Below are the R options and environment variables that are used by the future package and packages enhancing it.

WARNING: Note that the names and the default values of these options may change in future versions of the package. Please use with care until further notice.

Packages must not change future options

Just like for other R options, as a package developer you must not change any of the below future.* options. Only the end-user should set these. If you find yourself having to tweak one of the options, make sure to undo your changes immediately afterward. For example, if you want to bump up the future.globals.maxSize limit when creating a future, use something like the following inside your function:

oopts <- options(future.globals.maxSize = 1.0 * 1e9)  ## 1.0 GB
f <- future({ expr })  ## Launch a future with large objects

Settings moved to the 'parallelly' package

Several functions have been moved to the parallelly package:

The options and environment variables controlling those have been adjusted accordingly to have different prefixes. For example, option future.fork.enable has been renamed to parallelly.fork.enable and the corresponding environment variable R_FUTURE_FORK_ENABLE has been renamed to R_PARALLELLY_FORK_ENABLE. For backward compatibility reasons, the parallelly package will support both versions for a long foreseeable time. See the parallelly::parallelly.options page for the settings.

Options for controlling futures


(character string or future function) Default future strategy plan used unless otherwise specified via plan(). This will also be the future plan set when calling plan("default"). If not specified, this option may be set when the future package is loaded if command-line option --parallel=ncores (short -p ncores) is specified; if ncores > 1, then option future.plan is set to multisession otherwise sequential (in addition to option mc.cores being set to ncores, if ncores >= 1). (Default: sequential)


(numeric) Maximum allowed total size (in bytes) of global variables identified. Used to prevent too large exports. If set of +Inf, then the check for large globals is skipped. (Default: 500 * 1024 ^ 2 = 500 MiB)

future.globals.onReference: (beta feature - may change)

(character string) Controls whether the identified globals should be scanned for so called references (e.g. external pointers and connections) or not. It is unlikely that another R process ("worker") can use a global that uses a internal reference of the master R process---we call such objects non-exportable globals. If this option is "error", an informative error message is produced if a non-exportable global is detected. If "warning", a warning is produced, but the processing will continue; it is likely that the future will be resolved with a run-time error unless processed in the master R process (e.g. plan(sequential) and plan(multicore)). If "ignore", no scan is performed. (Default: "ignore" but may change)


(integer) An integer specifying the maximum recursive depth to which futures should be resolved. If negative, nothing is resolved. If 0, only the future itself is resolved. If 1, the future and any of its elements that are futures are resolved, and so on. If +Inf, infinite search depth is used. (Default: 0)

future.rng.onMisuse: (beta feature - may change)

(character string) If random numbers are used in futures, then parallel (L'Ecuyer-CMRG) RNG should be used in order to get statistical sound RNGs. The defaults in the future framework assume that no random number generation (RNG) is taken place in the future expression because L'Ecuyer-CMRG RNGs come with an unnecessary overhead if not needed. To protect against mistakes, the future framework attempts to detect when random numbers are used despite L'Ecuyer-CMRG RNGs are not in place. If this is detected, and future.rng.onMisuse = "error", then an informative error message is produced. If "warning", then a warning message is produced. If "ignore", no check is performed. (Default: "warning")

future.globalenv.onMisuse: (beta feature - may change)

(character string) Assigning variables to the global environment for the purpose of using the variable at a later time makes no sense with futures, because the next future may be evaluated in different R process. To protect against mistakes, the future framework attempts to detect when variables are added to the global environment. If this is detected, and future.globalenv.onMisuse = "error", then an informative error message is produced. If "warning", then a warning message is produced. If "ignore", no check is performed. (Default: "ignore")


(logical) If TRUE, a FutureCondition keeps a copy of the Future object that triggered the condition. If FALSE, it is dropped. (Default: TRUE)


(numeric) Maximum waiting time (in seconds) for a free worker before a timeout error is generated. (Default: 30 * 24 * 60 * 60 (= 30 days))


(numeric) Initial interval (in seconds) between polls. This controls the polling frequency for finding an available worker when all workers are currently busy. It also controls the polling frequency of resolve(). (Default: 0.01 = 0.01 seconds)


(numeric) Positive scale factor used to increase the interval after each poll. (Default: 1.01)

