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: set the
87 variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
88 at the start of a line). This will create a src/zonk target (for
89 appropriate value of src/). Unlike `all', these other targets only
90 exist in areas of the project where at least something mentions them.
91 So for example, if &TARGETS_zonk is set in src but not lib, `make
92 zonk' in lib will fail. If you want to make a target exist
93 everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
96 Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
97 --------------------------------------------------
99 The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
100 are automatically processed before and after each individual
101 directory's Subdir.sd.mk, and the &-substituted contents therefore
102 appear once for each subdirectory.
104 This lets you do per-directory boilerplate. Some useful boilerplate
105 is already provided in subdirmk, for you to reference like this:
106 &:include subdirmk/cdeps.sd.mk
107 &:include subdirmk/clean.sd.mk
108 For example you could put that in Suffix.sd.mk.
110 The top-level Subdir.sd.mk is the first makefile included after the
111 autogenerated `main.mk' which merely has some basic settings and
112 includes. So if you want to get in early and set global variables,
113 put them near the top of Subdir.sd.mk.
115 The file Final.sd.mk in the toplevel directory is processed and
116 included after all the other files.
118 subdirmk's filter script itself sets (only) these variables:
123 You are likely to want to define $(PWD), and shorter names for
124 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
129 If want to set global variables, such as CC, that should only be done
130 once. You can put them in your top-level Subdir.sd.mk, or a separate
131 file you `include' and declare using SUBDIRMK_MAKEFILES.
133 If you need different settings of variables like CC for different
134 subdirectories, you should probably do that with target-specific
135 variable settings. See the info node `(make) Target-specific'.
137 Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in'
138 --------------------------------------------------------------------
140 There are two kinds of template files.
142 Filename .sd.mk .mk.in
144 Processed by &-substitution, autoconf only
147 Instantiated Usu. once per subdir Once only
149 Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/
150 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
152 How to include `&:include foo.sd.mk' `include foo.mk'
153 in all relevant .sd.mk in only one
154 (but not needed for Subdir.sd.mk
155 Prefix, Suffix, Final)
157 If you `include subdirmk/regen.mk', dependency management and
158 automatic regeneration for all of this template substitution, and for
159 config.status etc. is done for you.
161 Tables of file reference syntaxes
162 ---------------------------------
164 In a nonrecursive makefile supporting out of tree builds there are
165 three separate important distinctions between different file
168 (i) In the build tree, or in the source tree ?
170 (ii) In (or relative to) the subdirectory to which this Subdir.sd.mk
171 relates, or relative to the project's top level ?
173 (iii) Absolute or relative pathname ? Usually relative pathnames
174 suffice. Where an absolute pathname is needed, it can be built
175 out of &/ and an appropriate make variable such as $(PWD).
177 Path construction &-expansions are built from the following:
185 In more detail, with all the various options laid out:
187 Recommended Relative paths in... Absolute paths in...
188 for build source build source
190 This lc &file &^file $(PWD)/&file $(abs_src)/&file
191 directory any &/file &^/file $(PWD)/&/file $(abs_src)/&/file
192 several & f g h &^ f g h $(addprefix...)
195 level any file &~/file $(PWD)/file $(abs_src)/file
196 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
197 several f g h &~ f g h $(addprefix...)
199 (This assumes you have appropriate make variables src, PWD and
205 In general & expands to the subdirectory name when used for a
206 filename, and to the subdirectory name with / replaced with _ for
209 Note that & is processed *even in makefile comments*. The substitutor
210 does not understand make syntax, or shell syntax, at all. However,
211 the substitution rules are chosen to work well with constructs which
212 are common in makefiles.
