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.
172 &:warn [!]WARNTAG ...
173 Suppress (with !) or re-enable (without !) warnings tagged
174 WARNTAG (see section `Warnings', below). The suppression list
175 is reset at the start of processing in each subdirectory.
176 Warnings that appear at the end of processing are controlled
177 by the final warning state after processing all the toplevel
178 input files (including Final.sd.mk).
180 &:local+global [!][&]VARIABLE ...
181 Suppresses any warnings relating to forthcoming mentions
182 to VARIABLE or &VARIABLE, as applicable. Scope ends at
183 the end of the current directory's Suffix.sd.mk.
184 Prefixing with ! removes [&]VARIABLE from the suppresion list.
186 &:changequote NEWQUOTE
187 changes the escape sequence from & to literally NEWQUOTE
188 NEWQUOTE may be any series of of non-whitespace characters,
189 and is terminated by EOL or lwsp. The whole line is
192 After this, write NEWQUOTE instead of &, everywhere.
193 The effect is unscoped and lasts until the next setting,
194 or until the end of the current directory's Suffix.sd.mk.
195 It takes effect on &:include'd files too, so maybe set
196 it back before using &:include.
199 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
200 NEWQUOTE\NEWQUOTE => NEWQUOTE
202 NEWQUOTE:changequote & set escape back to &
205 Dollar doubling and macro assistance
206 ------------------------------------
208 &$+ Starts dollar-doubling
209 &$- Stops dollar-doubling
210 Both are idempotent and local to the file or context.
212 This is useful both for make macrology involving $(eval ...), and
213 possibly for helping write complicated recipes involving shell
214 variables, inline Perl code, etc.
216 Sometimes we will show $'s being doubled inside another construct.
217 This means the content of the construct is $-doubled: $-doubling is
218 locally enabled, and restored afterwards.
220 &:macro NAME => define NAME
221 STUFF $ THINGS .. STUFF $$ THINGS
223 NAME is processed for &
225 &{..$..} => ${eval ${call ..$$..}}
226 (matches { } pairs to find the end)
227 content is $-doubled (unless it contains &$- to turn that off)
228 cf &(...), see "Convenience syntax for eval", below.
230 Together &:macro and &${...} provide a more reasonable macro facility
231 than raw make. They solve the problem that make expansions cannot
232 directly generate multiple rules, variable, etc.; instead, `$(eval )'
233 must be used, but that re-expands the argument, meaning that all the
234 literal text must be $-doubled. This applies to the macro text and to
235 the arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
236 Hence &:macro and &${...}.
238 While dollar-doubling:
239 - - - - - - - - - - -
241 $ => $$ including $'s produced by other
242 &-expansions not mentioned here
246 &$NN => ${NN} where N are digits
248 A few contexts do not support $-doubling, such as directive arguments
249 or places where this might imply $-quadrupling. (There is no way to
252 Convenience syntax for eval
253 - - - - - - - - - - - - - -
255 &(thing => $(call thing
256 &( thing => $(call thing
258 &(&lc => $(call sub_dir_lc or $(call TOP_lc
259 &( &lc => $(call sub_dir_lc or $(call TOP_lc
260 even though lc would normally be thought a filename
262 Unlike &{...}, this does not involve any dollar-doubling.
265 Invocation, "recursive" per-directory targets
266 ---------------------------------------------
268 Arrangements are made so that when you run `make foo' in a
269 subdirectory, it is like running the whole toplevel makefile, from the
270 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
271 be built, that builds it.
273 But `foo' can also be a conventional target like `all'.
275 Each subdirectory has its own `all' target. For example a
276 subdirectory `src' has a target `src/all'. The rules for these are
277 automatically generated from the settings of the per-directory
278 &TARGETS variables. &TARGETS is magic in this way. (In
279 src/Dir.sd.mk, &TARGETS of course refers to a make variable called
282 The `all' target in a parent directory is taken to imply the `all'
283 targets in all of its subdirectories, recursively. And in the
284 autogenerated stub Makefiles, `all' is the default target. So if you
285 just type `make' in the toplevel, you are asking for `&all'
286 (<subdir>/all) for every directory in the project.
288 In a parallel build, the rules for all these various subdirectory
289 targets may be in run in parallel: there is only one `make' invocation
290 at a time. There is no sequencing between subdirectories, only been
291 individual targets (as specified according to their dependencies).