Options for debugging futures


(logical) If TRUE, extensive debug messages are generated. (Default: FALSE)

Options for controlling package startup


(character vector or a logical) Specifies zero of more future startup scripts to be sourced when the future package is attached. It is only the first existing script that is sourced. If none of the specified files exist, nothing is sourced---there will be neither a warning nor an error. If this option is not specified, environment variable R_FUTURE_STARTUP_SCRIPT is considered, where multiple scripts may be separated by either a colon (:) or a semicolon (;). If neither is set, or either is set to TRUE, the default is to look for a .future.R script in the current directory and then in the user's home directory. To disable future startup scripts, set the option or the environment variable to FALSE. Importantly, this option is always set to FALSE if the future package is loaded as part of a future expression being evaluated, e.g. in a background process. In order words, they are sourced in the main R process but not in future processes. (Default: TRUE in main R process and FALSE in future processes / during future evaluation)


(character vector) Overrides commandArgs() when the future package is loaded.

Options for configuring low-level system behaviors

future.fork.multithreading.enable (beta feature - may change):

(logical) Enable or disable multi-threading while using forked parallel processing. If FALSE, different multi-thread library settings are overridden such that they run in single-thread mode. Specifically, multi-threading will be disabled for OpenMP (which requires the RhpcBLASctl package) and for RcppParallel. If TRUE, or not set (the default), multi-threading is allowed. Parallelization via multi-threaded processing (done in native code by some packages and external libraries) while at the same time using forked (aka "multicore") parallel processing is known to unstable. Note that this is not only true when using plan(multicore) but also when using, for instance, mclapply() of the parallel package. (Default: not set)

(logical) Enable or disable re-encoding of UTF-8 symbols that were incorrectly encoded while captured. In R (< 4.2.0) and on older versions of MS Windows, R cannot capture UTF-8 symbols as-is when they are captured from the standard output. For examples, a UTF-8 check mark symbol ("\u2713") would be relayed as "<U+2713>" (a string with eight ASCII characters). Setting this option to TRUE will cause value() to attempt to recover the intended UTF-8 symbols from <U+nnnn> string components, if, and only if, the string was captured by a future resolved on MS Windows. (Default: TRUE)

See also parallelly::parallelly.options.

Options for demos


(integer) Either a named list of mandelbrot() arguments or an integer in {1, 2, 3} specifying a predefined Mandelbrot region. (Default: 1L)


(integer) Number of rows and columns of tiles. (Default: 3L)

Deprecated or for internal prototyping

The following options exists only for troubleshooting purposes and must not be used in production. If used, there is a risk that the results are non-reproducible if processed elsewhere. To lower the risk of them being used by mistake, they are marked as deprecated and will produce warnings if set.


(character string) Action to take when non-existing global variables ("globals" or "unknowns") are identified when the future is created. If "error", an error is generated immediately. If "ignore", no action is taken and an attempt to evaluate the future expression will be made. The latter is useful when there is a risk for false-positive globals being identified, e.g. when future expression contains non-standard evaluation (NSE). (Default: "ignore")


(character string) Method used to identify globals. For details, see globalsOf(). (Default: "ordered")


(logical) If TRUE, globals that are Future objects (typically created as explicit futures) will be resolved and have their values (using value()) collected. Because searching for unresolved futures among globals (including their content) can be expensive, the default is not to do it and instead leave it to the run-time checks that assert proper ownership when resolving futures and collecting their values. (Default: FALSE)

Environment variables that set R options

All of the above R future.* options can be set by corresponding environment variable R_FUTURE_* when the future package is loaded. This means that those environment variables must be set before the future package is loaded in order to have an effect. For example, if R_FUTURE_RNG_ONMISUSE="ignore", then option future.rng.onMisuse is set to "ignore" (character string). Similarly, if R_FUTURE_GLOBALS_MAXSIZE="50000000", then option future.globals.maxSize is set to 50000000 (numeric).

See also

To set R options or environment variables when R starts (even before the future package is loaded), see the Startup help page. The startup package provides a friendly mechanism for configurating R's startup process.


# Allow at most 5 MB globals per futures
options(future.globals.maxSize = 5e6)

# Be strict; catch all RNG mistakes
options(future.rng.onMisuse = "error")