214 In the notation below, we suppose that the substitution is being in
215 done in a subdirectory sub/dir of the source tree. In the RH column
216 we describe the expansion at the top level, which is often a special
217 case (in general in variable names we call that TOP rather than the
220 &CAPS => sub_dir_CAPS or TOP_CAPS
221 &lc => sub/dir/lc or lc
222 Here CAPS is any ASCII letter A-Z and lc is a-z.
223 The assumption is that filenames are usually lowercase and
224 variables usually uppercase. Otherwise, use another syntax:
226 &/ => sub/dir/ or nothing
227 &_ => sub_dir_ or TOP_
229 (This implies that `&./' works roughly like `&/', although
230 it can produce a needless `./')
234 &^lc => $(top_srcdir)/sub/dir/lc
235 &^/ => $(top_srcdir)/sub/dir/
236 &^. => $(top_srcdir)/sub/dir
238 &~lc => $(top_srcdir)/lc
239 &~/ => $(top_srcdir)/
243 ^ pathname of this subdirectory in source tree
244 ~ pathname of top level of source tree
245 / terminates the path escape } needed if next is
246 _ terminates the var escape } not lwsp or space)
247 . terminates path escape giving dir name (excluding /)
248 = terminates var escape giving only prefix part (rarely needed)
249 lwsp starts multi-word processing (see below)
251 So pathname syntax is a subset of:
252 '&' [ '^' | '~' ] [ lc | '/' | '.' | '=' ]
254 && => && for convenience in shell runes
256 &\& => & general escaping mechanism
257 &\$ => $ provided for $-doubling regimes
258 &\NEWLINE eats the newline and vanishes
260 &$VARIABLE => $(sub_dir_VARIABLE) or $(TOP_VARIABLE)
261 VARIABLE is ASCII starting with a letter and matching \w+
266 Convenience syntax for prefixing multiple filenames.
267 Introduced by & followed by lwsp where lc could go.
268 Each lwsp-separated non-ws word is prefixed by &/ etc.
269 etc. respectively. No other & escapes are recognised.
270 This processing continues until & preceded by lwsp,
271 or until EOL (the end of the line), or \ then EOL.
273 &:<directive> <args>....
274 recognised at start of line only (possibly after lwsp)
276 &:include filename filename should usually be [&]foo.sd.mk
277 &:-include filename tolerate nonexistent file
278 filenames are relative to $(top_srcdir)
281 &!<lwsp> disables & until EOL (and then disappears)
283 &# delete everything to end of line
284 (useful if the RHS contains unrecognised & constructions)
286 &:changequote NEWQUOTE
287 changes the escape sequence from & to literally NEWQUOTE
288 NEWQUOTE may be any series of of non-whitespace characters,
289 and is terminated by EOL or lwsp. The whole line is
292 After this, write NEWQUOTE instead of &, everywhere.
293 The effect is global and lasts until the next setting.
294 It takes effect on &:include'd files too, so maybe set
295 it back before using &:include.
298 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
299 NEWQUOTE\NEWQUOTE => NEWQUOTE
301 NEWQUOTE:changequote & set escape back to &
304 Handled specially. If mentioned at the start of a line
305 (possibly following whitespace), declares that this
306 subdir ought to have a target `things'. The rule will be
307 &/things:: $(&TARGETS_things)
309 You may extend it by adding more :: rules for the target,
310 but the preferred style is to do things like this:
311 &TARGETS_check += & test-passed.stamp
313 It is important to mention &TARGETS_things at least once in
314 the context of each applicable directory, because doing so
315 arranges that the *parent* will also have a `things' target
316 which recursively implies this directory's `things'.
318 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
319 for example, is not magic. To make the target exist
320 without providing any prerequisites for it, write a line
321 containing just `&TARGETS_things +='.
323 `all' is extra special: every directory has an `all'
324 target, which corresponds to &TARGETS.
327 Dollar doubling and macro assistance
328 ------------------------------------
330 &$+ Starts dollar-doubling
331 &$- Stops dollar-doubling
332 Both are idempotent and local to the file or context.
334 Sometimes we will show $'s being doubled inside another construct.
335 This means the content of the construct is $-doubled: $-doubling is
336 locally enabled, and restored afterwards.
