X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ian/git?p=subdirmk.git;a=blobdiff_plain;f=subdirmk%2FREADME;h=16a33e373d937567550c611ae4184cb662b90710;hp=7cf84cf756597db3b2073c4c158172456fca59ed;hb=37ddf3c292b5dbf0ea56af4185d5a6faefac738d;hpb=3d62132e874bd2ceee3fb5960a172beb4594d6c9

diff --git a/subdirmk/README b/subdirmk/README
index 7cf84cf..16a33e3 100644
--- a/subdirmk/README
+++ b/subdirmk/README
@@ -1,3 +1,95 @@
+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.
+
+Per-directory targets, "recursion"
+----------------------------------
+
+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.
+
+There may also be other 
+
+
+
+
+
+
+  (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
 
@@ -16,6 +108,12 @@
 & ~ thing thing... &		& may be omitted before EOL or before \EOL
 				other &'s not recognised
 
+start of line (maybe after ws):
+&:<directive> <args>....
+args are processed for & first
+&:include filename		filename should usually be foo.mk.in
+&:-include filename
+
 CAPS is [A-Z][0-9_A-Z]*(?!\w)
 lc is [a-z][-+,0-9_a-z]*(?!\w)