The number of connections that can be open at the same time in R is typically 128, where the first three are occupied by the always open stdin(), stdout(), and stderr() connections, which leaves 125 slots available for other types of connections. Connections are used in many places, e.g. reading and writing to file, downloading URLs, communicating with parallel R processes over a socket connections, and capturing standard output via text connections.

availableConnections()

freeConnections()

## Value

A non-negative integer, or +Inf if the available number of connections is greated than 16384, which is a limit be set via option parallelly.availableConnections.tries.

## How to increase the limit

This limit of 128 connections can only be changed by rebuilding R from source. The limited is hardcoded as a

#define NCONNECTIONS 128

in src/main/connections.c.

## How the limit is identified

Since the limit might changed, for instance in custom R builds or in future releases of R, we do not want to assume that the limit is 128 for all R installation. Unfortunately, it is not possible to query R for what the limit is. Instead, availableConnections() infers it from trial-and-error. until it fails. For efficiency, the result is memoized throughout the current R session.

## References

1. 'WISH: Increase limit of maximum number of open connections (currently 125+3)', 2016-07-09, https://github.com/HenrikBengtsson/Wishlist-for-R/issues/28

base::showConnections().
total <- availableConnections()