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 ${TOP_VARIABLE}
118 VARIABLE is ASCII starting with a letter and matching \w+
123 Convenience syntax for prefixing multiple filenames.
124 Introduced by & followed by lwsp where lc could go.
125 Each lwsp-separated non-ws word is prefixed by &/ etc.
126 etc. respectively. No other & escapes are recognised.
127 This processing continues until & preceded by lwsp,
128 or until EOL (the end of the line), or \ then EOL.
130 &:<directive> <args>....
131 recognised at start of line only (possibly after lwsp)
134 for make multiple targets syntax
135 recognised anywhere *except* start of line
137 &:include filename filename should usually be [&]foo.sd.mk
138 &:-include filename tolerate nonexistent file
139 RHS is &-expanded but filenames are relative to the top
140 srcdir. This implies that unqualified names are like &~/
141 whereas &/ is like &^/. &^ and &~ do not work here because
142 they expand to constructions involving literally
143 `$(top_srcdir)', but the RHS is not make-expanded.
145 &!<lwsp> disables & until EOL (and then disappears)
147 &# delete everything to end of line
148 (useful if the RHS contains unrecognised & constructions)
151 Handled specially. If mentioned at the start of a line
152 (possibly following whitespace), declares that this
153 subdir ought to have a target `things'. The rule will be
154 &/things:: $(&TARGETS_things)
156 You may extend it by adding more :: rules for the target,
157 but the preferred style is to do things like this:
158 &TARGETS_check += & test-passed.stamp
160 It is important to mention &TARGETS_things at least once in
161 the context of each applicable directory, because doing so
162 arranges that the *parent* will also have a `things' target
163 which recursively implies this directory's `things'.
165 Must be spelled exactly &TARGETS_things. &_TARGETS_things,
166 for example, is not magic. To make the target exist
167 without providing any prerequisites for it, write a line
168 containing just `&TARGETS_things +='.
170 `all' is extra special: every directory has an `all'
171 target, which corresponds to &TARGETS.
176 &:warn [!]WARNTAG ...
177 Suppress (with !) or re-enable (without !) warnings tagged
178 WARNTAG (see section `Warnings', below). The suppression list
179 is reset at the start of processing in each subdirectory.
180 Warnings that appear at the end of processing are controlled
181 by the final warning state after processing all the toplevel
182 input files (including Final.sd.mk).
184 &:local+global [!][&]VARIABLE ...
185 Suppresses any warnings relating to forthcoming mentions
186 to VARIABLE or &VARIABLE, as applicable. Scope ends at
187 the end of the current directory's Suffix.sd.mk.
188 Prefixing with ! removes [&]VARIABLE from the suppresion list.
190 &:changequote NEWQUOTE
191 changes the escape sequence from & to literally NEWQUOTE
192 NEWQUOTE may be any series of of non-whitespace characters,
193 and is terminated by EOL or lwsp. The whole line is
196 After this, write NEWQUOTE instead of &, everywhere.
197 The effect is unscoped and lasts until the next setting,
198 or until the end of the current directory's Suffix.sd.mk.
199 It takes effect on &:include'd files too, so maybe set
200 it back before using &:include.
203 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
204 NEWQUOTE\NEWQUOTE => NEWQUOTE
206 NEWQUOTE:changequote & set escape back to &
209 Dollar doubling and macro assistance
210 ------------------------------------
212 &$+ Starts dollar-doubling
213 &$- Stops dollar-doubling
214 Both are idempotent and local to the file or context.
216 This is useful both for make macrology involving $(eval ...), and
217 possibly for helping write complicated recipes involving shell
218 variables, inline Perl code, etc.
220 Sometimes we will show $'s being doubled inside another construct.
221 This means the content of the construct is $-doubled: $-doubling is
222 locally enabled, and restored afterwards.
224 &:macro NAME => define NAME
225 STUFF $ THINGS .. STUFF $$ THINGS
227 NAME is processed for &
229 &{..$..} => ${eval ${call ..$$..}}
230 (matches { } pairs to find the end)
231 content is $-doubled (unless it contains &$- to turn that off)
232 cf &(...), see "Convenience syntax for eval", below.
