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.
111 If want to set global variables, such as CC, that should only be done
112 once. You can put them in your top-level Subdir.sd.mk, or a separate
113 file you `include' and declare using SUBDIRMK_MAKEFILES.
115 If you need different settings of variables like CC for different
116 subdirectories, you should probably do that with target-specific
117 variable settings. See the info node `(make) Target-specific'.
119 Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in'
120 --------------------------------------------------------------------
122 There are two kinds of template files.
124 Filename .sd.mk .mk.in
126 Processed by &-substitution, autoconf only
129 Instantiated Usu. once per subdir Once only
131 Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/
132 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
134 How to include `&:include foo.sd.mk' `include foo.mk'
135 in all relevant .sd.mk in only one
136 (but not needed for Subdir.sd.mk
139 If you `include subdirmk/regen.mk', dependency management and
140 automatic regeneration for all of this template substitution, and for
141 config.status etc. is done for you.
143 Tables of file reference syntaxes
144 ---------------------------------
146 In a nonrecursive makefile supporting out of tree builds there are
147 three separate important distinctions between different file
150 (i) In the build tree, or in the source tree ?
152 (ii) In (or relative to) the subdirectory to which this Subdir.sd.mk
153 relates, or relative to the project's top level ?
155 (iii) Absolute or relative pathname ? Usually relative pathnames
156 suffice. Where an absolute pathname is needed, it can be built
157 out of &/ and an appropriate make variable such as $(PWD).
159 Path construction &-expansions are built from the following:
167 In more detail, with all the various options laid out:
169 Recommended Relative paths in... Absolute paths in...
170 for build source build source
172 This lc &file &,file $(PWD)/&file $(abs_src)/&file
173 directory any &/file &,/file $(PWD)&/file $(abs_src)/&/file
174 several & f g h &, f g h $(addprefix...)
177 level any file &;/file $(PWD)/file $(abs_src)/file
178 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
179 several f g h &; f g h $(addprefix...)
181 (This assumes you have appropriate make variables src, PWD and
187 In general & expands to the subdirectory name when used for a
188 filename, and to the subdirectory name with / replaced with _ for
191 Note that & is processed *even in makefile comments*. The substitutor
192 does not understand make syntax, or shell syntax, at all. However,
193 the substitution rules are chosen to work well with constructs which
194 are common in makefiles.
196 In the notation below, we suppose that the substitution is being in
197 done in a subdirectory sub/dir of the source tree. In the RH column
198 we describe the expansion at the top level, which is often a special
199 case (in general in variable names we call that TOP rather than the
202 &CAPS => sub_dir_CAPS or TOP_CAPS
203 &lc => sub/dir/lc or lc
204 Here CAPS is any ASCII letter A-Z and lc is a-z.
205 The assumption is that filenames are usually lowercase and
206 variables usually uppercase. Otherwise, use another syntax:
208 &_ => sub_dir_ or TOP_
209 &=_ => sub_dir or TOP
211 &/ => sub/dir/ or nothing
214 &,lc => $(top_srcdir)/sub/dir/lc
215 &,/ => $(top_srcdir)/sub/dir/
217 &;lc => $(top_srcdir)/lc
218 &;/ => $(top_srcdir)/
221 = return subdir without delimiter (not allowed with `,' `;')
222 , pathname of this subdirectory in source tree
223 ; pathname of top level of source tree
224 / terminates the escape (needed if next is not lwsp or space)
225 lwsp starts multi-word processing (see below)
227 So pathname syntax is a subset of:
228 '&' [ ',' | ';' ] [ lc | '/' ]
230 && => && for convenience in shell runes
231 \& => & general escaping mechanism
236 Convenience syntax for prefixing multiple filenames.
237 Introduced by & followed by lwsp where lc could go.
238 Each lwsp-separated non-ws word is prefixed by &/ etc.
239 etc. respectively. No other & escapes are recognised.
240 This processing continues until & preceded by lwsp,
241 or until EOL (the end of the line), or \ then EOL.
243 &:<directive> <args>....
244 recognised at start of line only (possibly after lwsp)
245 args are processed for &
247 &:include filename filename should usually be foo.sd.mk
248 &:-include filename tolerate nonexistent file
249 filenames are relative to $(top_srcdir)
251 &!<lwsp> disables & until EOL (and then disappears)
253 &# delete everything to end of line
254 (useful if the RHS contains unrecognised & constructions)
257 changes the escape sequence from & to literally STUFF
258 STUFF may be any series of of non-whitespace characters,
259 and is terminated by EOL or lwsp. &!STUFF and the lwsp
262 After this, write STUFF instead of &, everywhere.
263 The effect is global and lasts until the next setting.
264 It takes effect on &:include'd files too, so maybe set
265 it back before using &:include.
