--- /dev/null
- &${..$..} => ${eval ${call ..$$..}}
+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, and helps a bit with metaprogramming and rule writing).
+
+Basic approach
+--------------
+
+The developer is expected to write a makefile fragment, in each
+relevant subdirectory, called `Dir.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 Dir.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 Dir.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 Dir.sd.mk
+can simply refer to files in other subdirectories directly.
+
+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. (If your variables start with capital letters and
+your filenames with lowercase. Otherwise, use &/ or &_.)
+
+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 nothing
+&_ => sub_dir_ or TOP_
+&. => sub/dir or .
+ (This implies that `&./' works roughly like `&/', although
+ it can produce a needless `./')
+
+&= => sub_dir or TOP
+
+&^lc => ${top_srcdir}/sub/dir/lc
+&^/ => ${top_srcdir}/sub/dir/
+&^. => ${top_srcdir}/sub/dir
+
+&~lc => ${top_srcdir}/lc
+&~/ => ${top_srcdir}/
+&~. => ${top_srcdir}
+
+In general:
+ ^ pathname of this subdirectory in source tree
+ ~ pathname of top level of source tree
+ / terminates the path escape } needed if next is
+ _ terminates the var escape } not letter or space)
+ . terminates path escape giving dir name (excluding /)
+ = terminates var escape giving only prefix part (rarely needed)
+ lwsp starts multi-word processing (see below)
+
+So pathname syntax is a subset of:
+ '&' [ '^' | '~' ] [ lc | '/' | '.' ]
+
+&& => && for convenience in shell runes
+
+&\& => & general escaping mechanism
+&\$ => $ provided for $-doubling regimes
+&\NEWLINE eats the newline and vanishes
+
+&$VARIABLE => ${sub_dir_VARIABLE} or ${TOP_VARIABLE}
+ VARIABLE is ASCII starting with a letter and matching \w+
+
+& 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.
+
+&:<directive> <args>....
+ recognised at start of line only (possibly after lwsp)
+
++&: => &:
++ for make multiple targets syntax
++ recognised anywhere *except* start of line
++
+&:include filename filename should usually be [&]foo.sd.mk
+&:-include filename tolerate nonexistent file
+ RHS is &-expanded but filenames are relative to the top
+ srcdir. This implies that unqualified names are like &~/
+ whereas &/ is like &^/. &^ and &~ do not work here because
+ they expand to constructions involving literally
+ `$(top_srcdir)', but the RHS is not make-expanded.
+
+&!<lwsp> disables & until EOL (and then disappears)
+
+&# delete everything to end of line
+ (useful if the RHS contains unrecognised & constructions)
+
+&TARGETS_things
+ Handled specially. If mentioned at the start of a line
+ (possibly following whitespace), 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. To make the target exist
+ without providing any prerequisites for it, write a line
+ containing just `&TARGETS_things +='.
+
+ `all' is extra special: every directory has an `all'
+ target, which corresponds to &TARGETS.
+
++Directives
++- - - - -
++
+&:warn [!]WARNTAG ...
+ Suppress (with !) or re-enable (without !) warnings tagged
+ WARNTAG (see section `Warnings', below). The suppression list
+ is reset at the start of processing in each subdirectory.
+ Warnings that appear at the end of processing are controlled
+ by the final warning state after processing all the toplevel
+ input files (including Final.sd.mk).
+
+&:local+global [!][&]VARIABLE ...
+ Suppresses any warnings relating to forthcoming mentions
+ to VARIABLE or &VARIABLE, as applicable. Scope ends at
+ the end of the current directory's Suffix.sd.mk.
+ Prefixing with ! removes [&]VARIABLE from the suppresion list.
+
+&:changequote NEWQUOTE
+ changes the escape sequence from & to literally NEWQUOTE
+ NEWQUOTE may be any series of of non-whitespace characters,
+ and is terminated by EOL or lwsp. The whole line is
+ discarded.
+
+ After this, write NEWQUOTE instead of &, everywhere.
+ The effect is unscoped and lasts until the next setting,
+ or until the end of the current directory's Suffix.sd.mk.
+ It takes effect on &:include'd files too, so maybe set
+ it back before using &:include.
+
+ Notably
+ NEWQUOTENEWQUOTE => NEWQUOTENEWQUOTE
+ NEWQUOTE\NEWQUOTE => NEWQUOTE
+ NEWQUOTE\$ => $
+ NEWQUOTE:changequote & set escape back to &
+
+
+Dollar doubling and macro assistance
+------------------------------------
+
+&$+ Starts dollar-doubling
+&$- Stops dollar-doubling
+ Both are idempotent and local to the file or context.
+
+This is useful both for make macrology involving $(eval ...), and
+possibly for helping write complicated recipes involving shell
+variables, inline Perl code, etc.
+
+Sometimes we will show $'s being doubled inside another construct.
+This means the content of the construct is $-doubled: $-doubling is
+locally enabled, and restored afterwards.
+
+&:macro NAME => define NAME
+STUFF $ THINGS .. STUFF $$ THINGS
+&:endm .. endef
+ NAME is processed for &
+
- Together &:macro and &${...} provide a more reasonable macro facility
++&{..$..} => ${eval ${call ..$$..}}
+ (matches { } pairs to find the end)
+ content is $-doubled (unless it contains &$- to turn that off)
++ contrast &(...), see "Convenience syntax for call", below.
