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. (`/' is legal in make
203 Note that & is processed *even in makefile comments*. The substitutor
204 does not understand make syntax, or shell syntax, at all. However,
205 the substitution rules are chosen to work well with constructs which
206 are common in makefiles.
208 In the notation below, we suppose that the substitution is being in
209 done in a subdirectory sub/dir of the source tree. In the RH column
210 we describe the expansion at the top level, which is often a special
211 case. (Even in variable names: top level's start `./'.)
213 &alpha => sub/dir/alpha or ./alpha
214 Here alpha is any ASCII letter A-Za-z.
215 The assumption is that filenames and variables usually
216 start with a letter. Otherwise, use another syntax:
218 &/ => sub/dir/ or nothing
220 (This implies that `&./' works much like `&/'. &./ is
221 suitable for variable name construction in a way &/ is not.)
223 &^alpha => $(top_srcdir)/sub/dir/alpha
224 &^/ => $(top_srcdir)/sub/dir/
225 &^. => $(top_srcdir)/sub/dir
227 &~alpha => $(top_srcdir)/alpha
228 &~/ => $(top_srcdir)/
232 ^ pathname of this subdirectory in source tree
233 ~ pathname of top level of source tree
234 / terminates the escape (needed if next is not lwsp, alpha or `.')
235 . terminates the escape, gives subdir without /
236 lwsp starts multi-word processing (see below)
238 &_ => ERROR (for compat with v1)
240 So pathname and variable syntax is a subset of:
241 '&' [ '^' | '~' ] [ alpha... | '/' | '.' ]
243 && => && for convenience in shell runes
244 &@ => & general escaping mechanism
245 &&@ => && @ after any number of & vanishes
247 &$VARIABLE $(sub/dir/VARIABLE)
248 VARIABLE is ASCII starting with a letter and matching \w+
254 Convenience syntax for prefixing multiple filenames.
255 Introduced by &... followed by lwsp where alpha could go.
256 Each lwsp-separated non-ws word is prefixed by &/ etc.
257 etc. respectively. No other & escapes are recognised.
258 This processing continues until & preceded by lwsp,
259 or until EOL (the end of the line), or \ then EOL.
261 &:<directive> <args>....
262 recognised at start of line only (possibly after lwsp)
263 args are processed for &
265 &:include filename filename should usually be foo.sd.mk
266 &:-include filename tolerate nonexistent file
267 filenames are relative to $(top_srcdir)
269 &:section number [ident]
270 Sections will be read by make in order, first by number
271 and only then by input file (in order passed
272 to generate ie that specified in configure.ac).
273 Number is \d+; order is lexical.
274 Default section number is `5'.
275 Section numbers may not contain only `0'.
276 Section is local to &:include'd files (reset to `5'
277 on entry, restored on return).
278 Ident is [A-Za-z][A-Z0-9a-z/_-]+ and is currently unused,
279 other than appearing in output. When used, will default
280 to source-relative input file name excluding .sd.mk.
282 &!<lwsp> disables & until EOL (and then disappears)
284 &# delete everything to end of line
285 (useful if the RHS contains unrecognised & constructions)
288 changes the escape sequence from & to literally STUFF
289 STUFF may be any series of of non-whitespace characters,
290 and is terminated by EOL or lwsp. The whole line is
293 After this, write STUFF instead of &, everywhere.
294 The effect is global and lasts until the next setting.
295 It takes effect on &:include'd files too, so maybe set
296 it back before using &:include.
299 STUFFSTUFF => STUFFSTUFF
301 STUFF: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:: $(&TARGETS_things)
309 You may extend it by adding more :: rules for the target,
310 but the preferred style is to do things like this:
311 &TARGETS_check += & test-passed.stamp
313 It is important to mention &TARGETS_things at least once in
314 the context of each applicable directory, because doing so
315 arranges that the *parent* will also have a `things' target
316 which recursively implies this directory's `things'.