338 &:macro NAME => define NAME
339 STUFF $ THINGS .. STUFF $$ THINGS
341 NAME is processed for &
343 &${..$..} => ${eval ${call ..$$..}}
344 (matches { } pairs to find the end)
345 content is $-doubled (unless it contains $- to turn that off)
347 Together &:macro and &${...} provide a more reasonable macro
348 facility than raw make. They solve the problem that make
349 expansions cannot directly generate multiple rules, variable,
350 etc.; instead, `$(eval )' must be used, but that re-expands
351 the argument, meaning that all the literal text must be
352 $-doubled. This applies to the macro text and to the
353 arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
354 Hence &:macro and &${...}.
356 While dollar-doubling:
357 - - - - - - - - - - -
359 $ => $$ including $'s produced by other
360 &-expansions not mentioned here
363 &$NN => $(NN) where N are digits
367 Subdirectory and variable naming
368 --------------------------------
370 The simple variable decoration scheme does not enforce a strict
371 namespace distinction between parts of variable names which come from
372 subdirectory names, and parts that mean something else.
374 So it is a good idea to be a bit careful with your directory naming.
375 `TOP', names that contain `_', and names that are similar to parts of
376 make variables (whether conventional ones, or ones used in your
377 project) are best avoided.
379 If you name your variables in ALL CAPS and your subdirectories in
380 lower case with `-' rather than `_', there will be no confusion.
382 Incorporating this into your project
383 ------------------------------------
385 Use `git-subtree' to merge the subdirmk/ directory. You may find it
386 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
387 symlinks as symlinks - just `git add' the link). And you probably
388 want to mention the situation in your top-level COPYING.
390 Symlink autogen.sh into your project toplevel.
392 In your configure.ac, say
394 m4_include([subdirmk/subdirmk.ac])
395 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
397 Write a Subdir.sd.mk in each directory. The toplevel one should
400 include subdirmk/usual.mk
401 include subdirmk/regen.mk
403 Write a Suffix.sd.mk in the toplevel, if you want. It should probably
406 &:include subdirmk/cdeps.sd.mk
407 &:include subdirmk/clean.sd.mk
412 You can convert your project incrementally. Start with the top-level
413 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
414 stuff to configure.ac, and fix everything up. Leave the existing
415 $(MAKE) -C for your existing subdirectories alone. Then you can
416 convert individual subdirectories, or classes of subdirectories, at
417 your leisure. (You must be /sure/ that each subdirectory will be
418 entered only once at a time, but your existing recursive make descent
419 system should already do that or you already have concurrency bugs.)
421 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
422 This is a frequent source of concurrency bugs in recursive make build
423 systems. When combined with nonrecursive make it's all in the same
424 directory and there is nothing stopping the different invocations
425 ending up trying to make the same targets at the same time. That
426 causes hideous racy lossage. There are ways to get this to work
427 reliably but it is advanced stuff.
429 If you make syntax errors, or certain kinds of other errors, in your
430 makefiles, you may find that just `make' is broken now and cannot get
431 far enough to regenerate a working set of makefiles. If this happens
432 just rerun ./config.status by hand.
434 If you go back and forth between different versions of your code you
435 can sometimes find that `make' complains that one of your Subdir.sd.mk
436 files is missing: typically, if iot was used and therefore a
437 dependency in some other version of your code. If you run `make
438 clean' (or `make realclean') these dependencies are suppressed, which
439 will clear up the problem.
446 Copyright 2019 Mark Wooding
447 Copyright 2019 Ian Jackson
449 subdirmk and its example is free software; you can redistribute it
450 and/or modify it under the terms of the GNU Library General Public
451 License as published by the Free Software Foundation; either
452 version 2 of the License, or (at your option) any later version.
454 This is distributed in the hope that it will be useful, but
455 WITHOUT ANY WARRANTY; without even the implied warranty of
456 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
457 Library General Public License for more details.
459 You should have received a copy of the GNU Library General Public
460 License along with this library as the file LGPL-2.
461 If not, see https://www.gnu.org/.
463 Individual files generally contain the following tag in the copyright
464 notice, instead of the full licence grant text:
465 SPDX-License-Identifier: LGPL-2.0-or-later
466 As is conventional, this should be read as a licence grant.
468 Contributions are accepted based on the git commit Signed-off-by
469 convention, by which the contributors' certify their contributions
470 according to the Developer Certificate of Origin version 1.1 - see
471 the file DEVELOPER-CERTIFICATE.
473 Where subdirmk is used by and incorporated into another project (eg
474 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
475 the rest of the project are under that other project's licence(s).
476 (The project's overall licence must be compatible with LGPL-2.0+.)