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 any warnings relating to forthcoming mentions
179 to VARIABLE or &VARIABLE, as applicable. Scope ends at
180 the end of the current directory's Suffix.sd.mk.
181 Prefixing with ! removes [&]VARIABLE from the suppresion list.
183 &:changequote NEWQUOTE
184 changes the escape sequence from & to literally NEWQUOTE
185 NEWQUOTE may be any series of of non-whitespace characters,
186 and is terminated by EOL or lwsp. The whole line is
189 After this, write NEWQUOTE instead of &, everywhere.
190 The effect is unscoped and lasts until the next setting,
191 or until the end of the current directory's Suffix.sd.mk.
192 It takes effect on &:include'd files too, so maybe set
193 it back before using &:include.
196 NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
197 NEWQUOTE\NEWQUOTE => NEWQUOTE
199 NEWQUOTE:changequote & set escape back to &
202 Dollar doubling and macro assistance
203 ------------------------------------
205 &$+ Starts dollar-doubling
206 &$- Stops dollar-doubling
207 Both are idempotent and local to the file or context.
209 This is useful both for make macrology involving $(eval ...), and
210 possibly for helping write complicated recipes involving shell
211 variables, inline Perl code, etc.
213 Sometimes we will show $'s being doubled inside another construct.
214 This means the content of the construct is $-doubled: $-doubling is
215 locally enabled, and restored afterwards.
217 &:macro NAME => define NAME
218 STUFF $ THINGS .. STUFF $$ THINGS
220 NAME is processed for &
222 &${..$..} => ${eval ${call ..$$..}}
223 (matches { } pairs to find the end)
224 content is $-doubled (unless it contains &$- to turn that off)
226 Together &:macro and &${...} provide a more reasonable macro facility
227 than raw make. They solve the problem that make expansions cannot
228 directly generate multiple rules, variable, etc.; instead, `$(eval )'
229 must be used, but that re-expands the argument, meaning that all the
230 literal text must be $-doubled. This applies to the macro text and to
231 the arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
232 Hence &:macro and &${...}.
234 While dollar-doubling:
235 - - - - - - - - - - -
237 $ => $$ including $'s produced by other
238 &-expansions not mentioned here
241 &$( => ${ (expands to { } so it is useable for shell too)
242 &$NN => ${NN} where N are digits
244 A few contexts do not support $-doubling, such as directive arguments
245 or places where this might imply $-quadrupling. (There is no way to
249 Invocation, "recursive" per-directory targets
250 ---------------------------------------------
252 Arrangements are made so that when you run `make foo' in a
253 subdirectory, it is like running the whole toplevel makefile, from the
254 toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
255 be built, that builds it.
257 But `foo' can also be a conventional target like `all'.
259 Each subdirectory has its own `all' target. For example a
260 subdirectory `src' has a target `src/all'. The rules for these are
261 automatically generated from the settings of the per-directory
262 &TARGETS variables. &TARGETS is magic in this way. (In
263 src/Dir.sd.mk, &TARGETS of course refers to a make variable called
266 The `all' target in a parent directory is taken to imply the `all'
267 targets in all of its subdirectories, recursively. And in the
268 autogenerated stub Makefiles, `all' is the default target. So if you
269 just type `make' in the toplevel, you are asking for `&all'
270 (<subdir>/all) for every directory in the project.
272 In a parallel build, the rules for all these various subdirectory
273 targets may be in run in parallel: there is only one `make' invocation
274 at a time. There is no sequencing between subdirectories, only been
275 individual targets (as specified according to their dependencies).
277 You can define other per-directory recursive targets too: set the
278 variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
279 at the start of a line). This will create a src/zonk target (for
280 appropriate value of src/). Unlike `all', these other targets only
281 exist in areas of the project where at least something mentions them.
282 So for example, if &TARGETS_zonk is set in src but not lib, `make
283 zonk' in lib will fail. If you want to make a target exist
284 everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
287 Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
288 --------------------------------------------------
290 The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
291 are automatically processed before and after each individual
292 directory's Dir.sd.mk, and the &-substituted contents therefore
293 appear once for each subdirectory.
