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 Relative paths in... Absolute paths in...
147 build source build source
149 This directory & &, &@ &@,
150 Top level . &; &@. &@;
152 Adding `@' means "absolute path". (`&.' is not allowed without @
153 because just `&./' is a silly way of writing `.'.) `/' terminates the
154 escape (needed if the next thing is not a lowercase character, or
155 space). `=' means "just the value, no /". Space starts multi-word
158 In more detail, with the various options for what comes next:
160 Recommended Relative paths in... Absolute paths in...
161 for build source build source
163 This lc &file &,file &@file &@,file
164 directory any &/file &,/file &@/file &@,/file
165 several & f g h &, f g h &@ f g h &@, f g h
167 Top lc file &;file &@.file &@;file
168 level any file &;/file &@./file &@;/file
169 several f g h &; f g h &@. f g h &@; f g h
170 .mk.in file $(src)/file $(abs)/file $(abs_src)/file
175 In general & expands to the subdirectory name when used for a
176 filename, and to the subdirectory name with / replaced with _ for
179 Note that & is processed *even in makefile comments*. The substitutor
180 does not understand make syntax, or shell syntax, at all. However,
181 the substitution rules are chosen to work well with constructs which
182 are common in makefiles.
184 In the notation below, we suppose that the substitution is being in
185 done in a subdirectory sub/dir of the source tree. In the RH column
186 we describe the expansion at the top level, which is often a special
187 case (in general in variable names we call that TOP rather than the
190 &CAPS => sub_dir_CAPS or TOP_CAPS
191 &lc => sub/dir/lc or lc
192 Here CAPS is any ASCII letter A-Z and lc is a-z.
193 The assumption is that filenames are usually lowercase and
194 variables usually uppercase. Otherwise, use another syntax:
196 &_ => sub_dir_ or TOP_
197 &=_ => sub_dir or TOP
199 &/ => sub/dir/ or nothing
202 &,lc => $(top_srcdir)/sub/dir/lc &,/ => $(top_srcdir)/sub/dir/
203 &;lc => $(top_srcdir)/lc &;/ => $(top_srcdir)/
205 &@lc => $(PWD)/sub/dir/lc &@/ => $(PWD)/sub/dir/
206 &@.lc => $(PWD)/lc &@./ => $(PWD)/
207 &@,lc => $(abs_top_srcdir)/sub/dir/lc &@,/ => $(abs_top_srcdir)/sub/dir/
208 &@;lc => $(abs_top_srcdir)/lc &@;/ => $(abs_top_srcdir)/
211 = return subdir without delimiter (not allowed with `,' `;' `@')
212 , pathname of this subdirectory in source tree
213 ; pathname of top level of source tree
214 . pathname of this directory in build tree, `@' must be specified
217 So pathname syntax is a subset of:
218 '&' [ '@' ] [ ',' | ';' | '.' ] [ lc | '/' ]
220 To avoid incomprehensible .sd.mk files, some combinations are not
221 allowed. For example `&=./' would mean `.' and `&./' would be the
222 empty string. Variations with `=' and one of `@' `,' `;' are
223 uncommon and must be written using make variables instead.
225 && => && for convenience in shell runes
226 \& => & general escaping mechanism
228 & thing thing... & &@ thing thing... &
230 &, thing thing... & &@, thing thing... &
231 &; thing thing... & &@; thing thing... &
232 Convenience syntax for prefixing multiple filenames.
233 Introduced by & followed by lwsp where lc could go.
234 Each lwsp-separated non-ws word is prefixed by &/ etc.
235 etc. respectively. No other & escapes are recognised.
236 This processing continues until & preceded by lwsp,
237 or until EOL (the end of the line), or \ then EOL.
239 &:<directive> <args>....
240 recognised at start of line only (possibly after lwsp)
241 args are processed for &
243 &:include filename filename should usually be foo.sd.mk
244 &:-include filename tolerate nonexistent file
245 filenames are relative to $(top_srcdir)
247 &!<lwsp> disables & until EOL (and then disappears)
249 &# delete everything to end of line
250 (useful if the RHS contains unrecognised & constructions)
253 changes the escape sequence from & to literally STUFF
254 STUFF may be any series of of non-whitespace characters,
255 and is terminated by EOL or lwsp. &!STUFF and the lwsp
258 After this, write STUFF instead of &, everywhere.
259 The effect is global and lasts until the next setting.
260 It takes effect on &:include'd files too, so maybe set
261 it back before using &:include.
