I can't see any mention in the docs of what happens when a character string NA is passed to an external function. What appears to happen is that it is converted to the two letter string 'NA'; I can't think of anything better to do, but this should be mentioned somewhere (or perhaps more prominently). The places I thought to look were the .C help file, and section 4.2 of the Writing Extensions manual. Duncan Murdoch -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-devel mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-devel-request@stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._