293 You can define other per-directory recursive targets too: set the
294 variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
295 at the start of a line). This will create a src/zonk target (for
296 appropriate value of src/). Unlike `all', these other targets only
297 exist in areas of the project where at least something mentions them.
298 So for example, if &TARGETS_zonk is set in src but not lib, `make
299 zonk' in lib will fail. If you want to make a target exist
300 everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
303 Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
304 --------------------------------------------------
306 The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
307 are automatically processed before and after each individual
308 directory's Dir.sd.mk, and the &-substituted contents therefore
309 appear once for each subdirectory.
311 This lets you do per-directory boilerplate. Some useful boilerplate
312 is already provided in subdirmk, for you to reference like this:
313 &:include subdirmk/cdeps.sd.mk
314 &:include subdirmk/clean.sd.mk
315 For example you could put that in Suffix.sd.mk.
317 The top-level Dir.sd.mk is the first makefile included after the
318 autogenerated `main.mk' which merely has some basic settings and
319 includes. So if you want to get in early and set global variables,
320 put them near the top of Dir.sd.mk.
322 The file Final.sd.mk in the toplevel directory is processed and
323 the result included after all the other files. Its subdirmk
324 filtering context inherits warning suppressions from the toplevel's
325 Dir.sd.mk etc., but not anything else.
327 subdirmk's filter script itself sets (only) these variables:
332 You are likely to want to define $(PWD), and shorter names for
333 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
338 subdirmk's `generate' program, which does the acual &-substitution,
339 can produce some warnings about your .sd.mk files. These can be
340 suppressed with the &:warn directive. The warning tags are:
343 The same VARNAME was used both with and without an & prefix.
344 This can be confusing. Also, if you avoid this then you will
345 get a warning iff you accidentally leave off a needed &.
346 The generation of this warning depends on scanning your
347 makefile for things that look like variable references, which
348 subdirmk does not do completely perfectly. Exciting make
349 syntax may evade this warning, or require suppressions.
350 (You can suppress this warning for a particular VARNAME with
351 the &:local+global directive.)
354 A variable expansion like $FBAR. make's expansion rules
355 interpret this as $(F)BAR. It's normally better to write
356 it this way, at least if the variable expansion is followed
357 by more letters. Note that &$FOO works differently to
358 raw make: it expands to ${sub_dir_FOO}.
361 An attempt at variable expansion looking like $&...
362 You probably expected this to mean $(TOP_F)BAR but it
363 expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.
366 &:warn was used to try to enable a warning that this version
367 of subdirmk does not understand. (Note that an attempt to
368 *dis*able an unknown warning is only reported if some other
369 warning was issued which might have been disabled.)
372 Guides, hints, and further explanations
373 =======================================
375 Incorporating this into your project
376 ------------------------------------
378 Use `git-subtree' to merge the subdirmk/ directory. You may find it
379 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
380 symlinks as symlinks - just `git add' the link). And you probably
381 want to mention the situation in your top-level COPYING and HACKING.
383 Symlink autogen.sh into your project toplevel.
385 In your configure.ac, say
387 m4_include([subdirmk/subdirmk.ac])
388 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
390 Write a Dir.sd.mk in each directory. See the substitution syntax
391 reference, above, and the example/ directory here. The toplevel
392 Dir.sd.mk should probably contain:
394 include subdirmk/usual.mk
395 include subdirmk/regen.mk
397 Write a Suffix.sd.mk in the toplevel, if you want. It should probably
400 &:include subdirmk/cdeps.sd.mk
401 &:include subdirmk/clean.sd.mk
407 You can convert your project incrementally. Start with the top-level
408 Makefile.in and rename it to Dir.sd.mk, and add the appropriate
409 stuff to configure.ac, and fix everything up. Leave the existing
410 $(MAKE) -C for your existing subdirectories alone. Then you can
411 convert individual subdirectories, or classes of subdirectories, at
412 your leisure. (You must be /sure/ that each recursive (non-subdirmk)
413 subdirectory will be entered only once at a time, but your existing
414 recursive make descent system should already do that or you already
415 have concurrency bugs.)
417 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
418 This is a frequent source of concurrency bugs in recursive make build
419 systems. When combined with nonrecursive make it's all in the same
420 directory and there is nothing stopping the different invocations
421 ending up trying to make the same targets at the same time. That
422 causes hideous racy lossage. There are ways to get this to work
423 reliably but it is advanced stuff.
