple variable decoration scheme does not enforce a strict
300 namespace distinction between parts of variable names which come from
301 subdirectory names, and parts that mean something else.
303 So it is a good idea to be a bit careful with your directory naming.
304 `TOP', names that contain `_', and names that are similar to parts of
305 make variables (whether conventional ones, or ones used in your
306 project) are best avoided.
308 If you name your variables in ALL CAPS and your subdirectories in
309 lower case with `-' rather than `_', there will be no confusion.
311 Incorporating this into your project
312 ------------------------------------
314 Use `git-subtree' to merge the subdirmk/ directory. You may find it
315 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
316 symlinks as symlinks - just `git add' the link). And you probably
317 want to mention the situation in your top-level COPYING.
319 Symlink autogen.sh into your project toplevel.
321 In your configure.ac, say
323 m4_include([subdirmk/subdirmk.ac])
324 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
326 Write a Subdir.sd.mk in each directory. The toplevel one should
329 include subdirmk/usual.mk
330 include subdirmk/regen.mk
332 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
335 &:include subdirmk/cdeps.sd.mk
336 &:include subdirmk/clean.sd.mk
341 You can convert your project incrementally. Start with the top-level
342 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
343 stuff to configure.ac, and fix everything up. Leave the existing
344 $(MAKE) -C for your existing subdirectories alone. Then you can
345 convert individual subdirectories, or classes of subdirectories, at
346 your leisure. (You must be /sure/ that each subdirectory will be
347 entered only once at a time, but your existing recursive make descent
348 system should already do that or you already have concurrency bugs.)
350 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
351 This is a frequent source of concurrency bugs in recursive make build
352 systems. When combined with nonrecursive make it's all in the same
353 directory and there is nothing stopping the different invocations
354 ending up trying to make the same targets at the same time. That
355 causes hideous racy lossage. There are ways to get this to work
356 reliably but it is advanced stuff.
358 If you make syntax errors, or certain kinds of other errors, in your
359 makefiles, you may find that just `make' is broken now and cannot get
360 far enough to regenerate a working set of makefiles. If this happens
361 just rerun ./config.status by hand.
368 Copyright 2019 Mark Wooding
369 Copyright 2019 Ian Jackson
371 subdirmk and its example is free software; you can redistribute it
372 and/or modify it under the terms of the GNU Library General Public
373 License as published by the Free Software Foundation; either
374 version 2 of the License, or (at your option) any later version.
376 This is distributed in the hope that it will be useful, but
377 WITHOUT ANY WARRANTY; without even the implied warranty of
378 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
379 Library General Public License for more details.
381 You should have received a copy of the GNU Library General Public
382 License along with this library as the file LGPL-2.
383 If not, see https://www.gnu.org/.
385 Individual files generally contain the following tag in the copyright
386 notice, instead of the full licence grant text:
387 SPDX-License-Identifier: LGPL-2.0-or-later
388 As is conventional, this should be read as a licence grant.
390 Contributions are accepted based on the git commit Signed-off-by
391 convention, by which the contributors' certify their contributions
392 according to the Developer Certificate of Origin version 1.1 - see
393 the file DEVELOPER-CERTIFICATE.
395 Where subdirmk is used by and incorporated into another project (eg
396 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
397 the rest of the project are under that other project's licence(s).
398 (The project's overall licence must be compatible with LGPL-2.0+.)