r - Modify variables outside function using mclapply -

it easy modify variables outside function using assign() or <<-, if function called lapply(). these tricks seem not working while calling function mclapply(), parallel version of lapply() in package multicore:

require(multicore) f <- function(i) {     x[i] <- x[i] + 1     y[i] <<- y[i] + 1 } x <- y <- 1:10 invisible(lapply(1:5, f)) x #  1  2  3  4  5  6  7  8  9 10, not changed y #  2  3  4  5  6  6  7  8  9 10, variables 1 5 changed  # running mclapply x <- y <- 1:10 invisible(mclapply(1:5, f)) x # 1  2  3  4  5  6  7  8  9 10, not changed y # 1  2  3  4  5  6  7  8  9 10, not changed again! 

how same task - modifying variables outside function - while calling function mclapply()?

i know should used care, because parallel running functions may try modify same variable can cause severe synchronization errors, helpful in cases.