+Dollar doubling and macro assistance
+------------------------------------
+
+&$+ Starts dollar-doubling
+&$- Stops dollar-doubling
+ Both are idempotent and local to the file or context.
+
+This is useful both for make macrology involving $(eval ...), and
+possibly for helping write complicated recipes involving shell
+variables, inline Perl code, etc.
+
+Sometimes we will show $'s being doubled inside another construct.
+This means the content of the construct is $-doubled: $-doubling is
+locally enabled, and restored afterwards.
+
+&:macro NAME => define NAME
+STUFF $ THINGS .. STUFF $$ THINGS
+&:endm .. endef
+ NAME is processed for &
+
+&{..$..} => ${eval ${call ..$$..}}
+ (matches { } pairs to find the end)
+ content is $-doubled (unless it contains &$- to turn that off)
+ cf &(...), see "Convenience syntax for eval", below.
+
+Together &:macro and &${...} provide a more reasonable macro facility
+than raw make. They solve the problem that make expansions cannot
+directly generate multiple rules, variable, etc.; instead, `$(eval )'
+must be used, but that re-expands the argument, meaning that all the
+literal text must be $-doubled. This applies to the macro text and to
+the arguments. Also `$(eval $(call ...))' is an unfortunate syntax.
+Hence &:macro and &${...}.
+
+While dollar-doubling:
+- - - - - - - - - - -
+
+$ => $$ including $'s produced by other
+ &-expansions not mentioned here
+
+&\$ => $
+&$( => $(
+&$NN => ${NN} where N are digits
+
+A few contexts do not support $-doubling, such as directive arguments
+or places where this might imply $-quadrupling. (There is no way to
+get $-quadrupling.)
+
+Convenience syntax for eval
+- - - - - - - - - - - - - -
+
+&(thing => $(call thing
+&( thing => $(call thing
+ and specially:
+&(&lc => $(call sub_dir_lc or $(call TOP_lc
+&( &lc => $(call sub_dir_lc or $(call TOP_lc
+ even though lc would normally be thought a filename
+
+Unlike &{...}, this does not involve any dollar-doubling.
+
+
+Invocation, "recursive" per-directory targets
+---------------------------------------------
+
+Arrangements are made so that when you run `make foo' in a
+subdirectory, it is like running the whole toplevel makefile, from the
+toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
+be built, that builds it.
+
+But `foo' can also be a conventional target like `all'.
+
+Each subdirectory has its own `all' target. For example a
+subdirectory `src' has a target `src/all'. The rules for these are
+automatically generated from the settings of the per-directory
+&TARGETS variables. &TARGETS is magic in this way. (In
+src/Dir.sd.mk, &TARGETS of course refers to a make variable called
+src_TARGETS.)
+
+The `all' target in a parent directory is taken to imply the `all'
+targets in all of its subdirectories, recursively. And in the
+autogenerated stub Makefiles, `all' is the default target. So if you
+just type `make' in the toplevel, you are asking for `&all'
+(<subdir>/all) for every directory in the project.
+
+In a parallel build, the rules for all these various subdirectory
+targets may be in run in parallel: there is only one `make' invocation
+at a time. There is no sequencing between subdirectories, only been
+individual targets (as specified according to their dependencies).
+
+You can define other per-directory recursive targets too: set the
+variable &TARGETS_zonk, or whatever (being sure to write &TARGETS_zonk
+at the start of a line). 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 set in src but not lib, `make
+zonk' in lib will fail. If you want to make a target exist
+everywhere, += it with nothing in Prefix.sd.mk or Suffix.sd.mk (see
+below).
+
+Prefix.sd.mk, Suffix.sd.mk, Final.sd.mk, inclusion
+--------------------------------------------------
+
+The files Prefix.sd.mk and Suffix.sd.mk in the toplevel of the source
+are automatically processed before and after each individual
+directory's Dir.sd.mk, and the &-substituted contents therefore
+appear once for each subdirectory.
+
+This lets you do per-directory boilerplate. Some useful boilerplate
+is already provided in subdirmk, for you to reference like this:
+ &:include subdirmk/cdeps.sd.mk
+ &:include subdirmk/clean.sd.mk
+For example you could put that in Suffix.sd.mk.
+
+The top-level Dir.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 Dir.sd.mk.
+
+The file Final.sd.mk in the toplevel directory is processed and
+the result included after all the other files. Its subdirmk
+filtering context inherits warning suppressions from the toplevel's
+Dir.sd.mk etc., but not anything else.
+
+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)).
+
+Warnings
+--------
+
+subdirmk's `generate' program, which does the acual &-substitution,
+can produce some warnings about your .sd.mk files. These can be
+suppressed with the &:warn directive. The warning tags are:
+
+ local+global
+ The same VARNAME was used both with and without an & prefix.
+ This can be confusing. Also, if you avoid this then you will
+ get a warning iff you accidentally leave off a needed &.
+ The generation of this warning depends on scanning your
+ makefile for things that look like variable references, which
+ subdirmk does not do completely perfectly. Exciting make
+ syntax may evade this warning, or require suppressions.
+ (You can suppress this warning for a particular VARNAME with
+ the &:local+global directive.)
+
+ single-char-var
+ A variable expansion like $FBAR. make's expansion rules
+ interpret this as $(F)BAR. It's normally better to write
+ it this way, at least if the variable expansion is followed
+ by more letters. Note that &$FOO works differently to
+ raw make: it expands to ${sub_dir_FOO}.
+
+ broken-var-ref
+ An attempt at variable expansion looking like $&...
+ You probably expected this to mean $(TOP_F)BAR but it
+ expands to $TOP_FBAR which make thinks means $(T)OP_FBAR.
+
+ unknown-warning
+ &:warn was used to try to enable a warning that this version
+ of subdirmk does not understand. (Note that an attempt to
+ *dis*able an unknown warning is only reported if some other
+ warning was issued which might have been disabled.)
+
+
+Guides, hints, and further explanations
+=======================================
~ian / subdirmk.git / blobdiff