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. Unqualified
32 filenames are relative to the build toplevel, and all commands all run
35 However, the sigil & is treated specially. By and large, it refers to
36 `the build directory corresponding to this .sd.mk file', etc.
37 There are a variety of convenient constructions.
39 The result is that to a large extent, the Subdir.sd.mk has an easy way
40 to namespace its "local" make variables, and an easy way to refer to
41 its "local" filenames (and filenames in general).
43 The Subdir.sd.mk's are filtered, fed through autoconf in the usual way
44 (for @..@-substitutions) and included by one autogenerated toplevel
47 So all of the input is combined and passed to one make invocation.
48 (A corollary is that there is no enforcement of the namespacing:
49 discipline is required to prefix relevant variable names with &, etc.)
51 Each subdirectory is also provided with an autogenerated `Makefile'
52 which exists purely to capture ordinary make invocations and arrange
53 for something suitable to happen.
55 Where there are dependencies between subdirectories, each Subdir.sd.mk
56 can simply refer to files in other subdirectories directly.
58 Invocation, "recursive" per-directory targets
59 ---------------------------------------------
61 Arrangements are made so that when you run `make foo' in a
62 subdirectory, it is like running the whole toplevel makefile, from the
63 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
64 be built, that builds it.
66 But `foo' can also be a conventional target like `all'.
68 Each subdirectory has its own `all' target. For example a
69 subdirectory `src' has a target `src/all'. The rules for these are
70 automatically generated from the settings of the per-directory
71 &TARGETS variables. &TARGETS is magic in this way. (In
72 src/Subdir.sd.mk, &TARGETS of course refers to a make variable called
75 The `all' target in a parent directory is taken to imply the `all'
76 targets in all of its subdirectories, recursively. And in the
77 autogenerated stub Makefiles, `all' is the default target. So if you
78 just type `make' in the toplevel, you are asking for `&all'
79 (<subdir>/all) for every directory in the project.
81 In a parallel build, the rules for all these various subdirectory
82 targets may be in run in parallel: there is only one `make' invocation
83 at a time. There is no sequencing between subdirectories, only been
84 individual targets (as specified according to their dependencies).
86 You can define other per-directory recursive targets too: simply
87 mention (usually, by setting) the variable &TARGETS_zonk, or whatever.
88 This will create a src/zonk target (for appropriate value of src/).
89 Unlike `all', these other targets only exist in areas of the project
90 where at least something mentions them. So for example, if
91 &TARGETS_zonk is mentioned in src but not lib, `make zonk' in
92 lib will fail. If you want to make a target exist everywhere,
93 mention its name in Perdir.sd.mk (see below).
95 Perdir.sd.mk, inclusion
96 -----------------------
98 The file Perdir.sd.mk in the toplevel of the source is automatically
99 processed after each individual directory's Subdir.sd.mk, and the
100 &-substituted contents therefore appear once for each subdirectory.
102 This lets you do per-directory boilerplate. Some useful boilerplate
103 is already provided in subdirmk, for you to reference like this:
104 &:include subdirmk/cdeps.sd.mk
105 &:include subdirmk/clean.sd.mk
106 For example you could put that in Perdir.sd.mk.
108 The top-level Subdir.sd.mk is the first makefile included after the
109 autogenerated `main.mk' which merely has some basic settings and
110 includes. So if you want to get in early and set global variables,
111 put them near the top of Subdir.sd.mk.
113 subdirmk's filter script itself sets (only) these variables:
118 You are likely to want to define $(PWD), and shorter names for
119 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
124 If want to set global variables, such as CC, that should only be done
125 once. You can put them in your top-level Subdir.sd.mk, or a separate
126 file you `include' and declare using SUBDIRMK_MAKEFILES.
128 If you need different settings of variables like CC for different
129 subdirectories, you should probably do that with target-specific
130 variable settings. See the info node `(make) Target-specific'.
132 Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in'
133 --------------------------------------------------------------------
135 There are two kinds of template files.
137 Filename .sd.mk .mk.in
139 Processed by &-substitution, autoconf only
142 Instantiated Usu. once per subdir Once only
144 Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/
145 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
147 How to include `&:include foo.sd.mk' `include foo.mk'
148 in all relevant .sd.mk in only one
149 (but not needed for Subdir.sd.mk
152 If you `include subdirmk/regen.mk', dependency management and
153 automatic regeneration for all of this template substitution, and for
154 config.status etc. is done for you.
