freePort: Find a TCP port that can be opened

In parallelly: Enhancing the 'parallel' Package

Find a TCP port that can be opened

Description

Find a TCP port that can be opened

Usage

freePort(ports = 1024:65535, default = "random", randomize = TRUE)

Arguments

ports	(integer vector, or character string) Zero or more TCP ports in [0, 65535] to scan. If "random", then a random set of ports is considered. If "auto", then the port given by environment variable R_PARALLEL_PORT is used, which may also specify "random". If R_PARALLEL_PORT is not set, the default is "random". The "auto" alternative is used for full backward compatibility with the parallelly package.
default	(integer) NA_integer_ or a port to returned if an available port could not be found. If "first", then ports[1]. If "random", then a random port among ports is used. If length(ports) == 0, then NA_integer_.
randomize	(logical) If TRUE, ports is randomly shuffled before searched. This shuffle does not forward the RNG seed.

Value

Returns an integer representing the first port among ports that can be opened. If none can be opened, then default is returned.

Validation of TCP port returned

This function attempts to validate that the TCP port returned is available and can be be opened. Thus, it is likely that this port is available momentarily after freePort() returns. However, due to race conditions, there is always a risk that the port is taken by another process on the same system after freePort() returns but before, say, makeClusterPSOCK(), opens it.

Controlling the random set of TCP ports

For backward compatibility with the parallel package, the default set of of ports freePort("random"), or freePort("auto") with environment variable R_PARALLEL_PORT=random samples from is 11000:11999. The set can be overriden by setting environment variable R_PARALLELLY_RANDOM_PORTS, e.g. R_PARALLELLY_RANDOM_PORTS="10001:10010,11000:54000".

Examples

# Make sure to undo any changes at the end
oenv <- Sys.getenv("R_PARALLELLY_RANDOM_PORTS")

# Get a random, free TCP port in 1024:65535
port <- freePort()
message("A free TCP port: ", port)

# Get a random, free TCP port in 11000:11999, which is what
# parallelly::makeClusterPSOCK() and parallel::makePSOCKcluster()
# default to (but the latter does not make sure it is available)
Sys.unsetenv("R_PARALLELLY_RANDOM_PORTS")
port <- freePort("random")
message("A free TCP port: ", port)

# Customize the range of ports to sample from to 30000:50000
Sys.setenv(R_PARALLELLY_RANDOM_PORTS = "30000:50000")
port <- freePort("random")
message("A free TCP port: ", port)

# Test if a specific port is free
isPortFree <- function(port) !is.na(freePort(port, default = NA_integer_))
message("TCP port 1024 is free: ", isPortFree(1024))

# Undo changes
Sys.setenv(R_PARALLELLY_RANDOM_PORTS = oenv)

parallelly documentation built on Dec. 18, 2025, 5:08 p.m.