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
23 builds well, and helps a bit with metaprogramming and rule writing).
28 The developer is expected to write a makefile fragment, in each
29 relevant subdirectory, called `Dir.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 Dir.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 Dir.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 Dir.sd.mk
56 can simply refer to files in other subdirectories directly.
61 In general & expands to the subdirectory name when used for a
62 filename, and to the subdirectory name with / replaced with _ for
63 variable names. (If your variables start with capital letters and
64 your filenames with lowercase. Otherwise, use &/ or &_.)
66 Note that & is processed *even in makefile comments*. The substitutor
67 does not understand make syntax, or shell syntax, at all. However,
68 the substitution rules are chosen to work well with constructs which
69 are common in makefiles.
71 In the notation below, we suppose that the substitution is being in
72 done in a subdirectory sub/dir of the source tree. In the RH column
73 we describe the expansion at the top level, which is often a special
74 case (in general in variable names we call that TOP rather than the
77 &CAPS => sub_dir_CAPS or TOP_CAPS
78 &lc => sub/dir/lc or lc
79 Here CAPS is any ASCII letter A-Z and lc is a-z.
80 The assumption is that filenames are usually lowercase and
81 variables usually uppercase. Otherwise, use another syntax:
83 &/ => sub/dir/ or nothing
84 &_ => sub_dir_ or TOP_
86 (This implies that `&./' works roughly like `&/', although
87 it can produce a needless `./')
91 &^lc => $(top_srcdir)/sub/dir/lc
92 &^/ => $(top_srcdir)/sub/dir/
93 &^. => $(top_srcdir)/sub/dir
95 &~lc => $(top_srcdir)/lc
100 ^ pathname of this subdirectory in source tree
101 ~ pathname of top level of source tree
102 / terminates the path escape } needed if next is
103 _ terminates the var escape } not letter or space)
104 . terminates path escape giving dir name (excluding /)
105 = terminates var escape giving only prefix part (rarely needed)
106 lwsp starts multi-word processing (see below)
108 So pathname syntax is a subset of:
109 '&' [ '^' | '~' ] [ lc | '/' | '.' ]
111 && => && for convenience in shell runes
113 &\& => & general escaping mechanism
114 &\$ => $ provided for $-doubling regimes
115 &\NEWLINE eats the newline and vanishes
117 &$VARIABLE => $(sub_dir_VARIABLE) or $(TOP_VARIABLE)
118 VARIABLE is ASCII starting with a letter and matching \w+
123 Convenience syntax for prefixing multiple filenames.
124 Introduced by & followed by lwsp where lc could go.
125 Each lwsp-separated non-ws word is prefixed by &/ etc.
126 etc. respectively. No other & escapes are recognised.
127 This processing continues until & preceded by lwsp,
128 or until EOL (the end of the line), or \ then EOL.
130 &:<directive> <args>....
131 recognised at start of line only (possibly after lwsp)
133 &:include filename filename should usually be [&]foo.sd.mk
134 &:-include filename tolerate nonexistent file
135 RHS is &-expanded but filenames are relative to the top
136 srcdir. This implies that unqualified names are like &~/
137 whereas &/ is like &^/. &^ and &~ do not work here because
138 they expand to constructions involving literally
139 `$(top_srcdir)', but the RHS is not make-expanded.
141 &!<lwsp> disables & until EOL (and then disappears)
143 &# delete everything to end of line
144 (useful if the RHS contains unrecognised & constructions)
147 Handled specially. If mentioned at the start of a line
148 (possibly following whitespace), declares that this
149 subdir ought to have a target `things'. The rule will be
150 &/things:: $(&TARGETS_things)
152 You may extend it by adding more :: rules for the target,
153 but the preferred style is to do things like this:
154 &TARGETS_check += & test-passed.stamp
156 It is important to mention &TARGETS_things at least once in
157 the context of each applicable directory, because doing so
158 arranges that the *parent* will also have a `things' target
159 which recursively implies this directory's `things'.
161 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
162 for example, is not magic. To make the target exist
163 without providing any prerequisites for it, write a line
164 containing just `&TARGETS_things +='.