156 Tables of file reference syntaxes
157 ---------------------------------
159 In a nonrecursive makefile supporting out of tree builds there are
160 three separate important distinctions between different file
163 (i) In the build tree, or in the source tree ?
165 (ii) In (or relative to) the subdirectory to which this Subdir.sd.mk
166 relates, or relative to the project's top level ?
168 (iii) Absolute or relative pathname ? Usually relative pathnames
169 suffice. Where an absolute pathname is needed, it can be built
170 out of &/ and an appropriate make variable such as $(PWD).
172 Path construction &-expansions are built from the following:
180 In more detail, with all the various options laid out:
182 Recommended Relative paths in... Absolute paths in...
183 for build source build source
185 This lc &file &^file $(PWD)/&file $(abs_src)/&file
186 directory any &/file &^/file $(PWD)&/file $(abs_src)/&/file
187 several & f g h &^ f g h $(addprefix...)
190 level any file &~/file $(PWD)/file $(abs_src)/file
191 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
192 several f g h &~ f g h $(addprefix...)
194 (This assumes you have appropriate make variables src, PWD and
200 In general & expands to the subdirectory name when used for a
201 filename, and to the subdirectory name with / replaced with _ for
204 Note that & is processed *even in makefile comments*. The substitutor
205 does not understand make syntax, or shell syntax, at all. However,
206 the substitution rules are chosen to work well with constructs which
207 are common in makefiles.
209 In the notation below, we suppose that the substitution is being in
210 done in a subdirectory sub/dir of the source tree. In the RH column
211 we describe the expansion at the top level, which is often a special
212 case (in general in variable names we call that TOP rather than the
215 &CAPS => sub_dir_CAPS or TOP_CAPS
216 &lc => sub/dir/lc or lc
217 Here CAPS is any ASCII letter A-Z and lc is a-z.
218 The assumption is that filenames are usually lowercase and
219 variables usually uppercase. Otherwise, use another syntax:
221 &_ => sub_dir_ or TOP_
222 &=_ => sub_dir or TOP
224 &/ => sub/dir/ or nothing
227 &^lc => $(top_srcdir)/sub/dir/lc
228 &^/ => $(top_srcdir)/sub/dir/
230 &~lc => $(top_srcdir)/lc
231 &~/ => $(top_srcdir)/
234 = return subdir without delimiter (not allowed with `^' `~')
235 ^ pathname of this subdirectory in source tree
236 ~ pathname of top level of source tree
237 / terminates the escape (needed if next is not lwsp or space)
238 lwsp starts multi-word processing (see below)
240 So pathname syntax is a subset of:
241 '&' [ '^' | '~' ] [ lc | '/' ]
243 && => && for convenience in shell runes
244 \& => & general escaping mechanism
249 Convenience syntax for prefixing multiple filenames.
250 Introduced by & followed by lwsp where lc could go.
251 Each lwsp-separated non-ws word is prefixed by &/ etc.
252 etc. respectively. No other & escapes are recognised.
253 This processing continues until & preceded by lwsp,
254 or until EOL (the end of the line), or \ then EOL.
256 &:<directive> <args>....
257 recognised at start of line only (possibly after lwsp)
258 args are processed for &
260 &:include filename filename should usually be foo.sd.mk
261 &:-include filename tolerate nonexistent file
262 filenames are relative to $(top_srcdir)
264 &!<lwsp> disables & until EOL (and then disappears)
266 &# delete everything to end of line
267 (useful if the RHS contains unrecognised & constructions)
270 changes the escape sequence from & to literally STUFF
271 STUFF may be any series of of non-whitespace characters,
272 and is terminated by EOL or lwsp. &!STUFF and the lwsp
275 After this, write STUFF instead of &, everywhere.
276 The effect is global and lasts until the next setting.
277 It takes effect on &:include'd files too, so maybe set
278 it back before using &:include.
