docs: Discuss variable settings and inclusion order

[subdirmk.git] / README

diff --git a/README b/README
index b22927beace665887ce11e60810a4f1c73d09112..cfd7ad03d1853cc3b8e68b6847ee1df079d401ca 100644 (file)
--- a/README
+++ b/README
@@ -28,15 +28,17 @@ Basic approach
 The developer is expected to write a makefile fragment, in each
 relevant subdirectory, called `Subdir.sd.mk'.
 
 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
 
 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
 
 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
 
 The Subdir.sd.mk's are filtered, fed through autoconf in the usual way
 (for @..@-substitutions) and included by one autogenerated toplevel


@@ -83,7 +85,7 @@ 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.
 
 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
 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


@@ -103,6 +105,19 @@ is already provided in subdirmk, for you to reference like this:
   &:include subdirmk/clean.sd.mk
 For example you could put that in Perdir.sd.mk.
 
   &: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
 ------------------
 
 Global definitions
 ------------------
 


@@ -138,6 +153,47 @@ 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.
 
 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
 -------------------
 
 Substitution syntax
 -------------------
 


@@ -163,22 +219,37 @@ empty string).
        variables usually uppercase.  Otherwise, use another syntax:
 
 &_             =>      sub_dir_                        or TOP_
        variables usually uppercase.  Otherwise, use another syntax:
 
 &_             =>      sub_dir_                        or TOP_

-&/             =>      sub/dir/                        or nothing

 &=_            =>      sub_dir                         or TOP
 &=_            =>      sub_dir                         or TOP

+
+&/             =>      sub/dir/                        or nothing

 &=/            =>      sub/dir                         or .
 &=/            =>      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... &
 
 &&             =>      &&              for convenience in shell runes
 \&             =>      &               general escaping mechanism
 
 & thing thing... &

-& ^ thing thing... &
-& ~ thing thing... &
+&, thing thing... &
+&; thing thing... &

        Convenience syntax for prefixing multiple filenames.
        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.
 
        This processing continues until & preceded by lwsp,
        or until EOL (the end of the line), or \ then EOL.
 


@@ -193,7 +264,7 @@ empty string).
 &!<lwsp>       disables & until EOL (and then disappears)
 
 &#     delete everything to end of line
 &!<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
 
 &!STUFF
        changes the escape sequence from & to literally STUFF


@@ -275,6 +346,31 @@ have:
   &:include subdirmk/cdeps.sd.mk
   &:include subdirmk/clean.sd.mk
 
   &: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
 -----------------
 
 Legal information
 -----------------