make a public `am_path`, more docs

1 files changed, 78 insertions, 18 deletions

diff --git a/automake.txt b/automake.txt
index 22a0b84..b6b9184 100644
--- a/automake.txt
+++ b/automake.txt
@@ -10,6 +10,34 @@ differently.  Yeah, I need a new name for it.
 High-level overview
 -------------------
 
+Now, what this does for you is:
+
+It makes it _easy_ to write non-recursive Makefiles--and ones that are
+similar to plain recursive Makefiles, at that! (search for the paper
+"Recursive Make Considered Harmful")  As harmful as recursive make is,
+it's historically been difficult to to write non-recursive Makefiles.
+This makes it easy.
+
+It also makes it easy to follow the GNU standards for your makefiles:
+it takes care of this entire table of .PHONY targets for you:
+
+| this | and this         | are aliases for this                                   |
+|------+------------------+--------------------------------------------------------|
+| all  | build            | $(outdir)/build                                        |
+|      | install          | $(outdir)/install                                      |
+|      | uninstall        | $(outdir)/uninstall                                    |
+|      | mostlyclean      | $(outdir)/mostlyclean                                  |
+|      | clean            | $(outdir)/clean                                        |
+|      | distclean        | $(outdir)/distclean                                    |
+|      | maintainer-clean | $(outdir)/maintainer-clean                             |
+|      | check            | $(outdir)/check (not implemented for you)              |
+|      | dist             | $(topoutdir)/$(PACKAGE)-$(VERSION).tar.gz (not .PHONY) |
+
+(You are still responsible for implementing the `$(outdir)/check`
+target in each of your Makefiles.)
+
+What you have to do is:
+
 In each source directory, you write a `Makefile`, very similarly to if
 you were writing for plain GNU Make, with
 
@@ -21,30 +49,40 @@ you were writing for plain GNU Make, with
     
     include $(topsrcdir)/automake.tail.mk
 
-Write your own `common.{each,once}.{head,tail}.mk` files that get
-included:
+And in the top-level output directory, you write a `config.mk` with:
+
+    ifeq ($(topsrcdir),)
+    # have your ./configure script adjust topsrcdir if doing an
+    # out-of-tree build
+    topsrcdir := $(patsubst %/,%,$(dir $(lastword $(MAKEFILE_LIST))))
+
+    # your configuration
+
+    endif
+
+And in the top-level source directory, Write your own helper makefiles
+that get included:
  - `common.once.head.mk`: before parsing any of your Makefiles
  - `common.each.head.mk`: before parsing each of your Makefiles
  - `common.each.tail.mk`: after parsing each of your Makefiles
  - `common.each.tail.mk`: after parsing all of your Makefiles
 
-Here is a table of all of the .PHONY targets that automake takes care
-of for you:
+The `common.*.mk` makefiles are nice for including generic pattern
+rules and variables that aren't specific to a directory.
 
-| this | and this         | are aliases for this                                   |
-|------+------------------+--------------------------------------------------------|
-| all  | build            | $(outdir)/build                                        |
-|      | install          | $(outdir)/install                                      |
-|      | uninstall        | $(outdir)/uninstall                                    |
-|      | mostlyclean      | $(outdir)/mostlyclean                                  |
-|      | clean            | $(outdir)/clean                                        |
-|      | distclean        | $(outdir)/distclean                                    |
-|      | maintainer-clean | $(outdir)/maintainer-clean                             |
-|      | check            | $(outdir)/check (not implemented for you)              |
-|      | dist             | $(topoutdir)/$(PACKAGE)-$(VERSION).tar.gz (not .PHONY) |
+You're probably thinking that this sounds too good to be true!
+Unfortunately, there are two major deviations from writing a plain
+recursive Makefile:
 
-You are responsible for implementing the `$(outdir)/check` target in
-each of your Makefiles.
+ 1. all targets and prerequisites (including .PHONY targets!) need to
+    be prefixed with
+    `$(srcdir)`/`$(outdir)`/`$(topsrcdir)`/`$(topoutdir)`.
+      * sub-gotcha: this means that if a pattern rule has a
+        prerequisite that may be in srcdir or outdir, then it must be
+        specified twice, once for each case.
+ 2. if a prerequisite is in a directory "owned" by another Makefile,
+    you must filter the pathname through `am_path`:
+    `$(call am_path,YOUR_PATH)`.
 
 Telling automake about your program
 -----------------------------------
@@ -53,6 +91,11 @@ You tell automake what to do for you by setting some variables.  They
 are all prefixed with `am_`; this prefix may be changed by editing the
 `_am` variable at the top of `automake.head.mk`.
 
+The exception to this is the `am_path` variable, which is a macro that
+is used to make a list of filenames relative to the appropriate
+directory, because unlike normal GNU (Auto)Make, $(outdir) isn't
+nescessarily equal to '.'.  See above.
+
 There are several commands that generate files; simply record the list
 of files that each command generates as the following variable
 variables:
@@ -97,9 +140,26 @@ between directories:
 |            | files in this directory, things in the dependency       |
 |            | directory will not be built.                            |
 
+Tips, notes
+-----------
+
+If you have a `./configure` script, don't have it modify the
+`Makefile`s; have everything you need modified be in
+`$(topoutdir)/config.mk` and have it generate that; then have it copy
+(or (sym?)link?) every `$(srcdir)/Makefile` into `$(outdir)/Makefile`.
+
+If you're wondering, `am_path` is defined equivalently to:
+
+    am_path = $(if $1,$(shell realpath -sm -- $1))`
+
+though it is implemented purely in Make, instead of calling out to
+another program.  Besides that older versions of coreutils don't have
+`realpath`, calling to an external program like that can have a
+_substantial_ slowdown on the parse time.
+
 ----
 Copyright (C) 2016  Luke Shumaker
 
 This documentation file is placed into the public domain.  If that is
 not possible in your legal system, I grant you permission to use it in
-absolutely every way that I can legally do so.
+absolutely every way that I can legally grant to you.