R help - Nov 2009 - When is a function call an expression? (multicore package)

I'm trying to understand how to use the multicore package.  In
particular, I'm trying to work out what is covered where it says
this in the help file for the parallel function:

    expr: expression to evaluate (do _not_ use any on-screen devices or
          GUI elements in this code)

Can a function that produces an EPS file be considered an "expression"
in this case?  I've tried to run two similar functions in parallel but
I'm obviously not using the term "expression" the way it's
intended.

I've made a trivial dummy function:

dummy <- function() {
  require(multicore)
  aa <- function() {
    postscript(file = "AA.eps", height = 220/25.4,
               width = 220/25.4, paper = "special", pointsize = 15)
    plot(1:10)
    dev.off()
  }
  bb <- function() {
    postscript(file = "BB.eps", height = 220/25.4,
               width = 220/25.4, paper = "special", pointsize = 15)
    plot(11:20)
    dev.off()
  }
  aaa <- mcparallel(aa())
  bbb <- mcparallel(bb())
}


The AA.eps file is created, but nothing else.  If I try a more
elaborate function, I get even less.

I tried using collect() after the two calls to mcparallel which gave
this puzzling message:

Error in !wait : invalid argument type
> Xlib: unexpected async reply (sequence 0x80)!
Am I trying to do something multicore wasn't designed to do, or am I
just going about it the wrong way?

******************************************
> sessionInfo()
R version 2.10.0 (2009-10-26)
i686-pc-linux-gnu

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=C              LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] splines   grDevices utils     stats     graphics  methods   base

other attached packages:
[1] multicore_0.1-3 gbm_1.6-3       survival_2.35-7 lattice_0.17-26

loaded via a namespace (and not attached):
[1] grid_2.10.0  plyr_0.1.9   tools_2.10.0
>
*******************************************************

TIA

Patrick






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