unix/mcfork: Fork a Copy of the Current R Process

mcforkR Documentation

Fork a Copy of the Current R Process


These are low-level functions, not available on Windows, and not exported from the namespace.

mcfork creates a new child process as a copy of the current R process.

mcexit closes the current child process, informing the master process as necessary.


mcfork(estranged = FALSE)

mcexit(exit.code = 0L, send = NULL)



logical, if TRUE then the new process has no ties to the parent process, will not show in the list of children and will not be killed on exit.


process exit code. By convention 0L signifies a clean exit, 1L an error.


if not NULL send this data before exiting (equivalent to using sendMaster).


The mcfork function provides an interface to the fork system call. In addition it sets up a pipe between the master and child process that can be used to send data from the child process to the master (see sendMaster) and child's ‘stdin’ is re-mapped to another pipe held by the master process (see sendChildStdin).

If you are not familiar with the fork system call, do not use this function directly as it leads to very complex inter-process interactions amongst the R processes involved.

In a nutshell fork spawns a copy (child) of the current process, that can work in parallel to the master (parent) process. At the point of forking both processes share exactly the same state including the workspace, global options, loaded packages etc. Forking is relatively cheap in modern operating systems and no real copy of the used memory is created, instead both processes share the same memory and only modified parts are copied. This makes mcfork an ideal tool for parallel processing since there is no need to setup the parallel working environment, data and code is shared automatically from the start.

mcexit is to be run in the child process. It sends send to the master (unless NULL) and then shuts down the child process. The child can also be shut down by sending it the signal SIGUSR1, as is done by the unexported function parallel:::rmChild.


mcfork returns an object of the class "childProcess" to the master and of class "masterProcess" to the child: both the classes inherit from class "process". If estranged is set to TRUE then the child process will be of the class "estrangedProcess" and cannot communicate with the master process nor will it show up on the list of children. These are lists with components pid (the process id of the other process) and a vector fd of the two file descriptor numbers for ends in the current process of the inter-process pipes.

mcexit never returns.

GUI/embedded environments

It is strongly discouraged to use mcfork and the higher-level functions which rely on it (e.g., mcparallel, mclapply and pvec) in GUI or embedded environments, because it leads to several processes sharing the same GUI which will likely cause chaos (and possibly crashes). Child processes should never use on-screen graphics devices. Some precautions have been taken to make this usable in R.app on macOS, but users of third-party front-ends should consult their documentation.

This can also apply to other connections (e.g., to an X server) created before forking, and to files opened by e.g. graphics devices.

Note that tcltk counts as a GUI for these purposes since Tcl runs an event loop. That event loop is inhibited in a child process but there could still be problems with Tk graphical connections.

It is strongly discouraged to use mcfork and the higher-level functions in any multi-threaded R process (with additional threads created by a third-party library or package). Such use can lead to deadlocks or crashes, because the child process created by mcfork may not be able to access resources locked in the parent or may see an inconsistent version of global data (mcfork runs system call fork without exec).

If in doubt, it is safer to use a non-FORK cluster (see makeCluster, clusterApply).


This is a very low-level API for expert use only.


Simon Urbanek and R Core.

Derived from the multicore package formerly on CRAN.

See Also

mcparallel, sendMaster


## This will work when run as an example, but not when pasted in.
p <- parallel:::mcfork()
if (inherits(p, "masterProcess")) {
    cat("I'm a child! ", Sys.getpid(), "\n")
    parallel:::mcexit(,"I was a child")
cat("I'm the master\n")