From simon.urbanek at r-project.org  Mon May 23 19:30:48 2011
From: simon.urbanek at r-project.org (Simon Urbanek)
Date: Mon, 23 May 2011 13:30:48 -0400
Subject: [Rd] Calling Rscript from Makevars
In-Reply-To: <CA000A16.4362%sean.mcguffee@gmail.com>
References: <CA000A16.4362%sean.mcguffee@gmail.com>
Message-ID: <97C76D4A-77B3-4767-AF81-73E1F04DB611@r-project.org>


On May 23, 2011, at 12:56 PM, Sean Robert McGuffee wrote:

> I'm not sure what you mean by, "any tests you run in configure will ignore it since autoconf only uses LIBS and not PKG_LIBS." I though autoconf used any variable names you tell it to, so that it would use PKG_LIBS if you tell it to. 
> 

No, many variables have a special meanings in autoconf, such as CC, CPP, CFLAGS, LIBS, LDFLAGS etc. When you run tests like AC_PROG*, AC_HEADER*, AC_CHECK_*, AC_COMPILE*, AC_LINK* then those use (and set) only the variables supported by configure and they have no idea about things like PKG_LIBS. So as far as autoconf is concerned PKG_LIBS has no effect.

Because R has its own building process, using autoconf with packages means you have to a) set autoconf's flags to match the R configuration before you do anything, b) use autoconf tests which implies using autoconf's flags, c) map resulting autoconf flags to special R package flags. If you skip any of a,b,c the result will not be reliable. In theory, you can handle R's flags and autoconf flags in parallel, i.e., updating both in the success branch of each test, but that's a bit too tedious to implement (you can't use the autoconf default actions) and unnecessary.


> Also, I'm still not clear as to what a Makevars file is. To clarify, I do understand a Makefile. When GNU's make program is run, it looks for a Makefile and interprets it in ways that are documented very well by GNU. I have yet to find any lead as to what a Makevars file is, what to put in it, what it does, or how it helps in installation with R packages. I can understand how to define variables in it, but the only way I know how to use variables that are defined is by using them in a Makefile. Where and how are variables defined in Makevars used?
> 

Makevars is simply a file included in R's Makefile when it is building the package. So, technically, it is a makefile. The difference between Makevars and Makefile is that Makevars doesn't need to specify any rules or variables that R already knows about, because they are already included by R. So it is much more convenient since it saves you a lot of trouble trying to setup the correct rules for compilation, linking etc. At the same time, it is a makefile so you can add targets, modify dependencies etc., you're not constrained to just setting variables although that is its primary use.

In practice Makevars can be used in several ways:

a) just set PKG_CFLAGS, PKG_LIBS
this is the most common use and it leaves the standard targets in place so R will use those to compile $(SHLIB) automatically

b) include additional targets:
if you have additional features to build, you can specify them in a target, for example this is form rJava:

all: $(SHLIB) @WANT_JRI_TRUE@ jri
jri: ... 

It builds the standard $(SHLIB) which is loaded by the package, but it also builds a separate target "jri" if enabled in configure

c) include dependencies:
you can add dependent builds that are needed for your $(SHLIB), for example from RSQLite:

all: $(SHLIB)
$(SHLIB): do_includes
do_includes:

This makes sure that do_includes is built before R attempts to build the .so/.dll


So, as you can see you Makevars gives you the flexibility of Makefile but without the hassle.

Cheers,
Simon



[Rd] Calling Rscript from Makevars
Simon Urbanek