318 Must be spelled exactly &TARGETS_things. &/TARGETS_things,
319 for example, is not magic. To make the target exist
320 without providing any prerequisites for it, write a line
321 containing just `&TARGETS_things +='.
323 `all' is extra special: every directory has an `all'
324 target, which corresponds to &TARGETS.
326 DRAFT - MACRO ASSISTANCE FACILITY
327 ---------------------------------
329 Thanks to Mark Wooding.
338 STUFF (with `$'s doubled except before a digit)
340 NAME = $(eval $(call %NAME,$1,$2,$3,$4,$5,$6,$7,$8,$9))
348 $(call NAME,ARG,ARG,...)
352 Subdirectory and variable naming
353 --------------------------------
355 The simple variable decoration scheme does not enforce a strict
356 namespace distinction between parts of variable names which come from
357 subdirectory names, and parts that mean something else.
359 So it is a good idea to be a bit careful with your directory naming.
360 `.', names that contain `/', and names that are similar to parts of
361 make variables (whether conventional ones, or ones used in your
362 project) are best avoided.
364 Incorporating this into your project
365 ------------------------------------
367 Use `git-subtree' to merge the subdirmk/ directory. You may find it
368 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
369 symlinks as symlinks - just `git add' the link). And you probably
370 want to mention the situation in your top-level COPYING.
372 Symlink autogen.sh into your project toplevel.
374 In your configure.ac, say
376 m4_include([subdirmk/subdirmk.ac])
377 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
379 Write a Subdir.sd.mk in each directory. The toplevel one should
382 include subdirmk/usual.mk
383 include subdirmk/regen.mk
385 Write a Perdir.sd.mk in the toplevel, if you want. It should probably
388 &:include subdirmk/cdeps.sd.mk
389 &:include subdirmk/clean.sd.mk
394 You can convert your project incrementally. Start with the top-level
395 Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
396 stuff to configure.ac, and fix everything up. Leave the existing
397 $(MAKE) -C for your existing subdirectories alone. Then you can
398 convert individual subdirectories, or classes of subdirectories, at
399 your leisure. (You must be /sure/ that each subdirectory will be
400 entered only once at a time, but your existing recursive make descent
401 system should already do that or you already have concurrency bugs.)
403 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
404 This is a frequent source of concurrency bugs in recursive make build
405 systems. When combined with nonrecursive make it's all in the same
406 directory and there is nothing stopping the different invocations
407 ending up trying to make the same targets at the same time. That
408 causes hideous racy lossage. There are ways to get this to work
409 reliably but it is advanced stuff.
411 If you make syntax errors, or certain kinds of other errors, in your
412 makefiles, you may find that just `make' is broken now and cannot get
413 far enough to regenerate a working set of makefiles. If this happens
414 just rerun ./config.status by hand.
421 Copyright 2019 Mark Wooding
422 Copyright 2019 Ian Jackson
424 subdirmk and its example is free software; you can redistribute it
425 and/or modify it under the terms of the GNU Library General Public
426 License as published by the Free Software Foundation; either
427 version 2 of the License, or (at your option) any later version.
429 This is distributed in the hope that it will be useful, but
430 WITHOUT ANY WARRANTY; without even the implied warranty of
431 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
432 Library General Public License for more details.
434 You should have received a copy of the GNU Library General Public
435 License along with this library as the file LGPL-2.
436 If not, see https://www.gnu.org/.
438 Individual files generally contain the following tag in the copyright
439 notice, instead of the full licence grant text:
440 SPDX-License-Identifier: LGPL-2.0-or-later
441 As is conventional, this should be read as a licence grant.
443 Contributions are accepted based on the git commit Signed-off-by
444 convention, by which the contributors' certify their contributions
445 according to the Developer Certificate of Origin version 1.1 - see
446 the file DEVELOPER-CERTIFICATE.
448 Where subdirmk is used by and incorporated into another project (eg
449 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
450 the rest of the project are under that other project's licence(s).
451 (The project's overall licence must be compatible with LGPL-2.0+.)