docs: New plan for directory references, new

[subdirmk.git] / README

diff --git a/README b/README
index e5ef4b4f5c28a1291e1d4b9c4e981d1ffb4f4eb7..0b95b98ff17f75152a7279503bc8139c56f86471 100644 (file)
--- a/README
+++ b/README
@@ -138,6 +138,35 @@ 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 directory reference syntaxes
+---------------------------------------
+
+Path construction &-expansions, meanings summary:
+
+                      In build tree            In source tree
+  This directory      just &                   &,
+  Top level           &.  implies absolute     &;
+
+Adding `@' means "absolute path".  This is not needed with &. because
+there is never any need to use &. since it would expand to nothing.
+`/' 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.
+
+      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
+             .mk.in   file      $(abs)/file     $(src)/file  $(abs_src)/file
+             several  f g h     &. f g h        &; f g h     &@; f g h
+                                &@. file
+                                &@. f g h
+
 Substitution syntax
 -------------------
 
@@ -169,6 +198,11 @@ empty string).
 &^             =>      $(top_srcdir)/sub/dir           or $(top_srcdir)
 &~             =>      $(abs_top_srcdir)/sub/dir       or $(abs_top_srcdir)
 
+In general:
+        ^       filenames in source tree rather than build tree
+        ~       filenames are absolute rather than relative
+        @       filenames do not contain subdir (useful with the above)
+
 &&             =>      &&              for convenience in shell runes
 \&             =>      &               general escaping mechanism
 
@@ -275,6 +309,31 @@ 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
 -----------------