425 If you make syntax errors, or certain kinds of other errors, in your
426 makefiles, you may find that just `make' is broken now and cannot get
427 far enough to regenerate a working set of makefiles. If this happens
428 just rerun ./config.status by hand.
430 If you go back and forth between different versions of your code you
431 can sometimes find that `make' complains that one of your Dir.sd.mk
432 files is missing: typically, if iot was used and therefore a
433 dependency in some other version of your code. If you run `make
434 clean' (or `make realclean') these dependencies are suppressed, which
435 will clear up the problem.
441 If want to set global variables, such as CC, that should only be done
442 once. You can put them in your top-level Dir.sd.mk, or a separate
443 file you `include' and declare using SUBDIRMK_MAKEFILES.
445 If you need different settings of variables like CC for different
446 subdirectories, you should probably do that with target-specific
447 variable settings. See the info node `(make) Target-specific'.
450 Directory templates `.sd.mk' vs plain autoconf templates `.mk.in'
451 --------------------------------------------------------------------
453 There are two kinds of template files.
455 Filename .sd.mk .mk.in
457 Processed by &-substitution, autoconf only
460 Instantiated Usu. once per subdir Once only
462 Need to be mentioned No, but Dir.sd.mk All not in subdirmk/
463 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
465 How to include `&:include foo.sd.mk' `include foo.mk'
466 in all relevant .sd.mk in only one
467 (but not needed for Dir.sd.mk
468 Prefix, Suffix, Final)
470 If you `include subdirmk/regen.mk', dependency management and
471 automatic regeneration for all of this template substitution, and for
472 config.status etc. is done for you.
475 Tables of file reference syntaxes
476 ---------------------------------
478 In a nonrecursive makefile supporting out of tree builds there are
479 three separate important distinctions between different file
482 (i) In the build tree, or in the source tree ?
484 (ii) In (or relative to) the subdirectory to which this Dir.sd.mk
485 relates, or relative to the project's top level ?
487 (iii) Absolute or relative pathname ? Usually relative pathnames
488 suffice. Where an absolute pathname is needed, it can be built
489 out of &/ and an appropriate make variable such as $(PWD).
491 Path construction &-expansions are built from the following:
499 In more detail, with all the various options laid out:
501 Recommended Relative paths in... Absolute paths in...
502 for build source build source
504 This lc &file &^file $(PWD)/&file $(abs_src)/&file
505 directory any &/file &^/file $(PWD)/&/file $(abs_src)/&/file
506 several & f g h &^ f g h $(addprefix...)
509 level any file &~/file $(PWD)/file $(abs_src)/file
510 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
511 several f g h &~ f g h $(addprefix...)
513 (This assumes you have appropriate make variables src, PWD and
517 Subdirectory and variable naming
518 --------------------------------
520 The simple variable decoration scheme does not enforce a strict
521 namespace distinction between parts of variable names which come from
522 subdirectory names, and parts that mean something else.
524 So it is a good idea to be a bit careful with your directory naming.
525 `TOP', names that contain `_', and names that are similar to parts of
526 make variables (whether conventional ones, or ones used in your
527 project) are best avoided.
529 If you name your variables in ALL CAPS and your subdirectories in
530 lower case with `-' rather than `_', there will be no confusion.
537 Copyright 2019 Mark Wooding
538 Copyright 2019 Ian Jackson
540 subdirmk and its example is free software; you can redistribute it
541 and/or modify it under the terms of the GNU Library General Public
542 License as published by the Free Software Foundation; either
543 version 2 of the License, or (at your option) any later version.
545 This is distributed in the hope that it will be useful, but
546 WITHOUT ANY WARRANTY; without even the implied warranty of
547 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
548 Library General Public License for more details.
550 You should have received a copy of the GNU Library General Public
551 License along with this library as the file LGPL-2.
552 If not, see https://www.gnu.org/.
554 Individual files generally contain the following tag in the copyright
555 notice, instead of the full licence grant text:
556 SPDX-License-Identifier: LGPL-2.0-or-later
557 As is conventional, this should be read as a licence grant.
559 Contributions are accepted based on the git commit Signed-off-by
560 convention, by which the contributors' certify their contributions
561 according to the Developer Certificate of Origin version 1.1 - see
562 the file DEVELOPER-CERTIFICATE.
564 Where subdirmk is used by and incorporated into another project (eg
565 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
566 the rest of the project are under that other project's licence(s).
567 (The project's overall licence must be compatible with LGPL-2.0+.)