X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ian/git?p=subdirmk.git;a=blobdiff_plain;f=subdirmk%2FREADME;h=e6da9eecd1f68b68ab930560236e802a741fb3cf;hp=7cf84cf756597db3b2073c4c158172456fca59ed;hb=e5c38d3c255e19c04378ef5358426ca827ff3282;hpb=3d62132e874bd2ceee3fb5960a172beb4594d6c9 diff --git a/subdirmk/README b/subdirmk/README index 7cf84cf..e6da9ee 100644 --- a/subdirmk/README +++ b/subdirmk/README @@ -1,40 +1,240 @@ -&CAPS => subdir_CAPS or TOP_CAPS -&lc => subdir/lc or lc +subdirmk - assistance for non-recursive use of make +=================================================== -&_ => subdir_ or TOP_ -&/ => subdir/ or nothing -&=_ => subdir or TOP -&=/ => subdir or . -&^ => $(top_srcdir)/subdir or $(top_srcdir) -&~ => $(abs_top_srcdir)/subdir or $(abs_top_srcdir) +Introduction +------------ -&& => && -\& => & +Peter Miller's 1997 essay _Recursive Make Considered Harmful_ +persuasively argues that it is better to arrannge to have a single +make invocation with the project's complete dependency tree, rather +than the currently conventional `$(MAKE) -C subdirectory' approach. -& thing thing... & => each thing prefixed by &/ &^/ &~/ resp -& ^ thing thing... & each thing is any non-ws -& ~ thing thing... & & may be omitted before EOL or before \EOL - other &'s not recognised +However, actually writing a project's build system in a non-recursive +style is not very ergonomic. The main difficulties are: + - constantly having to write out long file and directory names + - the lack of a per-directory make variable namespace means + long make variables (or namespace clashes) + - it is difficult to arrange that one can cd to a subdirectory + and say `make all' and have something reasonable happen + (to wit, build an appropriate subset) -CAPS is [A-Z][0-9_A-Z]*(?!\w) -lc is [a-z][-+,0-9_a-z]*(?!\w) +`subdirmk' is an attempt to solve these problems (and it also slightly +alleviates some of the boilerplate needed to support out-of-tree +builds well). -&! disables & *until* EOL (and disappears) +Basic approach +-------------- -&!STUFF STUFF is recognised instead of & - the terminating lwsp is discarded too - may also occur at eol +The developer is expected to write a makefile fragment, in each +relevant subdirectory, called `Subdir.sd.mk'. -eg notably - STUFF!& now & is recognised instead (ie back to normal) - STUFFSTUFF STUFF +These fragments may contain ordinary make language. -eg - &!@@@ @@@ is recognised instead of & - @@@!& go back to & +However, the sigil & is treated specially. By and large, it refers to +`the current directory'. There are a variety of convenient +constructions. -&TARGETS[_things] is handled specially - must be spelled precisely this way - if no _things, means _all +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. -Also, `all' is weird in that it is present even if not specified +The Subdir.sd.mk's are filtered, fed through autoconf in the usual way +(for @..@-substitutions) and included by one autogenerated toplevel +makefile. + +So all of the input is combined and passed to one make invocation. (A +corollary is that there is no enforcement of the namespacing: +discipline is required to prefix relevant variable names with &, etc. + +Each subdirectory is also provided with an autogenerated `Makefile' +which exists purely to capture ordinary make invocations and arrange +for something suitable to happen. + +Where there are dependencies between subdirectories, each Subdir.sd.mk +can simply refer to files in other subdirectories directly. + +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/Subdir.sd.mk, &TARGES 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' +(/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. + +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. +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 +lib will fail. If you want to make a target exist everywhere, +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 +processed after each individual directory's Subdir.