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 &~/ => $(top_srcdir)/
238 ^ pathname of this subdirectory in source tree
239 ~ pathname of top level of source tree
240 / terminates the path escape } needed if next is
241 _ terminates the var escape } not lwsp or space)
242 . terminates path escape giving dir name (excluding /)
243 = terminates var escape giving only prefix part (rarely needed)
244 lwsp starts multi-word processing (see below)
246 So pathname syntax is a subset of:
247 '&' [ '^' | '~' ] [ lc | '/' | '.' | '=' ]
249 && => && for convenience in shell runes
251 &\& => & general escaping mechanism
253 &\NEWLINE eats the newline and vanishes
255 &$VARIABLE => $(sub_dir_VARIABLE) or $(TOP_VARIABLE)
256 VARIABLE is ASCII starting with a letter and matching \w+
261 Convenience syntax for prefixing multiple filenames.
262 Introduced by & followed by lwsp where lc could go.
263 Each lwsp-separated non-ws word is prefixed by &/ etc.
264 etc. respectively. No other & escapes are recognised.
265 This processing continues until & preceded by lwsp,
266 or until EOL (the end of the line), or \ then EOL.
268 &:<directive> <args>....
269 recognised at start of line only (possibly after lwsp)
270 args are processed for &
272 &:include filename filename should usually be [&]foo.sd.mk
273 &:-include filename tolerate nonexistent file
274 filenames are relative to $(top_srcdir)
277 &!<lwsp> disables & until EOL (and then disappears)
279 &# delete everything to end of line
280 (useful if the RHS contains unrecognised & constructions)
282 &:changequote NEWQUOTE
283 changes the escape sequence from & to literally NEWQUOTE
284 NEWQUOTE may be any series of of non-whitespace characters,
285 and is terminated by EOL or lwsp. The whole line is
288 After this, write NEWQUOTE instead of &, everywhere.
289 The effect is global and lasts until the next setting.
290 It takes effect on &:include'd files too, so maybe set
291 it back before using &:include.
294 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
295 NEWQUOTE\NEWQUOTE => NEWQUOTE
296 NEWQUOTE:changequote & set escape back to &
299 Handled specially. If mentioned at the start of a line
300 (possibly following whitespace), declares that this
301 subdir ought to have a target `things'. The rule will be
302 &/things:: $(&TARGETS_things)
304 You may extend it by adding more :: rules for the target,
305 but the preferred style is to do things like this:
306 &TARGETS_check += & test-passed.stamp
308 It is important to mention &TARGETS_things at least once in
309 the context of each applicable directory, because doing so
310 arranges that the *parent* will also have a `things' target
311 which recursively implies this directory's `things'.
313 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
314 for example, is not magic. To make the target exist
315 without providing any prerequisites for it, write a line
316 containing just `&TARGETS_things +='.
318 `all' is extra special: every directory has an `all'
319 target, which corresponds to &TARGETS.
321 Subdirectory and variable naming
322 --------------------------------
324 The simple variable decoration scheme does not enforce a strict
325 namespace distinction between parts of variable names which come from
326 subdirectory names, and parts that mean something else.
328 So it is a good idea to be a bit careful with your directory naming.
329 `TOP', names that contain `_', and names that are similar to parts of
330 make variables (whether conventional ones, or ones used in your
331 project) are best avoided.
333 If you name your variables in ALL CAPS and your subdirectories in
334 lower case with `-' rather than `_', there will be no confusion.
336 Incorporating this into your project
337 ------------------------------------
339 Use `git-subtree' to merge the subdirmk/ directory. You may find it
340 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
341 symlinks as symlinks - just `git add' the link). And you probably
342 want to mention the situation in your top-level COPYING.
344 Symlink autogen.sh into your project toplevel.
346 In your configure.ac, say
348 m4_include([subdirmk/subdirmk.ac])
349 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
351 Write a Subdir.sd.mk in each directory. The toplevel one should
354 include subdirmk/usual.mk
355 include subdirmk/regen.mk
357 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
360 &:include subdirmk/cdeps.sd.mk
361 &:include subdirmk/clean.sd.mk
366 You can convert your project incrementally. Start with the top-level
367 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
368 stuff to configure.ac, and fix everything up. Leave the existing
369 $(MAKE) -C for your existing subdirectories alone. Then you can
370 convert individual subdirectories, or classes of subdirectories, at
371 your leisure. (You must be /sure/ that each subdirectory will be
372 entered only once at a time, but your existing recursive make descent
373 system should already do that or you already have concurrency bugs.)
375 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
376 This is a frequent source of concurrency bugs in recursive make build
377 systems. When combined with nonrecursive make it's all in the same
378 directory and there is nothing stopping the different invocations
379 ending up trying to make the same targets at the same time. That
380 causes hideous racy lossage. There are ways to get this to work
381 reliably but it is advanced stuff.
383 If you make syntax errors, or certain kinds of other errors, in your
384 makefiles, you may find that just `make' is broken now and cannot get
385 far enough to regenerate a working set of makefiles. If this happens
386 just rerun ./config.status by hand.
388 If you go back and forth between different versions of your code you
389 can sometimes find that `make' complains that one of your Subdir.sd.mk
390 files is missing: typically, if iot was used and therefore a
391 dependency in some other version of your code. If you run `make
392 clean' (or `make realclean') these dependencies are suppressed, which
393 will clear up the problem.
400 Copyright 2019 Mark Wooding
401 Copyright 2019 Ian Jackson
403 subdirmk and its example is free software; you can redistribute it
404 and/or modify it under the terms of the GNU Library General Public
405 License as published by the Free Software Foundation; either
406 version 2 of the License, or (at your option) any later version.
408 This is distributed in the hope that it will be useful, but
409 WITHOUT ANY WARRANTY; without even the implied warranty of
410 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
411 Library General Public License for more details.
413 You should have received a copy of the GNU Library General Public
414 License along with this library as the file LGPL-2.
415 If not, see https://www.gnu.org/.
417 Individual files generally contain the following tag in the copyright
418 notice, instead of the full licence grant text:
419 SPDX-License-Identifier: LGPL-2.0-or-later
420 As is conventional, this should be read as a licence grant.
422 Contributions are accepted based on the git commit Signed-off-by
423 convention, by which the contributors' certify their contributions
424 according to the Developer Certificate of Origin version 1.1 - see
425 the file DEVELOPER-CERTIFICATE.
427 Where subdirmk is used by and incorporated into another project (eg
428 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
429 the rest of the project are under that other project's licence(s).
430 (The project's overall licence must be compatible with LGPL-2.0+.)