[R] MC_CORES and mc.cores for parallel package

[Marc Girondot]

Hi,

 From the documentation of ?options

Options set in package parallel
These will be set when package parallel (or its namespace) is loaded if 
not already set.

mc.cores:
a integer giving the maximum allowed number of additional R processes 
allowed to be run in parallel to the current R process. Defaults to the 
setting of the environment variable MC_CORES if set. Most applications 
which use this assume a limit of 2 if it is unset.

Then I try:
 > getOption("mc.cores")
NULL

I suspect that my environment variable MC_CORES is not set. I test and 
that's right:
 > Sys.getenv("MC_CORES")
[1] ""

Then I do:
 > Sys.setenv(MC_CORES=4)
 > Sys.getenv("MC_CORES")
[1] "4"

But when I try again getOption("mc.cores"); it does not change:
 > getOption("mc.cores")
NULL

Probably I do something wrong but I don't see what !

Thanks

Marc