explict sharing of symbols between packages

[Duncan Temple Lang]

Oops. Thought I was postponing that mail and sent it instead.
So here is the alternative approach, i.e. b)

Rather than use the linker to connect the calls to the common routines
and their definitions in a secondary library (e.g. libCommon.so), you
can resolve and set the symbols yourself.  Specifically, you use
function pointers in the code that calls the shared routines and then
when you load the packages, you arrange to have these variables refer
to the actual routines that you want.

For example, suppose we have two packages - A and B and A has a
routine named foo() that we want to call in the routine bar() located
in B.

So A.c has
void
foo(void)  {
  ...
}

And B.c has

void (*foo_ptr)(void);

void 
bar()
{
  /* Want to call bar() */
 (*foo_ptr)();
}

Given this setup, you need to have an initialization
routine in B.c that sets the value of the variable foo_ptr.
There are two ways to do this.
You can call R_FindSymbol() within C code.

Using the initialization routine in package B
void R_init_B(DllInfo *dll)
{
   foo_ptr = R_FindSymbol("foo", "A", NULL);
}


Alternatively, you can do the lookup in R and pass
the address of foo() as an argument to a routine in B.

a <- getNativeSymbolInfo("foo", PACKAGE = "A")
.Call("setFoo", a,  PACKAGE = "B")

And the C routine setFoo()  looks something like

SEXP setFoo(SEXP address)
{
  foo_ptr = R_ExternalPtrAddr(address);
  return(R_NilValue);
}


This approach, compared to linking libraries, allows you to
dynamically change routines. This can be useful but is probably
overkill for your example. However, this approach does not rely 
on linkers idiosyncrasies.

 Hope this helps.

  D.






-- 
_______________________________________________________________

Duncan Temple Lang                duncan@research.bell-labs.com
Bell Labs, Lucent Technologies    office: (908)582-3217
700 Mountain Avenue, Room 2C-259  fax:    (908)582-3340
Murray Hill, NJ  07974-2070       
         http://cm.bell-labs.com/stat/duncan
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