234 Together &:macro and &${...} provide a more reasonable macro facility
235 than raw make. They solve the problem that make expansions cannot
236 directly generate multiple rules, variable, etc.; instead, `$(eval )'
237 must be used, but that re-expands the argument, meaning that all the
238 literal text must be $-doubled. This applies to the macro text and to
239 the arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
240 Hence &:macro and &${...}.
242 While dollar-doubling:
243 - - - - - - - - - - -
245 $ => $$ including $'s produced by other
246 &-expansions not mentioned here
250 &$NN => ${NN} where N are digits
252 A few contexts do not support $-doubling, such as directive arguments
253 or places where this might imply $-quadrupling. (There is no way to
256 Convenience syntax for eval
257 - - - - - - - - - - - - - -
259 &(thing => $(call thing
260 &( thing => $(call thing
262 &(&lc => $(call sub_dir_lc or $(call TOP_lc
263 &( &lc => $(call sub_dir_lc or $(call TOP_lc
264 even though lc would normally be thought a filename
266 Unlike &{...}, this does not involve any dollar-doubling.
269 Invocation, "recursive" per-directory targets
270 ---------------------------------------------
272 Arrangements are made so that when you run `make foo' in a
273 subdirectory, it is like running the whole toplevel makefile, from the
274 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
275 be built, that builds it.
277 But `foo' can also be a conventional target like `all'.
279 Each subdirectory has its own `all' target. For example a
280 subdirectory `src' has a target `src/all'. The rules for these are
281 automatically generated from the settings of the per-directory
282 &TARGETS variables. &TARGETS is magic in this way. (In
283 src/Dir.sd.mk, &TARGETS of course refers to a make variable called
286 The `all' target in a parent directory is taken to imply the `all'
287 targets in all of its subdirectories, recursively. And in the
288 autogenerated stub Makefiles, `all' is the default target. So if you
289 just type `make' in the toplevel, you are asking for `&all'
290 (<subdir>/all) for every directory in the project.
292 In a parallel build, the rules for all these various subdirectory
293 targets may be in run in parallel: there is only one `make' invocation
294 at a time. There is no sequencing between subdirectories, only been
295 individual targets (as specified according to their dependencies).
297 You can define other per-directory recursive targets too: set the
298 variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
299 at the start of a line). This will create a src/zonk target (for
300 appropriate value of src/). Unlike `all', these other targets only
301 exist in areas of the project where at least something mentions them.
302 So for example, if &TARGETS_zonk is set in src but not lib, `make
303 zonk' in lib will fail. If you want to make a target exist
304 everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
307 Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
308 --------------------------------------------------
310 The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
311 are automatically processed before and after each individual
312 directory's Dir.sd.mk, and the &-substituted contents therefore
313 appear once for each subdirectory.
315 This lets you do per-directory boilerplate. Some useful boilerplate
316 is already provided in subdirmk, for you to reference like this:
317 &:include subdirmk/cdeps.sd.mk
318 &:include subdirmk/clean.sd.mk
319 For example you could put that in Suffix.sd.mk.
321 The top-level Dir.sd.mk is the first makefile included after the
322 autogenerated `main.mk' which merely has some basic settings and
323 includes. So if you want to get in early and set global variables,
324 put them near the top of Dir.sd.mk.
326 The file Final.sd.mk in the toplevel directory is processed and
327 the result included after all the other files. Its subdirmk
328 filtering context inherits warning suppressions from the toplevel's
329 Dir.sd.mk etc., but not anything else.
331 subdirmk's filter script itself sets (only) these variables:
336 You are likely to want to define $(PWD), and shorter names for
337 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
342 subdirmk's `generate' program, which does the acual &-substitution,
343 can produce some warnings about your .sd.mk files. These can be
344 suppressed with the &:warn directive. The warning tags are:
347 The same VARNAME was used both with and without an & prefix.
348 This can be confusing. Also, if you avoid this then you will
349 get a warning iff you accidentally leave off a needed &.
350 The generation of this warning depends on scanning your
351 makefile for things that look like variable references, which
352 subdirmk does not do completely perfectly. Exciting make
353 syntax may evade this warning, or require suppressions.
354 (You can suppress this warning for a particular VARNAME with
355 the &:local+global directive.)
358 A variable expansion like $FBAR. make's expansion rules
359 interpret this as $(F)BAR. It's normally better to write
360 it this way, at least if the variable expansion is followed
361 by more letters. Note that &$FOO works differently to
362 raw make: it expands to ${sub_dir_FOO}.
365 An attempt at variable expansion looking like $&...
366 You probably expected this to mean $(TOP_F)BAR but it
367 expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.
370 &:warn was used to try to enable a warning that this version
371 of subdirmk does not understand. (Note that an attempt to
372 *dis*able an unknown warning is only reported if some other
373 warning was issued which might have been disabled.)
376 Guides, hints, and further explanations
377 =======================================
379 Incorporating this into your project
380 ------------------------------------
382 Use `git-subtree' to merge the subdirmk/ directory. You may find it
383 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
384 symlinks as symlinks - just `git add' the link). And you probably
385 want to mention the situation in your top-level COPYING and HACKING.