264 STUFFSTUFF => STUFFSTUFF
266 STUFF!& set escape back to &
269 Handled specially. If mentioned, declares that this
270 subdir ought to have a target `things'. The rule will be
271 &/things:: $(&TARGETS_things)
273 You may extend it by adding more :: rules for the target,
274 but the preferred style is to do things like this:
275 &TARGETS_check += & test-passed.stamp
277 It is important to mention &TARGETS_things at least once in
278 the context of each applicable directory, because doing so
279 arranges that the *parent* will also have a `things' target
280 which recursively implies this directory's `things'.
282 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
283 for example, is not magic. But mentioning &TARGETS_things in
284 a #-comment *does* work because the & filter does not care
287 `all' is extra special: every directory has an `all'
288 target, which corresponds to &TARGETS.
290 Subdirectory and variable naming
291 --------------------------------
293 The simple variable decoration scheme does not enforce a strict
294 namespace distinction between parts of variable names which come from
295 subdirectory names, and parts that mean something else.
297 So it is a good idea to be a bit careful with your directory naming.
298 `TOP', names that contain `_', and names that are similar to parts of
299 make variables (whether conventional ones, or ones used in your
300 project) are best avoided.
302 If you name your variables in ALL CAPS and your subdirectories in
303 lower case with `-' rather than `_', there will be no confusion.
305 Incorporating this into your project
306 ------------------------------------
308 Use `git-subtree' to merge the subdirmk/ directory. You may find it
309 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
310 symlinks as symlinks - just `git add' the link). And you probably
311 want to mention the situation in your top-level COPYING.
313 Symlink autogen.sh into your project toplevel.
315 In your configure.ac, say
317 m4_include([subdirmk/subdirmk.ac])
318 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
320 Write a Subdir.sd.mk in each directory. The toplevel one should
323 include subdirmk/usual.mk
324 include subdirmk/regen.mk
326 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
329 &:include subdirmk/cdeps.sd.mk
330 &:include subdirmk/clean.sd.mk
335 You can convert your project incrementally. Start with the top-level
336 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
337 stuff to configure.ac, and fix everything up. Leave the existing
338 $(MAKE) -C for your existing subdirectories alone. Then you can
339 convert individual subdirectories, or classes of subdirectories, at
340 your leisure. (You must be /sure/ that each subdirectory will be
341 entered only once at a time, but your existing recursive make descent
342 system should already do that or you already have concurrency bugs.)
344 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
345 This is a frequent source of concurrency bugs in recursive make build
346 systems. When combined with nonrecursive make it's all in the same
347 directory and there is nothing stopping the different invocations
348 ending up trying to make the same targets at the same time. That
349 causes hideous racy lossage. There are ways to get this to work
350 reliably but it is advanced stuff.
352 If you make syntax errors, or certain kinds of other errors, in your
353 makefiles, you may find that just `make' is broken now and cannot get
354 far enough to regenerate a working set of makefiles. If this happens
355 just rerun ./config.status by hand.
362 Copyright 2019 Mark Wooding
363 Copyright 2019 Ian Jackson
365 subdirmk and its example is free software; you can redistribute it
366 and/or modify it under the terms of the GNU Library General Public
367 License as published by the Free Software Foundation; either
368 version 2 of the License, or (at your option) any later version.
370 This is distributed in the hope that it will be useful, but
371 WITHOUT ANY WARRANTY; without even the implied warranty of
372 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
373 Library General Public License for more details.
375 You should have received a copy of the GNU Library General Public
376 License along with this library as the file LGPL-2.
377 If not, see https://www.gnu.org/.
379 Individual files generally contain the following tag in the copyright
380 notice, instead of the full licence grant text:
381 SPDX-License-Identifier: LGPL-2.0-or-later
382 As is conventional, this should be read as a licence grant.
384 Contributions are accepted based on the git commit Signed-off-by
385 convention, by which the contributors' certify their contributions
386 according to the Developer Certificate of Origin version 1.1 - see
387 the file DEVELOPER-CERTIFICATE.
389 Where subdirmk is used by and incorporated into another project (eg
390 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
391 the rest of the project are under that other project's licence(s).
392 (The project's overall licence must be compatible with LGPL-2.0+.)