295 This lets you do per-directory boilerplate. Some useful boilerplate
296 is already provided in subdirmk, for you to reference like this:
297 &:include subdirmk/cdeps.sd.mk
298 &:include subdirmk/clean.sd.mk
299 For example you could put that in Suffix.sd.mk.
301 The top-level Dir.sd.mk is the first makefile included after the
302 autogenerated `main.mk' which merely has some basic settings and
303 includes. So if you want to get in early and set global variables,
304 put them near the top of Dir.sd.mk.
306 The file Final.sd.mk in the toplevel directory is processed and
307 the result included after all the other files. Its subdirmk
308 filtering context inherits warning suppressions from the toplevel's
309 Dir.sd.mk etc., but not anything else.
311 subdirmk's filter script itself sets (only) these variables:
316 You are likely to want to define $(PWD), and shorter names for
317 top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)).
322 subdirmk's `generate' program, which does the acual &-substitution,
323 can produce some warnings about your .sd.mk files. These can be
324 suppressed with the &:warn directive. The warning tags are:
327 The same VARNAME was used both with and without an & prefix.
328 This can be confusing. Also, if you avoid this then you will
329 get a warning iff you accidentally leave off a needed &.
330 The generation of this warning depends on scanning your
331 makefile for things that look like variable references, which
332 subdirmk does not do completely perfectly. Exciting make
333 syntax may evade this warning, or require suppressions.
334 (You can suppress this warning for a particular VARNAME with
335 the &:local+global directive.)
338 A variable expansion like $FBAR. make's expansion rules
339 interpret this as $(F)BAR. It's normally better to write
340 it this way, at least if the variable expansion is followed
341 by more letters. Note that &$FOO works differently to
342 raw make: it expands to ${sub_dir_FOO}.
345 An attempt at variable expansion looking like $&...
346 You probably expected this to mean $(TOP_F)BAR but it
347 expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.
350 &:warn was used to try to enable a warning that this version
351 of subdirmk does not understand. (Note that an attempt to
352 *dis*able an unknown warning is only reported if some other
353 warning was issued which might have been disabled.)
356 Guides, hints, and further explanations
357 =======================================
359 Incorporating this into your project
360 ------------------------------------
362 Use `git-subtree' to merge the subdirmk/ directory. You may find it
363 useful to symlink the DEVELOPER-CERTIFICATE file (git can store
364 symlinks as symlinks - just `git add' the link). And you probably
365 want to mention the situation in your top-level COPYING and HACKING.
367 Symlink autogen.sh into your project toplevel.
369 In your configure.ac, say
371 m4_include([subdirmk/subdirmk.ac])
372 SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
374 Write a Dir.sd.mk in each directory. See the substitution syntax
375 reference, above, and the example/ directory here. The toplevel
376 Dir.sd.mk should probably contain:
378 include subdirmk/usual.mk
379 include subdirmk/regen.mk
381 Write a Suffix.sd.mk in the toplevel, if you want. It should probably
384 &:include subdirmk/cdeps.sd.mk
385 &:include subdirmk/clean.sd.mk
391 You can convert your project incrementally. Start with the top-level
392 Makefile.in and rename it to Dir.sd.mk, and add the appropriate
393 stuff to configure.ac, and fix everything up. Leave the existing
394 $(MAKE) -C for your existing subdirectories alone. Then you can
395 convert individual subdirectories, or classes of subdirectories, at
396 your leisure. (You must be /sure/ that each recursive (non-subdirmk)
397 subdirectory will be entered only once at a time, but your existing
398 recursive make descent system should already do that or you already
399 have concurrency bugs.)
401 Aside from this, be very wary of any invocation of $(MAKE) anywhere.
402 This is a frequent source of concurrency bugs in recursive make build
403 systems. When combined with nonrecursive make it's all in the same
404 directory and there is nothing stopping the different invocations
405 ending up trying to make the same targets at the same time. That
406 causes hideous racy lossage. There are ways to get this to work
407 reliably but it is advanced stuff.
409 If you make syntax errors, or certain kinds of other errors, in your
410 makefiles, you may find that just `make' is broken now and cannot get
411 far enough to regenerate a working set of makefiles. If this happens
412 just rerun ./config.status by hand.
