The developer is expected to write a makefile fragment, in each
relevant subdirectory, called `Subdir.sd.mk'.
-These fragments may contain ordinary make language.
+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 current directory'. There are a variety of convenient
-constructions.
+`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.
+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
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.)
+
+Path construction &-expansions are built from the following:
+
+ Relative paths in... Absolute paths in...
+ build source build source
+
+ This directory & &, &@ &@,
+ Top level . &; &@. &@;
+
+In more detail, with the various options for what comes next:
+
+ Recommended Relative paths in... Absolute paths in...
+ for build source build source
+
+ This lc &file &,file &@file &@,file
+ directory any &/file &,/file &@/file &@,/file
+ several & f g h &, f g h &@ f g h &@, f g h
+
+ Top lc file &;file &@.file &@;file
+ level any file &;/file &@./file &@;/file
+ several f g h &; f g h &@. f g h &@; f g h
+ .mk.in file $(src)/file $(abs)/file $(abs_src)/file
+
Substitution syntax
-------------------
variables usually uppercase. Otherwise, use another syntax:
&_ => sub_dir_ or TOP_
-&/ => sub/dir/ or nothing
&=_ => sub_dir or TOP
+
+&/ => sub/dir/ or nothing
&=/ => sub/dir or .
-&^ => $(top_srcdir)/sub/dir or $(top_srcdir)
-&~ => $(abs_top_srcdir)/sub/dir or $(abs_top_srcdir)
+
+&,lc => $(top_srcdir)/sub/dir/lc &,/ => $(top_srcdir)/sub/dir/
+&;lc => $(top_srcdir)/lc &;/ => $(top_srcdir)/
+
+&@lc => $(PWD)/sub/dir/lc &@/ => $(PWD)/sub/dir/
+&@.lc => $(PWD)/lc &@./ => $(PWD)/
+&@,lc => $(abs_top_srcdir)/sub/dir/lc &@,/ => $(abs_top_srcdir)/sub/dir/
+&@;lc => $(abs_top_srcdir)/lc &@;/ => $(abs_top_srcdir)/
+
+In general:
+ = return subdir without delimiter (not allowed with `,' `;' `@')
+ , pathname of this subdirectory in source tree
+ ; pathname of top level of source tree
+ . pathname of this directory in build tree, `@' must be specified
+ @ absolute pathnames
+ / terminates the escape (needed if next is not lwsp or space)
+ lwsp starts multi-word processing (see below)
+
+So pathname syntax is a subset of:
+ '&' [ '@' ] [ ',' | ';' | '.' ] [ lc | '/' ]
+
+ To avoid incomprehensible .sd.mk files, some combinations are not
+ allowed. For example `&=./' would mean `.' and `&./' would be the
+ empty string. Variations with `=' and one of `@' `,' `;' are
+ uncommon and must be written using make variables instead.
&& => && for convenience in shell runes
\& => & general escaping mechanism
-& thing thing... &
-& ^ thing thing... &
-& ~ thing thing... &
+& thing thing... & &@ thing thing... &
+ &. thing thing... &
+&, thing thing... & &@, thing thing... &
+&; thing thing... & &@; thing thing... &
Convenience syntax for prefixing multiple filenames.
- Introduced by & followed by lwsp (space or tab).
- Each lwsp-separated non-ws word is prefixed by &/ &^/ &~/
- respectively. No other & escapes are recognised.
+ Introduced by & followed by lwsp where lc could go.
+ Each lwsp-separated non-ws word is prefixed by &/ etc.
+ etc. respectively. No other & escapes are recognised.
This processing continues until & preceded by lwsp,
or until EOL (the end of the line), or \ then EOL.
&:include subdirmk/cdeps.sd.mk
&:include subdirmk/clean.sd.mk
+Hints
+-----
+
+You can convert your project incrementally. Start with the top-level
+Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
+stuff to configure.ac, and fix everything up. Leave the existing
+$(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.
+
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