X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ian/git?p=subdirmk.git;a=blobdiff_plain;f=README;h=0ade775ed3d954378b7268c1e2487db91a9309c8;hp=e5ef4b4f5c28a1291e1d4b9c4e981d1ffb4f4eb7;hb=2e6f4d3e9d48db5fbdbe87c7076e1ae56fc66a67;hpb=f1789a0ae66174a95c8d629738eae98d40c084ac

diff --git a/README b/README
index e5ef4b4..0ade775 100644
--- a/README
+++ b/README
@@ -138,6 +138,37 @@ 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.
 
+Summary of recommended directory reference syntaxes
+---------------------------------------------------
+
+Path construction &-expansions, meanings summary:
+
+      Recommended     In build tree             In source tree
+             when     Relative  Absolute        Relative     Absolute
+                                          
+  This directory      &         &@              &,           &@,
+  Top level           .         &@.             &;           &@;
+
+Adding `@' means "absolute path".  (`&.' is not allowed without @
+because just `&./' is a silly way of writing `.'.)  `/' terminates the
+escape (needed if the next thing is not a lowercase character, or
+space).  `=' means "just the value, no /".  Space starts multi-word
+processing.
+
+In more detail, with the various options for what comes next:
+
+      Recommended     In build tree             In source tree
+             when     Relative  Absolute        Relative     Absolute
+                                          
+  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
+  level      any      file      &@./file        &;/file      &@;/file
+             several  f g h     &@. f g h       &; f g h     &@; f g h
+             .mk.in   file      $(abs)/file     $(src)/file  $(abs_src)/file
+
 Substitution syntax
 -------------------
 
@@ -163,22 +194,45 @@ empty string).
 	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
+
+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.
 
@@ -275,6 +329,31 @@ 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 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.
+
 
 Legal information
 -----------------