[Rd] read.table() code fails outside of the utils package

[Simon Urbanek]

Andrew,

On Apr 21, 2014, at 11:53 AM, Andrew Piskorski <atp at piskorski.com> wrote:

> One of the great things about R is how readable and re-usable much of
> its own implementation is.  If an R function doesn't do quite what you
> want but is close, it is usually very easy to read its code and start
> adapting that as the base for a modified version.
> 
> In the 2.x versions of R, that was the case with read.table().  It was
> easy to experiment with its source code, as it all worked just fine
> when run at the top level or from inside any other package.
> 
> In R 3.1.0, that is no longer true.  The read.table() source ONLY works
> when run from inside the "utils" package.  The (only) culprit is this:
> 
>  .External(C_readtablehead, file, 1L, comment.char, blank.lines.skip, quote, sep, skipNul)
> 
> Older versions of read.table() instead did this, which ran fine from
> any package; this entry point no longer exists:
> 
>  .Internal(readTableHead(file, nlines, comment.char, blank.lines.skip, quote, sep)) 
> 
> The C implementation of readTableHead is in utils.so, but the symbol
> is marked as local.

And that's how it should be - there is not reason why any other code should link to it. Why don't you just use

.External(utils:::C_readtablehead, ...)

if you need to call it?

Cheers,
Simon


>  .External(C_readtablehead, file, 1L, comment.char, blank.lines.skip, quote, sep, skipNul)
>  I tried adding "attribute_visible" to its
> function definition in "src/library/utils/src/io.c" and recompiling,
> which DID make the symbol globally visible.  With that change, my own
> C code works just fine when calling readTableHead.  But interestingly,
> R code using .External() like this still fails:
> 
>   .External("readtablehead", ..., PACKAGE="utils") 
>   Error: "readtablehead" not available for .External() for package "utils" 
> 
> Why is that?  Apparently the C symbol being visible isn't enough, but
> what else is needed for .External() to work?
> (Clearly there's something here about how R C programming works that I
> don't understand.)
> 
> Finally, since it is generally useful to be able to experiment with
> and re-use parts of the stock read.table() implementation, I suggest:
> 
> 1. R add "attribute_visible" or otherwise make readtablehead callable
>   from user C code.
> 2. R make readtablehead callable from user R code via .External().
> 
> What do you think?  Note that I'm not asking that the current
> interface or behavior of readtablehead necessarily be SUPPORTED in any
> way, just that it be callable for experimental purposes, much as the
> old .Internal(readTableHead()) was in earlier versions of R.
> 
> -- 
> Andrew Piskorski <atp at piskorski.com>
> 
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