------------
Peter Miller's 1997 essay _Recursive Make Considered Harmful_
-persuasively argues that it is better to arrannge to have a single
+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.
- 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
+ - 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)
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
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.
+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
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, &TARGES of course refers to a make variable called
+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'
(<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.
+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.
+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
Perdir.sd.mk, inclusion
-----------------------
-The file Perdir.sd.mk in the toplevel of fthe source is automatically
+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.
&: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
------------------
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'
--------------------------------------------------------------------
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
+ 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
-------------------
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
+&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 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)/
+
+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... &
+&^ 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.
- This processing continues until a & preceded by lwsp,
+ 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>....
&:include filename filename should usually be foo.sd.mk
&:-include filename tolerate nonexistent file
+ filenames are relative to $(top_srcdir)
&!<lwsp> disables & until EOL (and then disappears)
+&# delete everything to end of line
+ (useful if the RHS contains unrecognised & constructions)
+
&!STUFF
changes the escape sequence from & to literally STUFF
STUFF may be any series of of non-whitespace characters,
&:include subdirmk/cdeps.sd.mk
&:include subdirmk/clean.sd.mk
+Hints
+-----
+
+You can convert your project incrementally. Start with the top-level
+Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
+stuff to configure.ac, and fix everything up. Leave the existing
+$(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
-----------------
~ian / secnet.git / blobdiff