387 Symlink autogen.sh into your project toplevel.
389 In your configure.ac, say
391 m4_include([subdirmk/subdirmk.ac])
392 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
394 Write a Dir.sd.mk in each directory. See the substitution syntax
395 reference, above, and the example/ directory here. The toplevel
396 Dir.sd.mk should probably contain:
398 include subdirmk/usual.mk
399 include subdirmk/regen.mk
401 Write a Suffix.sd.mk in the toplevel, if you want. It should probably
404 &:include subdirmk/cdeps.sd.mk
405 &:include subdirmk/clean.sd.mk
411 You can convert your project incrementally. Start with the top-level
412 Makefile.in and rename it to Dir.sd.mk, and add the appropriate
413 stuff to configure.ac, and fix everything up. Leave the existing
414 $(MAKE) -C for your existing subdirectories alone. Then you can
415 convert individual subdirectories, or classes of subdirectories, at
416 your leisure. (You must be /sure/ that each recursive (non-subdirmk)
417 subdirectory will be entered only once at a time, but your existing
418 recursive make descent system should already do that or you already
419 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 Dir.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.
445 If want to set global variables, such as CC, that should only be done
446 once. You can put them in your top-level Dir.sd.mk, or a separate
447 file you `include' and declare using SUBDIRMK_MAKEFILES.
449 If you need different settings of variables like CC for different
450 subdirectories, you should probably do that with target-specific
451 variable settings. See the info node `(make) Target-specific'.
454 Directory templates `.sd.mk' vs plain autoconf templates `.mk.in'
455 --------------------------------------------------------------------
457 There are two kinds of template files.
459 Filename .sd.mk .mk.in
461 Processed by &-substitution, autoconf only
464 Instantiated Usu. once per subdir Once only
466 Need to be mentioned No, but Dir.sd.mk All not in subdirmk/
467 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
469 How to include `&:include foo.sd.mk' `include foo.mk'
470 in all relevant .sd.mk in only one
471 (but not needed for Dir.sd.mk
472 Prefix, Suffix, Final)
474 If you `include subdirmk/regen.mk', dependency management and
475 automatic regeneration for all of this template substitution, and for
476 config.status etc. is done for you.
479 Tables of file reference syntaxes
480 ---------------------------------
482 In a nonrecursive makefile supporting out of tree builds there are
483 three separate important distinctions between different file
486 (i) In the build tree, or in the source tree ?
488 (ii) In (or relative to) the subdirectory to which this Dir.sd.mk
489 relates, or relative to the project's top level ?
491 (iii) Absolute or relative pathname ? Usually relative pathnames
492 suffice. Where an absolute pathname is needed, it can be built
493 out of &/ and an appropriate make variable such as $(PWD).
495 Path construction &-expansions are built from the following:
503 In more detail, with all the various options laid out:
505 Recommended Relative paths in... Absolute paths in...
506 for build source build source
508 This lc &file &^file $(PWD)/&file $(abs_src)/&file
509 directory any &/file &^/file $(PWD)/&/file $(abs_src)/&/file
510 several & f g h &^ f g h $(addprefix...)
513 level any file &~/file $(PWD)/file $(abs_src)/file
514 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
515 several f g h &~ f g h $(addprefix...)
517 (This assumes you have appropriate make variables src, PWD and
521 Subdirectory and variable naming
522 --------------------------------
524 The simple variable decoration scheme does not enforce a strict
525 namespace distinction between parts of variable names which come from
526 subdirectory names, and parts that mean something else.
528 So it is a good idea to be a bit careful with your directory naming.
529 `TOP', names that contain `_', and names that are similar to parts of
530 make variables (whether conventional ones, or ones used in your
531 project) are best avoided.
533 If you name your variables in ALL CAPS and your subdirectories in
534 lower case with `-' rather than `_', there will be no confusion.
541 Copyright 2019 Mark Wooding
542 Copyright 2019 Ian Jackson
544 subdirmk and its example is free software; you can redistribute it
545 and/or modify it under the terms of the GNU Library General Public
546 License as published by the Free Software Foundation; either
547 version 2 of the License, or (at your option) any later version.
549 This is distributed in the hope that it will be useful, but
550 WITHOUT ANY WARRANTY; without even the implied warranty of
551 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
552 Library General Public License for more details.
554 You should have received a copy of the GNU Library General Public
555 License along with this library as the file LGPL-2.
556 If not, see https://www.gnu.org/.
558 Individual files generally contain the following tag in the copyright
559 notice, instead of the full licence grant text:
560 SPDX-License-Identifier: LGPL-2.0-or-later
561 As is conventional, this should be read as a licence grant.
563 Contributions are accepted based on the git commit Signed-off-by
564 convention, by which the contributors' certify their contributions
565 according to the Developer Certificate of Origin version 1.1 - see
566 the file DEVELOPER-CERTIFICATE.
568 Where subdirmk is used by and incorporated into another project (eg
569 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
570 the rest of the project are under that other project's licence(s).
571 (The project's overall licence must be compatible with LGPL-2.0+.)