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 Perdir.sd.mk. + +Global definitions +------------------ + +If want to set global variables, such as CC, that should only be done +once. You can put them in your top-level Subdir.sd.mk, or a separate +file you `include' and declare using SUBDIRMK_MAKEFILES. + +Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in' +-------------------------------------------------------------------- + +There are two kinds of template files. + + Filename .sd.mk .mk.in + + Processed by &-substitution, autoconf only + then autoconf + + Instantiated Usu. once per subdir Once only + + Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/ + in configure.ac? via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES + + How to include `&:include foo.sd.mk' `include foo.mk' + in all relevant .sd.mk in only one + (but not needed for Subdir.sd.mk + Subdir and Perdir) + +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. + +Substitution syntax +------------------- + +In general & expands to the subdirectory name when used for a +filename, and to the subdirectory name with / replaced with _ for +variable names. + +Note that & is processed *even in makefile comments*. The substitutor +does not understand make syntax, or shell syntax, at all. However, +the substitution rules are chosen to work well with constructs which +are common in makefiles. + +In the notation below, we suppose that the substitution is being in +done in a subdirectory sub/dir of the source tree. In the RH column +we describe the expansion at the top level, which is often a special +case (in general in variable names we call that TOP rather than the +empty string). + +&CAPS => sub_dir_CAPS or TOP_CAPS +&lc => sub/dir/lc or lc + Here CAPS is any ASCII letter A-Z and lc is a-z. + The assumption is that filenames are usually lowercase and + variables usually uppercase. Otherwise, use another syntax: + +&_ => sub_dir_ or TOP_ +&/ => sub/dir/ or nothing +&=_ => sub_dir or TOP +&=/ => sub/dir or . +&^ => $(top_srcdir)/sub/dir or $(top_srcdir) +&~ => $(abs_top_srcdir)/sub/dir or $(abs_top_srcdir) + +&& => && for convenience in shell runes +\& => & general escaping mechanism + +& thing thing... & +& ^ thing thing... & +& ~ thing thing... & + 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. + This processing continues until a & preceded by lwsp, + or until EOL (the end of the line), or \ then EOL. + +&: .... + recognised at start of line only (possibly after lwsp) + args are processed for & + +&:include filename filename should usually be foo.sd.mk +&:-include filename tolerate nonexistent file + +&! disables & until EOL (and then disappears) + +&!STUFF + changes the escape sequence from & to literally STUFF + STUFF may be any series of of non-whitespace characters, + and is terminated by EOL or lwsp. STUFF and the lwsp + is discarded. + + After this, write STUFF instead of &, everywhere. + The effect is global and lasts until the next setting. + It takes effect on &:include'd files too, so maybe set + it back before using &:include. + + Notably + STUFFSTUFF => STUFF + \STUFF => STUFF + STUFF!& set escape back to & + +&TARGETS_things + Handled specially. If mentioned, declares that + this subdirectory ought to have a target `things'. + (`all' if not specified). The rule will be + &/things:: $(&TARGETS_things) + + You may extend it by adding more :: rules for the target, + but the preferred style is to do things like this: + &TARGETS_check += & test-passed.stamp + + It is important to mention &TARGETS_things at least once in + the context of each applicable directory, because it arranges + that the *parent* will also have a `things' target which + recursively implies this directory's `things'. + + Must be spelled exactly &TARGETS_things. &_TARGETS_things, + for example, does not work. But mentioning it in a #-comment + *does* work because the & filter does not care about comments. + + `all' is extra special: every directory has an `all' + target, which corresponds to &TARGETS. + +Subdirectory and variable naming +-------------------------------- + +The simple variable decoration scheme does not enforce a strict +namespace distinction between parts of variable names which come from +subdirectory names, and parts that mean something else. + +So it is a good idea to be a bit careful with your directory naming. +`TOP', names that contain `_', and names that are similar to parts of +make variables (whether conventional ones, or ones used in your +project) are best avoided. + +If you name your variables in ALL CAPS and your subdirectories in +lower case with `-' rather than `_', there will be no confusion.