166 `all' is extra special: every directory has an `all'
167 target, which corresponds to &TARGETS.
169 &:warn [!]WARNTAG ...
170 Suppress (with !) or re-enable (without !) warnings tagged
171 WARNTAG (see section `Warnings', below). The suppression list
172 is reset at the start of processing in each subdirectory.
173 Warnings that appear at the end of processing are controlled
174 by the final warning state after processing all the toplevel
175 input files (including Final.sd.mk).
177 &:local+global [&]VARIABLE ...
178 Suppresses the warning about seeing both VARIABLE and
179 &VARIABLE. Any & specified in the RHS is redundant: this
180 always affects both versions identically.
182 &:changequote NEWQUOTE
183 changes the escape sequence from & to literally NEWQUOTE
184 NEWQUOTE may be any series of of non-whitespace characters,
185 and is terminated by EOL or lwsp. The whole line is
188 After this, write NEWQUOTE instead of &, everywhere.
189 The effect is unscoped and lasts until the next setting,
190 or until the end of the current directory's Suffix.sd.mk.
191 It takes effect on &:include'd files too, so maybe set
192 it back before using &:include.
195 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
196 NEWQUOTE\NEWQUOTE => NEWQUOTE
198 NEWQUOTE:changequote & set escape back to &
201 Dollar doubling and macro assistance
202 ------------------------------------
204 &$+ Starts dollar-doubling
205 &$- Stops dollar-doubling
206 Both are idempotent and local to the file or context.
208 Sometimes we will show $'s being doubled inside another construct.
209 This means the content of the construct is $-doubled: $-doubling is
210 locally enabled, and restored afterwards.
212 &:macro NAME => define NAME
213 STUFF $ THINGS .. STUFF $$ THINGS
215 NAME is processed for &
217 &${..$..} => ${eval ${call ..$$..}}
218 (matches { } pairs to find the end)
219 content is $-doubled (unless it contains &$- to turn that off)
221 Together &:macro and &${...} provide a more reasonable macro
222 facility than raw make. They solve the problem that make
223 expansions cannot directly generate multiple rules, variable,
224 etc.; instead, `$(eval )' must be used, but that re-expands
225 the argument, meaning that all the literal text must be
226 $-doubled. This applies to the macro text and to the
227 arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
228 Hence &:macro and &${...}.
230 While dollar-doubling:
231 - - - - - - - - - - -
233 $ => $$ including $'s produced by other
234 &-expansions not mentioned here
237 &$NN => $(NN) where N are digits
240 A few contexts do not support $-doubling, such as directive arguments
241 or places where this might imply $-quadrupling. (There is no way to
245 Invocation, "recursive" per-directory targets
246 ---------------------------------------------
248 Arrangements are made so that when you run `make foo' in a
249 subdirectory, it is like running the whole toplevel makefile, from the
250 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
251 be built, that builds it.
253 But `foo' can also be a conventional target like `all'.
255 Each subdirectory has its own `all' target. For example a
256 subdirectory `src' has a target `src/all'. The rules for these are
257 automatically generated from the settings of the per-directory
258 &TARGETS variables. &TARGETS is magic in this way. (In
259 src/Dir.sd.mk, &TARGETS of course refers to a make variable called
262 The `all' target in a parent directory is taken to imply the `all'
263 targets in all of its subdirectories, recursively. And in the
264 autogenerated stub Makefiles, `all' is the default target. So if you
265 just type `make' in the toplevel, you are asking for `&all'
266 (<subdir>/all) for every directory in the project.
268 In a parallel build, the rules for all these various subdirectory
269 targets may be in run in parallel: there is only one `make' invocation
270 at a time. There is no sequencing between subdirectories, only been
271 individual targets (as specified according to their dependencies).
273 You can define other per-directory recursive targets too: set the
274 variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
275 at the start of a line). This will create a src/zonk target (for
276 appropriate value of src/). Unlike `all', these other targets only
277 exist in areas of the project where at least something mentions them.
