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 directory reference syntaxes
142 ---------------------------------------
144 In source tree In build tree
145 Relative Absolute Relative Absolute
148 This directory &^/file &~/file &/file $(abs)/&/file
149 & ^ f g h & ~ f g h & f g h
151 Top level $(ts)/file $(ats)/file file $(abs)/file
157 In general & expands to the subdirectory name when used for a
158 filename, and to the subdirectory name with / replaced with _ for
161 Note that & is processed *even in makefile comments*. The substitutor
162 does not understand make syntax, or shell syntax, at all. However,
163 the substitution rules are chosen to work well with constructs which
164 are common in makefiles.
166 In the notation below, we suppose that the substitution is being in
167 done in a subdirectory sub/dir of the source tree. In the RH column
168 we describe the expansion at the top level, which is often a special
169 case (in general in variable names we call that TOP rather than the
172 &CAPS => sub_dir_CAPS or TOP_CAPS
173 &lc => sub/dir/lc or lc
174 Here CAPS is any ASCII letter A-Z and lc is a-z.
175 The assumption is that filenames are usually lowercase and
176 variables usually uppercase. Otherwise, use another syntax:
178 &_ => sub_dir_ or TOP_
179 &/ => sub/dir/ or nothing
180 &=_ => sub_dir or TOP
182 &^ => $(top_srcdir)/sub/dir or $(top_srcdir)
183 &~ => $(abs_top_srcdir)/sub/dir or $(abs_top_srcdir)
185 && => && for convenience in shell runes
186 \& => & general escaping mechanism
191 Convenience syntax for prefixing multiple filenames.
192 Introduced by & followed by lwsp (space or tab).
193 Each lwsp-separated non-ws word is prefixed by &/ &^/ &~/
194 respectively. No other & escapes are recognised.
195 This processing continues until & preceded by lwsp,
196 or until EOL (the end of the line), or \ then EOL.
198 &:<directive> <args>....
199 recognised at start of line only (possibly after lwsp)
200 args are processed for &
202 &:include filename filename should usually be foo.sd.mk
203 &:-include filename tolerate nonexistent file
204 filenames are relative to $(top_srcdir)
206 &!<lwsp> disables & until EOL (and then disappears)
208 &# delete everything to end of line
209 (useful if the RHS contains unrecognised & constructions)
212 changes the escape sequence from & to literally STUFF
213 STUFF may be any series of of non-whitespace characters,
214 and is terminated by EOL or lwsp. &!STUFF and the lwsp
217 After this, write STUFF instead of &, everywhere.
218 The effect is global and lasts until the next setting.
219 It takes effect on &:include'd files too, so maybe set
220 it back before using &:include.
223 STUFFSTUFF => STUFFSTUFF
225 STUFF!& set escape back to &
228 Handled specially. If mentioned, declares that this
229 subdir ought to have a target `things'. The rule will be
230 &/things:: $(&TARGETS_things)
232 You may extend it by adding more :: rules for the target,
233 but the preferred style is to do things like this:
234 &TARGETS_check += & test-passed.stamp
236 It is important to mention &TARGETS_things at least once in
237 the context of each applicable directory, because doing so
238 arranges that the *parent* will also have a `things' target
239 which recursively implies this directory's `things'.
241 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
242 for example, is not magic. But mentioning &TARGETS_things in
243 a #-comment *does* work because the & filter does not care
246 `all' is extra special: every directory has an `all'
247 target, which corresponds to &TARGETS.
249 Subdirectory and variable naming
250 --------------------------------
252 The simple variable decoration scheme does not enforce a strict
253 namespace distinction between parts of variable names which come from
254 subdirectory names, and parts that mean something else.
256 So it is a good idea to be a bit careful with your directory naming.
257 `TOP', names that contain `_', and names that are similar to parts of
258 make variables (whether conventional ones, or ones used in your
259 project) are best avoided.
261 If you name your variables in ALL CAPS and your subdirectories in
262 lower case with `-' rather than `_', there will be no confusion.
264 Incorporating this into your project
265 ------------------------------------
267 Use `git-subtree' to merge the subdirmk/ directory. You may find it
268 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
269 symlinks as symlinks - just `git add' the link). And you probably
270 want to mention the situation in your top-level COPYING.
272 Symlink autogen.sh into your project toplevel.
274 In your configure.ac, say
276 m4_include([subdirmk/subdirmk.ac])
277 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
279 Write a Subdir.sd.mk in each directory. The toplevel one should
282 include subdirmk/usual.mk
283 include subdirmk/regen.mk
285 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
288 &:include subdirmk/cdeps.sd.mk
289 &:include subdirmk/clean.sd.mk
294 You can convert your project incrementally. Start with the top-level
295 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
296 stuff to configure.ac, and fix everything up. Leave the existing
297 $(MAKE) -C for your existing subdirectories alone. Then you can
298 convert individual subdirectories, or classes of subdirectories, at
299 your leisure. (You must be /sure/ that each subdirectory will be
300 entered only once at a time, but your existing recursive make descent
301 system should already do that or you already have concurrency bugs.)
303 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
304 This is a frequent source of concurrency bugs in recursive make build
305 systems. When combined with nonrecursive make it's all in the same
306 directory and there is nothing stopping the different invocations
307 ending up trying to make the same targets at the same time. That
308 causes hideous racy lossage. There are ways to get this to work
309 reliably but it is advanced stuff.
311 If you make syntax errors, or certain kinds of other errors, in your
312 makefiles, you may find that just `make' is broken now and cannot get
313 far enough to regenerate a working set of makefiles. If this happens
314 just rerun ./config.status by hand.
321 Copyright 2019 Mark Wooding
322 Copyright 2019 Ian Jackson
324 subdirmk and its example is free software; you can redistribute it
325 and/or modify it under the terms of the GNU Library General Public
326 License as published by the Free Software Foundation; either
327 version 2 of the License, or (at your option) any later version.
329 This is distributed in the hope that it will be useful, but
330 WITHOUT ANY WARRANTY; without even the implied warranty of
331 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
332 Library General Public License for more details.
334 You should have received a copy of the GNU Library General Public
335 License along with this library as the file LGPL-2.
336 If not, see https://www.gnu.org/.
338 Individual files generally contain the following tag in the copyright
339 notice, instead of the full licence grant text:
340 SPDX-License-Identifier: LGPL-2.0-or-later
341 As is conventional, this should be read as a licence grant.
343 Contributions are accepted based on the git commit Signed-off-by
344 convention, by which the contributors' certify their contributions
345 according to the Developer Certificate of Origin version 1.1 - see
346 the file DEVELOPER-CERTIFICATE.
348 Where subdirmk is used by and incorporated into another project (eg
349 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
350 the rest of the project are under that other project's licence(s).
351 (The project's overall licence must be compatible with LGPL-2.0+.)