From: Ian Jackson Date: Mon, 30 Dec 2019 12:06:12 +0000 (+0000) Subject: Merge subdirmk X-Git-Tag: v0.6.0~250 X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=secnet.git;a=commitdiff_plain;h=24eb6b672f1095d5b362b6615f22c17f1136274a Merge subdirmk --- 24eb6b672f1095d5b362b6615f22c17f1136274a diff --cc subdirmk/README index 27dd855,0000000..fcf8c00 mode 100644,000000..100644 --- a/subdirmk/README +++ b/subdirmk/README @@@ -1,416 -1,0 +1,416 @@@ +subdirmk - assistance for non-recursive use of make +=================================================== + +Introduction +------------ + +Peter Miller's 1997 essay _Recursive Make Considered Harmful_ +persuasively argues that it is better to arrange to have a single +make invocation with the project's complete dependency tree, rather +than the currently conventional `$(MAKE) -C subdirectory' approach. + +However, actually writing a project's build system in a non-recursive +style is not very ergonomic. The main difficulties are: + - constantly having to write out long file and directory names + - the lack of a per-directory make variable namespace means + long make variables (or namespace clashes) + - it is difficult to arrange that one can cd to a subdirectory + and say `make all' and have something reasonable happen + (to wit, build an appropriate subset) + +`subdirmk' is an attempt to solve these problems (and it also slightly +alleviates some of the boilerplate needed to support out-of-tree +builds well). + +Basic approach +-------------- + +The developer is expected to write a makefile fragment, in each +relevant subdirectory, called `Subdir.sd.mk'. + +These fragments may contain ordinary make language. Unqualified +filenames are relative to the build toplevel, and all commands all run +there. + +However, the sigil & is treated specially. By and large, it refers to +`the build directory corresponding to this .sd.mk file', etc. +There are a variety of convenient constructions. + +The result is that to a large extent, the Subdir.sd.mk has an easy way +to namespace its "local" make variables, and an easy way to refer to +its "local" filenames (and filenames in general). + +The Subdir.sd.mk's are filtered, fed through autoconf in the usual way +(for @..@-substitutions) and included by one autogenerated toplevel +makefile. + +So all of the input is combined and passed to one make invocation. +(A corollary is that there is no enforcement of the namespacing: +discipline is required to prefix relevant variable names with &, etc.) + +Each subdirectory is also provided with an autogenerated `Makefile' +which exists purely to capture ordinary make invocations and arrange +for something suitable to happen. + +Where there are dependencies between subdirectories, each Subdir.sd.mk +can simply refer to files in other subdirectories directly. + +Invocation, "recursive" per-directory targets +--------------------------------------------- + +Arrangements are made so that when you run `make foo' in a +subdirectory, it is like running the whole toplevel makefile, from the +toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might +be built, that builds it. + +But `foo' can also be a conventional target like `all'. + +Each subdirectory has its own `all' target. For example a +subdirectory `src' has a target `src/all'. The rules for these are +automatically generated from the settings of the per-directory +&TARGETS variables. &TARGETS is magic in this way. (In +src/Subdir.sd.mk, &TARGETS of course refers to a make variable called +src_TARGETS.) + +The `all' target in a parent directory is taken to imply the `all' +targets in all of its subdirectories, recursively. And in the +autogenerated stub Makefiles, `all' is the default target. So if you +just type `make' in the toplevel, you are asking for `&all' +(/all) for every directory in the project. + +In a parallel build, the rules for all these various subdirectory +targets may be in run in parallel: there is only one `make' invocation +at a time. There is no sequencing between subdirectories, only been +individual targets (as specified according to their dependencies). + +You can define other per-directory recursive targets too: simply +mention (usually, by setting) the variable &TARGETS_zonk, or whatever. +This will create a src/zonk target (for appropriate value of src/). +Unlike `all', these other targets only exist in areas of the project +where at least something mentions them. So for example, if +&TARGETS_zonk is mentioned in src but not lib, `make zonk' in +lib will fail. If you want to make a target exist everywhere, +mention its name in Perdir.sd.mk (see below). + +Perdir.sd.mk, inclusion +----------------------- + +The file Perdir.sd.mk in the toplevel of the source is automatically +processed after each individual directory's Subdir.sd.mk, and the +&-substituted contents therefore appear once for each subdirectory. + +This lets you do per-directory boilerplate. Some useful boilerplate +is already provided in subdirmk, for you to reference like this: + &:include subdirmk/cdeps.sd.mk + &:include subdirmk/clean.sd.mk +For example you could put that in Perdir.sd.mk. + +The top-level Subdir.sd.mk is the first makefile included after the +autogenerated `main.mk' which merely has some basic settings and +includes. So if you want to get in early and set global variables, +put them near the top of Subdir.sd.mk. + +subdirmk's filter script itself sets (only) these variables: + top_srcdir + abs_top_srcdir + SUBDIRMK_MAKEFILES + MAKEFILE_TEMPLATES +You are likely to want to define $(PWD), and shorter names for +top_srdir and abs_top_srcdir (we suggest $(src) and $(abs_src)). + +Global definitions +------------------ + +If want to set global variables, such as CC, that should only be done +once. You can put them in your top-level Subdir.sd.mk, or a separate +file you `include' and declare using SUBDIRMK_MAKEFILES. + +If you need different settings of variables like CC for different +subdirectories, you should probably do that with target-specific +variable settings. See the info node `(make) Target-specific'. + +Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in' +-------------------------------------------------------------------- + +There are two kinds of template files. + + Filename .sd.mk .mk.in + + Processed by &-substitution, autoconf only + then autoconf + + Instantiated Usu. once per subdir Once only + + Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/ + in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES + + How to include `&:include foo.sd.mk' `include foo.mk' + in all relevant .sd.mk in only one + (but not needed for Subdir.sd.mk + Subdir and Perdir) + +If you `include subdirmk/regen.mk', dependency management and +automatic regeneration for all of this template substitution, and for +config.status etc. is done for you. + +Tables of file reference syntaxes +--------------------------------- + +In a nonrecursive makefile supporting out of tree builds there are +three separate important distinctions between different file +locations: + + (i) In the build tree, or in the source tree ? + + (ii) In (or relative to) the subdirectory to which this Subdir.sd.mk + relates, or relative to the project's top level ? + + (iii) Absolute or relative pathname ? Usually relative pathnames + suffice. Where an absolute pathname is needed, it can be built + out of &/ and an appropriate make variable such as $(PWD). + +Path construction &-expansions are built from the following: + + Relative paths in... + build source + + This directory & &^ + Top level . &~ + +In more detail, with all the various options laid out: + + Recommended Relative paths in... Absolute paths in... + for build source build source + + This lc &file &^file $(PWD)/&file $(abs_src)/&file - directory any &/file &^/file $(PWD)&/file $(abs_src)/&/file ++ directory any &/file &^/file $(PWD)/&file $(abs_src)/&/file + several & f g h &^ f g h $(addprefix...) + + Top lc file &~file + level any file &~/file $(PWD)/file $(abs_src)/file + .mk.in file $(src)/file $(PWD)/file $(abs_src)/file + several f g h &~ f g h $(addprefix...) + +(This assumes you have appropriate make variables src, PWD and +abs_src.) + +Substitution syntax +------------------- + +In general & expands to the subdirectory name when used for a +filename, and to the subdirectory name with / replaced with _ for +variable names. + +Note that & is processed *even in makefile comments*. The substitutor +does not understand make syntax, or shell syntax, at all. However, +the substitution rules are chosen to work well with constructs which +are common in makefiles. + +In the notation below, we suppose that the substitution is being in +done in a subdirectory sub/dir of the source tree. In the RH column +we describe the expansion at the top level, which is often a special +case (in general in variable names we call that TOP rather than the +empty string). + +&CAPS => sub_dir_CAPS or TOP_CAPS +&lc => sub/dir/lc or lc + Here CAPS is any ASCII letter A-Z and lc is a-z. + The assumption is that filenames are usually lowercase and + variables usually uppercase. Otherwise, use another syntax: + +&_ => sub_dir_ or TOP_ +&=_ => sub_dir or TOP + +&/ => sub/dir/ or nothing +&=/ => sub/dir or . + +&^lc => $(top_srcdir)/sub/dir/lc +&^/ => $(top_srcdir)/sub/dir/ + +&~lc => $(top_srcdir)/lc +&~/ => $(top_srcdir)/ + +In general: + = return subdir without delimiter (not allowed with `^' `~') + ^ pathname of this subdirectory in source tree + ~ pathname of top level of source tree + / terminates the escape (needed if next is not lwsp or space) + lwsp starts multi-word processing (see below) + +So pathname syntax is a subset of: + '&' [ '^' | '~' ] [ lc | '/' ] + +&& => && for convenience in shell runes +\& => & general escaping mechanism + +& thing thing... & +&^ thing thing... & +&~ thing thing... & + Convenience syntax for prefixing multiple filenames. + Introduced by & followed by lwsp where lc could go. + Each lwsp-separated non-ws word is prefixed by &/ etc. + etc. respectively. No other & escapes are recognised. + This processing continues until & preceded by lwsp, + or until EOL (the end of the line), or \ then EOL. + +&: .... + recognised at start of line only (possibly after lwsp) + args are processed for & + +&:include filename filename should usually be foo.sd.mk +&:-include filename tolerate nonexistent file + filenames are relative to $(top_srcdir) + +&! disables & until EOL (and then disappears) + +&# delete everything to end of line + (useful if the RHS contains unrecognised & constructions) + +&!STUFF + changes the escape sequence from & to literally STUFF + STUFF may be any series of of non-whitespace characters, + and is terminated by EOL or lwsp. &!STUFF and the lwsp + are discarded. + + After this, write STUFF instead of &, everywhere. + The effect is global and lasts until the next setting. + It takes effect on &:include'd files too, so maybe set + it back before using &:include. + + Notably + STUFFSTUFF => STUFFSTUFF + \STUFF => STUFF + STUFF!