On 02/11/2010 07:40 PM, Simon Urbanek wrote:
On Feb 11, 2010, at 12:24 PM, Romain Francois wrote:
Thanks.
On 02/11/2010 05:55 PM, Simon Urbanek wrote:
Romain,
I think your'e confusing two entirely different concepts here:
Yes. The name "LinkingTo" probably helped my confusion.
Admittedly, it's probably not the best name ;).
1) LinkingTo: allows a package to provide C-level functions to other packages
(see R-ext 5.4). Let's say package A provides a function foo by calling
R_RegisterCCallable for that function. If a package B wants to use that
function, it uses LinkingTo: and calls R_GetCCallable to obtain the function
pointer. It does not actually link to package A because that is in general not
possible - it simply obtains the pointers through R. In addition, LinkingTo:
makes sure that you have access to the header files of package A which help you
to cast the functions and define any data structures you may need. Since C++ is
a superset of C you can use this facility with C++ as long as you don't depend
on anything outside of the header files.
2) linking directly to another package's shared object is in general not
possible, because packages are not guaranteed to be dynamic libraries. They are
usually shared objects which may or may not be compatible with a dynamic
library on a given platform. Therefore the R-ext describes other way in which
you may provide some library independently of the package shared object to
other packages (see R-ext 5.8). The issue is that you have to create a separate
library (PKG/libs[/arch]/PKG.so won't work in general!) and provide this to
other packages. As 5.8 says, this is in general not trivial because it is very
platform dependent and the most portable way is to offer a static library.
To come back to your example, LinkingTo: A and B will work if you remove
Makevars from B (you don't want to link)and put your hello method into the A.h
header:
Sure. but in real life I can't realistically put everything in the header files.
It was just an example based on your example ;) - which was not very realistic,
either. It practice it is reasonable, because it is sufficient to declare in
the headers whatever you're providing so the only homework is to cast function
pointers you have obtained via R_GetCCallable to the declarations from the
header file.
I suspect what you meant is not as much related to LinkingTo: (since the mess
C++ creates at the binary level is rather hard to pass through dl pointers -
but if someone has a working solution it may be worth to create a package), but
rather to provide a library. That is not covered by R at this point so you're
in realm of R-ext 5.8. Given how non-trivial task this is (to get it right) it
may be worthwhile thinking about a portable solution and add it to R, but I
don't think anyone has done that yet (mainly due to the low benefit/cost ratio
I suspect). For all cases so far it was sufficient to create C or R level API
for other package to use.
Cheers,
Simon
Yes. The goal is to provide a library that other packages can just use.
I thought LinkingTo would help, but now I guess not. or maybe just so
that B can find headers of A.
The R_RegisterCCallable/R_GetCCallable business seems to be only
applicable when you are developping both A and B and you deal with plain
functions, otherwise the name mangling, overloading of methods, etc
would make the task hard and not fun at all.
So I guess, yes the question is how to reliably and portably provide a
library for package A so that package B can just use it. This would be
very valuable.
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/NrTG : Rcpp 0.7.5
|- http://tr.im/MPYc : RProtoBuf: protocol buffers for R
`- http://tr.im/KfKn : Rcpp 0.7.2
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