281 STUFFSTUFF => STUFFSTUFF
283 STUFF!& set escape back to &
286 Handled specially. If mentioned, declares that this
287 subdir ought to have a target `things'. The rule will be
288 &/things:: $(&TARGETS_things)
290 You may extend it by adding more :: rules for the target,
291 but the preferred style is to do things like this:
292 &TARGETS_check += & test-passed.stamp
294 It is important to mention &TARGETS_things at least once in
295 the context of each applicable directory, because doing so
296 arranges that the *parent* will also have a `things' target
297 which recursively implies this directory's `things'.
299 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
300 for example, is not magic. But mentioning &TARGETS_things in
301 a #-comment *does* work because the & filter does not care
304 `all' is extra special: every directory has an `all'
305 target, which corresponds to &TARGETS.
307 Subdirectory and variable naming
308 --------------------------------
310 The simple variable decoration scheme does not enforce a strict
311 namespace distinction between parts of variable names which come from
312 subdirectory names, and parts that mean something else.
314 So it is a good idea to be a bit careful with your directory naming.
315 `TOP', names that contain `_', and names that are similar to parts of
316 make variables (whether conventional ones, or ones used in your
317 project) are best avoided.
319 If you name your variables in ALL CAPS and your subdirectories in
320 lower case with `-' rather than `_', there will be no confusion.
322 Incorporating this into your project
323 ------------------------------------
325 Use `git-subtree' to merge the subdirmk/ directory. You may find it
326 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
327 symlinks as symlinks - just `git add' the link). And you probably
328 want to mention the situation in your top-level COPYING.
330 Symlink autogen.sh into your project toplevel.
332 In your configure.ac, say
334 m4_include([subdirmk/subdirmk.ac])
335 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
337 Write a Subdir.sd.mk in each directory. The toplevel one should
340 include subdirmk/usual.mk
341 include subdirmk/regen.mk
343 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
346 &:include subdirmk/cdeps.sd.mk
347 &:include subdirmk/clean.sd.mk
352 You can convert your project incrementally. Start with the top-level
353 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
354 stuff to configure.ac, and fix everything up. Leave the existing
355 $(MAKE) -C for your existing subdirectories alone. Then you can
356 convert individual subdirectories, or classes of subdirectories, at
357 your leisure. (You must be /sure/ that each subdirectory will be
358 entered only once at a time, but your existing recursive make descent
359 system should already do that or you already have concurrency bugs.)
361 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
362 This is a frequent source of concurrency bugs in recursive make build
363 systems. When combined with nonrecursive make it's all in the same
364 directory and there is nothing stopping the different invocations
365 ending up trying to make the same targets at the same time. That
366 causes hideous racy lossage. There are ways to get this to work
367 reliably but it is advanced stuff.
369 If you make syntax errors, or certain kinds of other errors, in your
370 makefiles, you may find that just `make' is broken now and cannot get
371 far enough to regenerate a working set of makefiles. If this happens
372 just rerun ./config.status by hand.
379 Copyright 2019 Mark Wooding
380 Copyright 2019 Ian Jackson
382 subdirmk and its example is free software; you can redistribute it
383 and/or modify it under the terms of the GNU Library General Public
384 License as published by the Free Software Foundation; either
385 version 2 of the License, or (at your option) any later version.
387 This is distributed in the hope that it will be useful, but
388 WITHOUT ANY WARRANTY; without even the implied warranty of
389 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
390 Library General Public License for more details.
392 You should have received a copy of the GNU Library General Public
393 License along with this library as the file LGPL-2.
394 If not, see https://www.gnu.org/.
396 Individual files generally contain the following tag in the copyright
397 notice, instead of the full licence grant text:
398 SPDX-License-Identifier: LGPL-2.0-or-later
399 As is conventional, this should be read as a licence grant.
401 Contributions are accepted based on the git commit Signed-off-by
402 convention, by which the contributors' certify their contributions
403 according to the Developer Certificate of Origin version 1.1 - see
404 the file DEVELOPER-CERTIFICATE.
406 Where subdirmk is used by and incorporated into another project (eg
407 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
408 the rest of the project are under that other project's licence(s).
409 (The project's overall licence must be compatible with LGPL-2.0+.)