278 So for example, if &TARGETS_zonk is set in src but not lib, `make
279 zonk' in lib will fail. If you want to make a target exist
280 everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
283 Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
284 --------------------------------------------------
286 The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
287 are automatically processed before and after each individual
288 directory's Dir.sd.mk, and the &-substituted contents therefore
289 appear once for each subdirectory.
291 This lets you do per-directory boilerplate. Some useful boilerplate
292 is already provided in subdirmk, for you to reference like this:
293 &:include subdirmk/cdeps.sd.mk
294 &:include subdirmk/clean.sd.mk
295 For example you could put that in Suffix.sd.mk.
297 The top-level Dir.sd.mk is the first makefile included after the
298 autogenerated `main.mk' which merely has some basic settings and
299 includes. So if you want to get in early and set global variables,
300 put them near the top of Dir.sd.mk.
302 The file Final.sd.mk in the toplevel directory is processed and
303 the result included after all the other files. Its subdirmk
304 filtering context inherits warning suppressions from the toplevel's
305 Dir.sd.mk etc., but not anything else.
307 subdirmk's filter script itself sets (only) these variables:
312 You are likely to want to define $(PWD), and shorter names for
313 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
318 If want to set global variables, such as CC, that should only be done
319 once. You can put them in your top-level Dir.sd.mk, or a separate
320 file you `include' and declare using SUBDIRMK_MAKEFILES.
322 If you need different settings of variables like CC for different
323 subdirectories, you should probably do that with target-specific
324 variable settings. See the info node `(make) Target-specific'.
326 Directory templates `.sd.mk' vs plain autoconf templates `.mk.in'
327 --------------------------------------------------------------------
329 There are two kinds of template files.
331 Filename .sd.mk .mk.in
333 Processed by &-substitution, autoconf only
336 Instantiated Usu. once per subdir Once only
338 Need to be mentioned No, but Dir.sd.mk All not in subdirmk/
339 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
341 How to include `&:include foo.sd.mk' `include foo.mk'
342 in all relevant .sd.mk in only one
343 (but not needed for Dir.sd.mk
344 Prefix, Suffix, Final)
346 If you `include subdirmk/regen.mk', dependency management and
347 automatic regeneration for all of this template substitution, and for
348 config.status etc. is done for you.
351 Tables of file reference syntaxes
352 ---------------------------------
354 In a nonrecursive makefile supporting out of tree builds there are
355 three separate important distinctions between different file
358 (i) In the build tree, or in the source tree ?
360 (ii) In (or relative to) the subdirectory to which this Dir.sd.mk
361 relates, or relative to the project's top level ?
363 (iii) Absolute or relative pathname ? Usually relative pathnames
364 suffice. Where an absolute pathname is needed, it can be built
365 out of &/ and an appropriate make variable such as $(PWD).
367 Path construction &-expansions are built from the following:
375 In more detail, with all the various options laid out:
377 Recommended Relative paths in... Absolute paths in...
378 for build source build source
380 This lc &file &^file $(PWD)/&file $(abs_src)/&file
381 directory any &/file &^/file $(PWD)/&/file $(abs_src)/&/file
382 several & f g h &^ f g h $(addprefix...)
385 level any file &~/file $(PWD)/file $(abs_src)/file
386 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
387 several f g h &~ f g h $(addprefix...)
389 (This assumes you have appropriate make variables src, PWD and
395 subdirmk's `generate' program, which does the acual &-substitution,
396 can produce some warnings about your .sd.mk files. These can be
397 suppressed with the &:warn directive. The warning tags are:
400 The same VARNAME was used both with and without an & prefix.
401 This can be confusing. Also, if you avoid this then you will
402 get a warning iff you accidentally leave off a needed &.
403 The generation of this warning depends on scanning your
404 makefile for things that look like variable references, which
405 subdirmk does not do completely perfectly. Exciting make
406 syntax may evade this warning, or require suppressions.
407 (You can suppress this warning for a particular VARNAME with
408 the &:local+global directive.)
411 A variable expansion like $FBAR. make's expansion rules
412 interpret this as $(F)BAR. It's normally better to write
413 it this way, at least if the variable expansion is followed
414 by more letters. Note that &$FOO works differently to
415 raw make: it expands to $(sub_dir_FOO).
