[cffi-devel] Re: Returning errno as a second value

[Stelian Ionescu]

On Tue, 2014-01-28 at 16:11 +0100, Stelian Ionescu wrote:
> On Tue, 2014-01-28 at 15:33 +0100, Felix Filozov wrote:
> > Hi Luís,
> > I am hoping to resurrect this thread. Did you by any chance take a
> > look at the pull request I made a few months ago relating to this
> > feature?
> 
> I strongly object to anything that slows down the common call path,
> which is the case here because capture-errno is always called when not
> using libffi.
> 
> In addition, IMO this is a hairy-enough case that we should ask the
> implementors to add this feature and not do it ourselves.
> 
> As for the interface, the simplest thing might be to add it as last
> value - not second because custom foreign types can return multiple
> values, e.g. string+ptr - but that will turn out to be slow because I
> don't think our current type system knows about arity of custom types
> and marshalling unknown numbers of multiple values will cons.

For example

(cffi:defcfun foo :string+ptr (a :int))

on SBCL expands to

(DEFUN FOO (A)
   (LET ((#:G1087 A))
     (CFFI:TRANSLATE-FROM-FOREIGN
      (CFFI-SYS:%FOREIGN-FUNCALL "foo" (:INT #:G1087 :POINTER)
                                 :CONVENTION :CDECL :LIBRARY :DEFAULT)
      #<CFFI::FOREIGN-STRING+PTR-TYPE :UTF-8>)))

Since translate-from-foreign is in a tail position we currently don't
care about the number of values it returns, but with errno and knowledge
of multiple values we must make it expand to this:

(defun foo (a)
  (let ((#:g1087 a))
    (multiple-value-bind (ret errno)
        (cffi-sys:%foreign-funcall "foo" (:int #:g1087 :pointer)
                                   :convention :cdecl :library :default)
      (multiple-value-bind (val0 val1)
          (cffi:translate-from-foreign
            ptr #<cffi::foreign-string+ptr-type :utf-8>)))
    (values val0 val1 errno)))

-- 
Stelian Ionescu a.k.a. fe[nl]ix
Quidquid latine dictum sit, altum videtur.

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