414 If you go back and forth between different versions of your code you
415 can sometimes find that `make' complains that one of your Dir.sd.mk
416 files is missing: typically, if iot was used and therefore a
417 dependency in some other version of your code. If you run `make
418 clean' (or `make realclean') these dependencies are suppressed, which
419 will clear up the problem.
425 If want to set global variables, such as CC, that should only be done
426 once. You can put them in your top-level Dir.sd.mk, or a separate
427 file you `include' and declare using SUBDIRMK_MAKEFILES.
429 If you need different settings of variables like CC for different
430 subdirectories, you should probably do that with target-specific
431 variable settings. See the info node `(make) Target-specific'.
434 Directory templates `.sd.mk' vs plain autoconf templates `.mk.in'
435 --------------------------------------------------------------------
437 There are two kinds of template files.
439 Filename .sd.mk .mk.in
441 Processed by &-substitution, autoconf only
444 Instantiated Usu. once per subdir Once only
446 Need to be mentioned No, but Dir.sd.mk All not in subdirmk/
447 in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
449 How to include `&:include foo.sd.mk' `include foo.mk'
450 in all relevant .sd.mk in only one
451 (but not needed for Dir.sd.mk
452 Prefix, Suffix, Final)
454 If you `include subdirmk/regen.mk', dependency management and
455 automatic regeneration for all of this template substitution, and for
456 config.status etc. is done for you.
459 Tables of file reference syntaxes
460 ---------------------------------
462 In a nonrecursive makefile supporting out of tree builds there are
463 three separate important distinctions between different file
466 (i) In the build tree, or in the source tree ?
468 (ii) In (or relative to) the subdirectory to which this Dir.sd.mk
469 relates, or relative to the project's top level ?
471 (iii) Absolute or relative pathname ? Usually relative pathnames
472 suffice. Where an absolute pathname is needed, it can be built
473 out of &/ and an appropriate make variable such as $(PWD).
475 Path construction &-expansions are built from the following:
483 In more detail, with all the various options laid out:
485 Recommended Relative paths in... Absolute paths in...
486 for build source build source
488 This lc &file &^file $(PWD)/&file $(abs_src)/&file
489 directory any &/file &^/file $(PWD)/&/file $(abs_src)/&/file
490 several & f g h &^ f g h $(addprefix...)
493 level any file &~/file $(PWD)/file $(abs_src)/file
494 .mk.in file $(src)/file $(PWD)/file $(abs_src)/file
495 several f g h &~ f g h $(addprefix...)
497 (This assumes you have appropriate make variables src, PWD and
501 Subdirectory and variable naming
502 --------------------------------
504 The simple variable decoration scheme does not enforce a strict
505 namespace distinction between parts of variable names which come from
506 subdirectory names, and parts that mean something else.
508 So it is a good idea to be a bit careful with your directory naming.
509 `TOP', names that contain `_', and names that are similar to parts of
510 make variables (whether conventional ones, or ones used in your
511 project) are best avoided.
513 If you name your variables in ALL CAPS and your subdirectories in
514 lower case with `-' rather than `_', there will be no confusion.
521 Copyright 2019 Mark Wooding
522 Copyright 2019 Ian Jackson
524 subdirmk and its example is free software; you can redistribute it
525 and/or modify it under the terms of the GNU Library General Public
526 License as published by the Free Software Foundation; either
527 version 2 of the License, or (at your option) any later version.
529 This is distributed in the hope that it will be useful, but
530 WITHOUT ANY WARRANTY; without even the implied warranty of
531 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
532 Library General Public License for more details.
534 You should have received a copy of the GNU Library General Public
535 License along with this library as the file LGPL-2.
536 If not, see https://www.gnu.org/.
538 Individual files generally contain the following tag in the copyright
539 notice, instead of the full licence grant text:
540 SPDX-License-Identifier: LGPL-2.0-or-later
541 As is conventional, this should be read as a licence grant.
543 Contributions are accepted based on the git commit Signed-off-by
544 convention, by which the contributors' certify their contributions
545 according to the Developer Certificate of Origin version 1.1 - see
546 the file DEVELOPER-CERTIFICATE.
548 Where subdirmk is used by and incorporated into another project (eg
549 via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and
550 the rest of the project are under that other project's licence(s).
551 (The project's overall licence must be compatible with LGPL-2.0+.)