docs: Summarise directory reference syntaxes

[subdirmk.git] / README

diff --git a/README b/README
index cc309f6ae1ec796febb92b7b4fad8083f2d9669d..5897e308644050ec8006fe42e830816ed03f0fb2 100644 (file)
--- a/README
+++ b/README
@@ -83,7 +83,7 @@ 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
@@ -93,7 +93,7 @@ mention its name in Perdir.sd.mk (see below).
 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.
 
@@ -138,6 +138,19 @@ 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
+---------------------------------------
+
+                  In source tree             In build tree
+                  Relative     Absolute      Relative  Absolute
+
+                                             &file     $(abs)/&file
+ This directory   &^/file      &~/file       &/file    $(abs)/&/file
+                  & ^ f g h    & ~ f g h     & f g h
+
+ Top level        $(ts)/file   $(ats)/file   file      $(abs)/file
+                                             f g h
+
 Substitution syntax
 -------------------
 
@@ -193,7 +206,7 @@ empty string).
 &!<lwsp>       disables & until EOL (and then disappears)
 
 &#     delete everything to end of line
-       (useful if the RHS contains unrecognise & constructions)
+       (useful if the RHS contains unrecognised & constructions)
 
 &!STUFF
        changes the escape sequence from & to literally STUFF
@@ -275,6 +288,31 @@ 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
 -----------------