+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 arrannge 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.mk.in'.
+
+These fragments may contain ordinary make language.
+
+However, the sigil & is treated specially. By and large, it refers to
+`the current directory'. There are a variety of convenient
+constructions.
+
+The result is that to a large extent, the Subdir.mk.in has an easy way
+to namespace its "local" make variables, and an easy way to refer to
+its "local" filenames.
+
+The Subdir.mk.in'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.mk.in
+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. (In src/Subdir.mk.in, this 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.
+
+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. (&TARGETS is magic.)
+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.mk.in (see below).
+
+Perdir.mk.in, inclusion
+-----------------------
+
+The file Perdir.mk.in in the toplevel of fthe source is automatically
+processed after each individual directory's Subdir.mk.in, 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.mk.in
+ &:include subdirmk/clean.mk.in
+
+Note that you must use &:include, which is an include processed during
+the generation of the per-directory Subdir.mk files. That ensures
+that the contents of these files is replicated, with appropriate
+per-directory substitutions, for each directory.
+
+Global definitions
+------------------
+
+If want to set global variables, such as CC
+
+
+
+ subdirmk/cdeps.mk.in
+ subdirmk/cdeps.mk.in
+
+
+
+
+
+
+
+ (None of this prevents
+
+
+
+
+
+ https://web.archive.org/web/20150330111905/http://miller.emu.id.au/pmiller/books/rmch/
+
+
+
+
&CAPS => subdir_CAPS or TOP_CAPS
&lc => subdir/lc or lc
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