& set escape back to & + +&TARGETS_things + Handled specially. If mentioned, declares that this + subdir ought to have a target `things'. The rule will be + &/things:: $(&TARGETS_things) + + You may extend it by adding more :: rules for the target, + but the preferred style is to do things like this: + &TARGETS_check += & test-passed.stamp + + It is important to mention &TARGETS_things at least once in + the context of each applicable directory, because doing so + arranges that the *parent* will also have a `things' target + which recursively implies this directory's `things'. + + Must be spelled exactly &TARGETS_things. &_TARGETS_things, + for example, is not magic. But mentioning &TARGETS_things in + a #-comment *does* work because the & filter does not care + about comments. + + `all' is extra special: every directory has an `all' + target, which corresponds to &TARGETS. + +Subdirectory and variable naming +-------------------------------- + +The simple variable decoration scheme does not enforce a strict +namespace distinction between parts of variable names which come from +subdirectory names, and parts that mean something else. + +So it is a good idea to be a bit careful with your directory naming. +`TOP', names that contain `_', and names that are similar to parts of +make variables (whether conventional ones, or ones used in your +project) are best avoided. + +If you name your variables in ALL CAPS and your subdirectories in +lower case with `-' rather than `_', there will be no confusion. + +Incorporating this into your project +------------------------------------ + +Use `git-subtree' to merge the subdirmk/ directory. You may find it +useful to symlink the DEVELOPER-CERTIFICATE file (git can store +symlinks as symlinks - just `git add' the link). And you probably +want to mention the situation in your top-level COPYING. + +Symlink autogen.sh into your project toplevel. + +In your configure.ac, say + + m4_include([subdirmk/subdirmk.ac]) + SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...]) + +Write a Subdir.sd.mk in each directory. The toplevel one should +probably contain: + + include subdirmk/usual.mk + include subdirmk/regen.mk + +Write a Perdir.sd.mk in the toplevel, if you want. It should probably +have: + + &:include subdirmk/cdeps.sd.mk + &:include subdirmk/clean.sd.mk + +Hints +----- + +You can convert your project incrementally. Start with the top-level +Makefile.in and rename it to Subdir.sd.mk, and add the appropriate +stuff to configure.ac, and fix everything up. Leave the existing +$(MAKE) -C for your existing subdirectories alone. Then you can +convert individual subdirectories, or classes of subdirectories, at +your leisure. (You must be /sure/ that each subdirectory will be +entered only once at a time, but your existing recursive make descent +system should already do that or you already have concurrency bugs.) + +Aside from this, be very wary of any invocation of $(MAKE) anywhere. +This is a frequent source of concurrency bugs in recursive make build +systems. When combined with nonrecursive make it's all in the same +directory and there is nothing stopping the different invocations +ending up trying to make the same targets at the same time. That +causes hideous racy lossage. There are ways to get this to work +reliably but it is advanced stuff. + +If you make syntax errors, or certain kinds of other errors, in your +makefiles, you may find that just `make' is broken now and cannot get +far enough to regenerate a working set of makefiles. If this happens +just rerun ./config.status by hand. + +If you go back and forth between different versions of your code you +can sometimes find that `make' complains that one of your Subdir.sd.mk +files is missing: typically, if iot was used and therefore a +dependency in some other version of your code. If you run `make +clean' (or `make realclean') these dependencies are suppressed, which +will clear up the problem. + + +Legal information +----------------- + +subdirmk is + Copyright 2019 Mark Wooding + Copyright 2019 Ian Jackson + + subdirmk and its example is free software; you can redistribute it + and/or modify it under the terms of the GNU Library General Public + License as published by the Free Software Foundation; either + version 2 of the License, or (at your option) any later version. + + This is distributed in the hope that it will be useful, but + WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Library General Public License for more details. + + You should have received a copy of the GNU Library General Public + License along with this library as the file LGPL-2. + If not, see https://www.gnu.org/. + +Individual files generally contain the following tag in the copyright +notice, instead of the full licence grant text: + SPDX-License-Identifier: LGPL-2.0-or-later +As is conventional, this should be read as a licence grant. + +Contributions are accepted based on the git commit Signed-off-by +convention, by which the contributors' certify their contributions +according to the Developer Certificate of Origin version 1.1 - see +the file DEVELOPER-CERTIFICATE. + +Where subdirmk is used by and incorporated into another project (eg +via git subtree), the directory subdirmk/ is under GNU LGPL-2.0+, and +the rest of the project are under that other project's licence(s). +(The project's overall licence must be compatible with LGPL-2.0+.)