[R] How to detach binary objects/libraries?

[Prof Brian D Ripley]

On 25 Mar 2002, Peter Dimitrov wrote:

> On Mon, 2002-03-25 at 13:46, ripley at stats.ox.ac.uk wrote:
> > On 25 Mar 2002, Peter Dimitrov wrote:
> >
> > > Hi all,
> > >
> > > First, I want to apologize if this question has been already answered. I
> > > have a RedHat 7.2. system with recently patched version of R-1.4.1. I'm
> > > in a process of writing R package, say X, that loads some C++ code.
> > > Currently, while adding functionality to it in both R and C++ sources,
> > > I'm getting crashes every time I detach/load/access the C++ function(s).
> > > Typical transcript follows:
> > >
> > > > library(X)
> > > Loading required package: mva
> > > > is.loaded("exp_dist_fast") # The name of the C stub function
> > > [1] TRUE
> > > > detach(package:X)
> > > > is.loaded("exp_dist_fast")
> > > [1] TRUE # Problem?
> > > > library(X) # Load X after changes in the C/C++ code
> > > > is.loaded("exp_dist_fast") # or .C("exp_dist_fast",...)
> > >
> > > Process R:1 segmentation fault at Mon Mar 25 12:17:57 2002
> > >
> > > What is the way to cleanly unload the object code and is there better
> > > mechanism than detach(package:X)?
> >
> > Does you package use .Last.lib to unload your shared library, X.so?
> > If not, that will probably be your problem, as library.dynam will not
> > reload it (and you probably use library.dynam).
> >
> > Take a look at package tcltk for an example.
>
> Thank you, Prof. Ripley!
>
> Adding .Last.lib solved the segmentantion fault problem. On the other
> hand, it created the following puzzle. Transcript follows:
>
> > library(X)
> Loading required package: mva
> > is.loaded("exp_dist_fast")
> [1] TRUE
> > detach(package:X)
> > is.loaded("exp_dist_fast")
> [1] FALSE
> > library(X)
> > is.loaded("exp_dist_fast")
> [1] FALSE
> > .C("exp_dist_fast",...)
> Error in .C("exp_dist_fast", ...),  :
> 	C/Fortran function name not in load table
> > detach(package:X)
> Error in dyn.unload(x) : dynamic/shared library
> "/opt/lib/R/library/X/libs/X.so" was not loaded
>
> X's zzz.R follows:
>
> .First.lib <- function( lib, pkg )
>   library.dynam( "X", pkg, lib )
>
> .Last.lib <- function( libpath )
>   dyn.unload(
>              file.path( libpath,
>                        "libs",
>                        paste( "X", .Platform$"dynlib.ext", sep = "" )
>                        )
>              )
>
> Is the solution just to change library.dynam with dyn.load( file.path(
> ..))?

No, it is to follow the example I pointed you too.  You need code like

.Last.lib <- function(libpath) {
    if(is.loaded(symbol.C("tcltk_end"))) {
        .C("tcltk_end", PACKAGE="tcltk")
        dyn.unload(file.path(libpath, "libs", "tcltk.dll"))
        num <- match("tcltk", get(".Dyn.libs", envir = NULL))
        assign(".Dyn.libs",
               get(".Dyn.libs", envir = NULL)[-num],
               envir = NULL)
    }
}


> > I'm not sure what Linux does if you change a shared library that is in
> > use, but it is not a good idea (and fatal on Solaris, for example).
>
> It's probably quite wrong, but I was hoping that it will work much the
> same way as in sourcing the original not-yet-in-a-package R file that
> dynamically loads X.so. In Linux it reloads X.so without a problem.

But my point was: what does changing the file copy of an already loaded
shared library do?   It's not allowed on Windows, and on Solaris it will
lead to a crash.  I don't know about Linux, as I would never try it
given my experience.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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