warn on data pointers of zero-length vectore

git-svn-id: https://svn.r-project.org/R/trunk@88219 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/manual/R-exts.texi

@@ -14639,6 +14639,17 @@ directly would violate assumptions the memory manager depends on.
 @code{DATAPTR_RO} returns a generic read-only data pointer for any
 vector object.
 
+@strong{N.B.} These will return a valid data pointer only for vectors of
+positive length. Zero-length vectors have no `data' and these accessors
+will usually return an invalid pointer, for example to address
+@code{0x000000000001}.  So usages such as
+@example
+memcpy(REAL(newx), REAL_RO(x), LENGTH(x) * sizeof(double));
+@end example
+@noindent
+are undefined behaviour without a prior check on the length of @code{x}.
+@c Returning 0x000000000001 was inroduced in R 4.5.0.
+
 Formerly it was possible for packages to obtain internal versions of
 some accessors by defining @samp{USE_RINTERNALS} before including
 @file{Rinternals.h}. This is no longer the case. Defining