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 nothing
222 &_ => sub_dir_ or TOP_
224 (This implies that `&./' works roughly like `&/', although
225 it can produce a needless `./')
229 &^lc => $(top_srcdir)/sub/dir/lc
230 &^/ => $(top_srcdir)/sub/dir/
231 &^. => $(top_srcdir)/sub/dir
233 &~lc => $(top_srcdir)/lc
234 &~/ => $(top_srcdir)/
238 ^ pathname of this subdirectory in source tree
239 ~ pathname of top level of source tree
240 / terminates the path escape } needed if next is
241 _ terminates the var escape } not lwsp or space)
242 . terminates path escape giving dir name (excluding /)
243 = terminates var escape giving only prefix part (rarely needed)
244 lwsp starts multi-word processing (see below)
246 So pathname syntax is a subset of:
247 '&' [ '^' | '~' ] [ lc | '/' | '.' | '=' ]
249 && => && for convenience in shell runes
251 &\& => & general escaping mechanism
257 Convenience syntax for prefixing multiple filenames.
258 Introduced by & followed by lwsp where lc could go.
259 Each lwsp-separated non-ws word is prefixed by &/ etc.
260 etc. respectively. No other & escapes are recognised.
261 This processing continues until & preceded by lwsp,
262 or until EOL (the end of the line), or \ then EOL.
264 &:<directive> <args>....
265 recognised at start of line only (possibly after lwsp)
266 args are processed for &
268 &:include filename filename should usually be foo.sd.mk
269 &:-include filename tolerate nonexistent file
270 filenames are relative to $(top_srcdir)
272 &!<lwsp> disables & until EOL (and then disappears)
274 &# delete everything to end of line
275 (useful if the RHS contains unrecognised & constructions)
277 &:changequote NEWQUOTE
278 changes the escape sequence from & to literally NEWQUOTE
279 NEWQUOTE may be any series of of non-whitespace characters,
280 and is terminated by EOL or lwsp. The whole line is
283 After this, write NEWQUOTE instead of &, everywhere.
284 The effect is global and lasts until the next setting.
285 It takes effect on &:include'd files too, so maybe set
286 it back before using &:include.
289 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
290 NEWQUOTE\NEWQUOTE => NEWQUOTE
291 NEWQUOTE:changequote & set escape back to &
294 Handled specially. If mentioned, declares that this
295 subdir ought to have a target `things'. The rule will be
296 &/things:: $(&TARGETS_things)
298 You may extend it by adding more :: rules for the target,
299 but the preferred style is to do things like this:
300 &TARGETS_check += & test-passed.stamp
302 It is important to mention &TARGETS_things at least once in
303 the context of each applicable directory, because doing so
304 arranges that the *parent* will also have a `things' target
305 which recursively implies this directory's `things'.
307 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
308 for example, is not magic. But mentioning &TARGETS_things in
309 a #-comment *does* work because the & filter does not care
312 `all' is extra special: every directory has an `all'
313 target, which corresponds to &TARGETS.
315 Subdirectory and variable naming
316 --------------------------------
318 The simple variable decoration scheme does not enforce a strict
319 namespace distinction between parts of variable names which come from
320 subdirectory names, and parts that mean something else.
322 So it is a good idea to be a bit careful with your directory naming.
323 `TOP', names that contain `_', and names that are similar to parts of
324 make variables (whether conventional ones, or ones used in your
325 project) are best avoided.
327 If you name your variables in ALL CAPS and your subdirectories in
328 lower case with `-' rather than `_', there will be no confusion.
330 Incorporating this into your project
331 ------------------------------------
333 Use `git-subtree' to merge the subdirmk/ directory. You may find it
334 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
335 symlinks as symlinks - just `git add' the link). And you probably
336 want to mention the situation in your top-level COPYING.
338 Symlink autogen.sh into your project toplevel.
340 In your configure.ac, say
342 m4_include([subdirmk/subdirmk.ac])
343 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
345 Write a Subdir.sd.mk in each directory. The toplevel one should
348 include subdirmk/usual.mk
349 include subdirmk/regen.mk
351 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
354 &:include subdirmk/cdeps.sd.mk
355 &:include subdirmk/clean.sd.mk
360 You can convert your project incrementally. Start with the top-level
361 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
362 stuff to configure.ac, and fix everything up. Leave the existing
363 $(MAKE) -C for your existing subdirectories alone. Then you can
364 convert individual subdirectories, or classes of subdirectories, at
365 your leisure. (You must be /sure/ that each subdirectory will be
366 entered only once at a time, but your existing recursive make descent
367 system should already do that or you already have concurrency bugs.)
369 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
370 This is a frequent source of concurrency bugs in recursive make build
371 systems. When combined with nonrecursive make it's all in the same
372 directory and there is nothing stopping the different invocations
373 ending up trying to make the same targets at the same time. That
374 causes hideous racy lossage. There are ways to get this to work
375 reliably but it is advanced stuff.
377 If you make syntax errors, or certain kinds of other errors, in your
378 makefiles, you may find that just `make' is broken now and cannot get
379 far enough to regenerate a working set of makefiles. If this happens
380 just rerun ./config.status by hand.
382 If you go back and forth between different versions of your code you
383 can sometimes find that `make' complains that one of your Subdir.sd.mk
384 files is missing: typically, if iot was used and therefore a
385 dependency in some other version of your code. If you run `make
386 clean' (or `make realclean') these dependencies are suppressed, which
387 will clear up the problem.
394 Copyright 2019 Mark Wooding
395 Copyright 2019 Ian Jackson
397 subdirmk and its example is free software; you can redistribute it
398 and/or modify it under the terms of the GNU Library General Public
399 License as published by the Free Software Foundation; either
400 version 2 of the License, or (at your option) any later version.
402 This is distributed in the hope that it will be useful, but
403 WITHOUT ANY WARRANTY; without even the implied warranty of
404 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
405 Library General Public License for more details.
407 You should have received a copy of the GNU Library General Public
408 License along with this library as the file LGPL-2.
409 If not, see https://www.gnu.org/.
411 Individual files generally contain the following tag in the copyright
412 notice, instead of the full licence grant text:
413 SPDX-License-Identifier: LGPL-2.0-or-later
414 As is conventional, this should be read as a licence grant.
416 Contributions are accepted based on the git commit Signed-off-by
417 convention, by which the contributors' certify their contributions
418 according to the Developer Certificate of Origin version 1.1 - see
419 the file DEVELOPER-CERTIFICATE.
421 Where subdirmk is used by and incorporated into another project (eg
422 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
423 the rest of the project are under that other project's licence(s).
424 (The project's overall licence must be compatible with LGPL-2.0+.)