Mark H Weaver authored Jan 05, 2016 and Andy Wingo committed Dec 18, 2016

Implements wishlist item <https://debbugs.gnu.org/18592>.
Requested by Frank Terbeck <ft@bewatermyfriend.org>.
Based on a proposed patch by Nala Ginrut <nalaginrut@gmail.com>.
Patch ported to 2.2 by Andy Wingo <wingo@pobox.com>.

* libguile/foreign.c (cif_to_procedure): Add 'with_errno' argument.
  If true, truncate result to only one return value.
  (scm_i_foreign_call): Separate the arguments.  Always return errno.
  (pointer_to_procedure): New static function.
  (scm_pointer_to_procedure_with_errno): New C API function, implemented
  in terms of 'pointer_to_procedure'.
  (scm_pointer_to_procedure): Reimplement in terms of
  'pointer_to_procedure', no longer bound to "pointer->procedure".  See
  below.
  (scm_i_pointer_to_procedure): New C function bound to
  "pointer->procedure" which now accepts the optional #:return-errno?
  keyword argument, implemented in terms of 'pointer_to_procedure'.
  (k_return_errno): New keyword #:return-errno?.
* libguile/foreign.h (scm_pointer_to_procedure_with_errno): Add prototype.
* doc/ref/api-foreign.texi (Dynamic FFI): Adjust documentation.
* libguile/vm-engine.c (foreign-call): Return two values.