+
- directly generate multiple rules, variable, etc.; instead, `$(eval )'
++Together &:macro and &{...} provide a more reasonable macro facility
+than raw make. They solve the problem that make expansions cannot
- Hence &:macro and &${...}.
++directly generate multiple rules, variables, etc.; instead, `$(eval )'
+must be used, but that re-expands the argument, meaning that all the
+literal text must be $-doubled. This applies to the macro text and to
+the arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
- &$( => ${ (expands to { } so it is useable for shell too)
++Hence &:macro and &{...}.
+
+While dollar-doubling:
+- - - - - - - - - - -
+
+$ => $$ including $'s produced by other
+ &-expansions not mentioned here
+
+&\$ => $
- Copyright 2019 Ian Jackson
++&$( => $(
+&$NN => ${NN} where N are digits
+
+A few contexts do not support $-doubling, such as directive arguments
+or places where this might imply $-quadrupling. (There is no way to
+get $-quadrupling.)
+
++Convenience syntax for call
++- - - - - - - - - - - - - -
++
++&(thing => $(call thing
++&( thing => $(call thing
++ and specially:
++&(&lc => $(call sub_dir_lc or $(call TOP_lc
++&( &lc => $(call sub_dir_lc or $(call TOP_lc
++ even though lc would normally be thought a filename
++
++Unlike &{...}, this does not involve any dollar-doubling.
++
++Use this when the expansion is going to be a piece of text to be used
++as part of a rule, filename, etc. When the expansion is top-level
++make text (eg, rules), use &:macro and &{...}.
++
+
+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/Dir.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'
+(<subdir>/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: set the
+variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
+at the start of a line). 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 set in src but not lib, `make
+zonk' in lib will fail. If you want to make a target exist
+everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
+below).
+
+Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
+--------------------------------------------------
+
+The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
+are automatically processed before and after each individual
+directory's Dir.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 Suffix.sd.mk.
+
+The top-level Dir.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 Dir.sd.mk.
+
+The file Final.sd.mk in the toplevel directory is processed and
+the result included after all the other files. Its subdirmk
+filtering context inherits warning suppressions from the toplevel's
+Dir.sd.mk etc., but not anything else.
+
+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)).
+
+Warnings
+--------
+
+subdirmk's `generate' program, which does the acual &-substitution,
+can produce some warnings about your .sd.mk files. These can be
+suppressed with the &:warn directive. The warning tags are:
+
+ local+global
+ The same VARNAME was used both with and without an & prefix.
+ This can be confusing. Also, if you avoid this then you will
+ get a warning iff you accidentally leave off a needed &.
+ The generation of this warning depends on scanning your
+ makefile for things that look like variable references, which
+ subdirmk does not do completely perfectly. Exciting make
+ syntax may evade this warning, or require suppressions.
+ (You can suppress this warning for a particular VARNAME with
+ the &:local+global directive.)
+
+ single-char-var
+ A variable expansion like $FBAR. make's expansion rules
+ interpret this as $(F)BAR. It's normally better to write
+ it this way, at least if the variable expansion is followed
+ by more letters. Note that &$FOO works differently to
+ raw make: it expands to ${sub_dir_FOO}.
+
+ broken-var-ref
+ An attempt at variable expansion looking like $&...
+ You probably expected this to mean $(TOP_F)BAR but it
+ expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.
+
+ unknown-warning
+ &:warn was used to try to enable a warning that this version
+ of subdirmk does not understand. (Note that an attempt to
+ *dis*able an unknown warning is only reported if some other
+ warning was issued which might have been disabled.)
+
+
+Guides, hints, and further explanations
+=======================================
+
+Incorporating this into your project
+------------------------------------
+
+Use `git-subtree' to merge the subdirmk/ directory. You may find it
+useful to symlink the DEVELOPER-CERTIFICATE file (git can store
+symlinks as symlinks - just `git add' the link). And you probably
+want to mention the situation in your top-level COPYING and HACKING.
+
+Symlink autogen.sh into your project toplevel.
+
+In your configure.ac, say
+
+ m4_include([subdirmk/subdirmk.ac])
+ SUBDIRMK_SUBDIRS([...list of subdirectories in relative syntax...])
+
+Write a Dir.sd.mk in each directory. See the substitution syntax
+reference, above, and the example/ directory here. The toplevel
+Dir.sd.mk should probably contain:
+
+ include subdirmk/usual.mk
+ include subdirmk/regen.mk
+
+Write a Suffix.sd.mk in the toplevel, if you want. It should probably
+have:
+
+ &:include subdirmk/cdeps.sd.mk
+ &:include subdirmk/clean.sd.mk
+
+
+Hints
+-----
+
+You can convert your project incrementally. Start with the top-level
+Makefile.in and rename it to Dir.sd.mk, and add the appropriate
+stuff to configure.ac, and fix everything up. Leave the existing
+$(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 recursive (non-subdirmk)
+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 Dir.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.
+
+
+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 Dir.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'.
+
+
+Directory 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 Dir.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 Dir.sd.mk
+ Prefix, Suffix, Final)
+
+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 Dir.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
+ 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.)
+
+
+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.
+
+
+Legal information
+=================
+
+subdirmk is
++ Copyright 2019-2020 Ian Jackson
+ Copyright 2019 Mark Wooding
+
+ 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+.)
~ianmdlvl / secnet.git / commitdiff