418 &:warn was used to try to enable a warning that this version
419 of subdirmk does not understand. (Note that an attempt to
420 *dis*able an unknown warning is only reported if some other
421 warning was issued which might have been disabled.)
423 Subdirectory and variable naming
424 --------------------------------
426 The simple variable decoration scheme does not enforce a strict
427 namespace distinction between parts of variable names which come from
428 subdirectory names, and parts that mean something else.
430 So it is a good idea to be a bit careful with your directory naming.
431 `TOP', names that contain `_', and names that are similar to parts of
432 make variables (whether conventional ones, or ones used in your
433 project) are best avoided.
435 If you name your variables in ALL CAPS and your subdirectories in
436 lower case with `-' rather than `_', there will be no confusion.
438 Incorporating this into your project
439 ------------------------------------
441 Use `git-subtree' to merge the subdirmk/ directory. You may find it
442 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
443 symlinks as symlinks - just `git add' the link). And you probably
444 want to mention the situation in your top-level COPYING and HACKING.
446 Symlink autogen.sh into your project toplevel.
448 In your configure.ac, say
450 m4_include([subdirmk/subdirmk.ac])
451 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
453 Write a Dir.sd.mk in each directory. The toplevel one should
456 include subdirmk/usual.mk
457 include subdirmk/regen.mk
459 Write a Suffix.sd.mk in the toplevel, if you want. It should probably
462 &:include subdirmk/cdeps.sd.mk
463 &:include subdirmk/clean.sd.mk
468 You can convert your project incrementally. Start with the top-level
469 Makefile.in and rename it to Dir.sd.mk, and add the appropriate
470 stuff to configure.ac, and fix everything up. Leave the existing
471 $(MAKE) -C for your existing subdirectories alone. Then you can
472 convert individual subdirectories, or classes of subdirectories, at
473 your leisure. (You must be /sure/ that each recursive (non-subdirmk)
474 subdirectory will be entered only once at a time, but your existing
475 recursive make descent system should already do that or you already
476 have concurrency bugs.)
478 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
479 This is a frequent source of concurrency bugs in recursive make build
480 systems. When combined with nonrecursive make it's all in the same
481 directory and there is nothing stopping the different invocations
482 ending up trying to make the same targets at the same time. That
483 causes hideous racy lossage. There are ways to get this to work
484 reliably but it is advanced stuff.
486 If you make syntax errors, or certain kinds of other errors, in your
487 makefiles, you may find that just `make' is broken now and cannot get
488 far enough to regenerate a working set of makefiles. If this happens
489 just rerun ./config.status by hand.
491 If you go back and forth between different versions of your code you
492 can sometimes find that `make' complains that one of your Subdir.sd.mk
493 files is missing: typically, if iot was used and therefore a
494 dependency in some other version of your code. If you run `make
495 clean' (or `make realclean') these dependencies are suppressed, which
496 will clear up the problem.
503 Copyright 2019 Mark Wooding
504 Copyright 2019 Ian Jackson
506 subdirmk and its example is free software; you can redistribute it
507 and/or modify it under the terms of the GNU Library General Public
508 License as published by the Free Software Foundation; either
509 version 2 of the License, or (at your option) any later version.
511 This is distributed in the hope that it will be useful, but
512 WITHOUT ANY WARRANTY; without even the implied warranty of
513 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
514 Library General Public License for more details.
516 You should have received a copy of the GNU Library General Public
517 License along with this library as the file LGPL-2.
518 If not, see https://www.gnu.org/.
520 Individual files generally contain the following tag in the copyright
521 notice, instead of the full licence grant text:
522 SPDX-License-Identifier: LGPL-2.0-or-later
523 As is conventional, this should be read as a licence grant.
525 Contributions are accepted based on the git commit Signed-off-by
526 convention, by which the contributors' certify their contributions
527 according to the Developer Certificate of Origin version 1.1 - see
528 the file DEVELOPER-CERTIFICATE.
530 Where subdirmk is used by and incorporated into another project (eg
531 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
532 the rest of the project are under that other project's licence(s).
533 (The project's overall licence must be compatible with LGPL-2.0+.)