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
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.
A non-negative integer, or
+Inf if the available number of connections
is greated than 16384, which is a limit be set via option
This limit of 128 connections can only be changed by rebuilding R from source. The limited is hardcoded as a
#define NCONNECTIONS 128
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.
availableConnections() infers it from trial-and-error.
until it fails. For efficiency, the result is memoized throughout the
current R session.
'WISH: Increase limit of maximum number of open connections (currently 125+3)', 2016-07-09, https://github.com/HenrikBengtsson/Wishlist-for-R/issues/28