268 STUFFSTUFF => STUFFSTUFF
270 STUFF!& set escape back to &
273 Handled specially. If mentioned, declares that this
274 subdir ought to have a target `things'. The rule will be
275 &/things:: $(&TARGETS_things)
277 You may extend it by adding more :: rules for the target,
278 but the preferred style is to do things like this:
279 &TARGETS_check += & test-passed.stamp
281 It is important to mention &TARGETS_things at least once in
282 the context of each applicable directory, because doing so
283 arranges that the *parent* will also have a `things' target
284 which recursively implies this directory's `things'.
286 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
287 for example, is not magic. But mentioning &TARGETS_things in
288 a #-comment *does* work because the & filter does not care
291 `all' is extra special: every directory has an `all'
292 target, which corresponds to &TARGETS.
294 Subdirectory and variable naming
295 --------------------------------
297 The simple variable decoration scheme does not enforce a strict
298 namespace distinction between parts of variable names which come from
299 subdirectory names, and parts that mean something else.
301 So it is a good idea to be a bit careful with your directory naming.
302 `TOP', names that contain `_', and names that are similar to parts of
303 make variables (whether conventional ones, or ones used in your
304 project) are best avoided.
306 If you name your variables in ALL CAPS and your subdirectories in
307 lower case with `-' rather than `_', there will be no confusion.
309 Incorporating this into your project
310 ------------------------------------
312 Use `git-subtree' to merge the subdirmk/ directory. You may find it
313 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
314 symlinks as symlinks - just `git add' the link). And you probably
315 want to mention the situation in your top-level COPYING.
317 Symlink autogen.sh into your project toplevel.
319 In your configure.ac, say
321 m4_include([subdirmk/subdirmk.ac])
322 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
324 Write a Subdir.sd.mk in each directory. The toplevel one should
327 include subdirmk/usual.mk
328 include subdirmk/regen.mk
330 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
333 &:include subdirmk/cdeps.sd.mk
334 &:include subdirmk/clean.sd.mk
339 You can convert your project incrementally. Start with the top-level
340 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
341 stuff to configure.ac, and fix everything up. Leave the existing
342 $(MAKE) -C for your existing subdirectories alone. Then you can
343 convert individual subdirectories, or classes of subdirectories, at
344 your leisure. (You must be /sure/ that each subdirectory will be
345 entered only once at a time, but your existing recursive make descent
346 system should already do that or you already have concurrency bugs.)
348 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
349 This is a frequent source of concurrency bugs in recursive make build
350 systems. When combined with nonrecursive make it's all in the same
351 directory and there is nothing stopping the different invocations
352 ending up trying to make the same targets at the same time. That
353 causes hideous racy lossage. There are ways to get this to work
354 reliably but it is advanced stuff.
356 If you make syntax errors, or certain kinds of other errors, in your
357 makefiles, you may find that just `make' is broken now and cannot get
358 far enough to regenerate a working set of makefiles. If this happens
359 just rerun ./config.status by hand.
366 Copyright 2019 Mark Wooding
367 Copyright 2019 Ian Jackson
369 subdirmk and its example is free software; you can redistribute it
370 and/or modify it under the terms of the GNU Library General Public
371 License as published by the Free Software Foundation; either
372 version 2 of the License, or (at your option) any later version.
374 This is distributed in the hope that it will be useful, but
375 WITHOUT ANY WARRANTY; without even the implied warranty of
376 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
377 Library General Public License for more details.
379 You should have received a copy of the GNU Library General Public
380 License along with this library as the file LGPL-2.
381 If not, see https://www.gnu.org/.
383 Individual files generally contain the following tag in the copyright
384 notice, instead of the full licence grant text:
385 SPDX-License-Identifier: LGPL-2.0-or-later
386 As is conventional, this should be read as a licence grant.
388 Contributions are accepted based on the git commit Signed-off-by
389 convention, by which the contributors' certify their contributions
390 according to the Developer Certificate of Origin version 1.1 - see
391 the file DEVELOPER-CERTIFICATE.
393 Where subdirmk is used by and incorporated into another project (eg
394 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
395 the rest of the project are under that other project's licence(s).
396 (The project's overall licence must be compatible with LGPL-2.0+.)