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\name{ipcmutex}
\alias{ipclocked}
\alias{ipclock}
\alias{ipctrylock}
\alias{ipcunlock}
\alias{ipcid}
\alias{ipcremove}
\alias{ipcyield}
\alias{ipcvalue}
\alias{ipcreset}
\title{Inter-process locks and counters}
\description{
Functions documented on this page enable locks and counters between
processes on the \emph{same} computer.
Use \code{ipcid()} to generate a unique mutex or counter identifier. A
mutex or counter with the same \code{id}, including those in different
processes, share the same state.
\code{ipcremove()} removes external state associated with mutex or
counters created with \code{id}.
\code{ipclock()} blocks until the lock is
obtained. \code{ipctrylock()} tries to obtain the lock, returning
immediately if it is not available. \code{ipcunlock()} releases the
lock. \code{ipclocked()} queries the lock to determine whether it is
currently held.
\code{ipcyield()} returns the current counter, and increments the
value for subsequent calls. \code{ipcvalue()} returns the current
counter without incrementing. \code{ipcreset()} sets the counter to
\code{n}, such that the next call to \code{ipcyield()} or
\code{ipcvalue()} returns \code{n}.
}
\usage{
## Utilities
ipcid(id)
ipcremove(id)
## Locks
ipclock(id)
ipctrylock(id)
ipcunlock(id)
ipclocked(id)
## Counters
ipcyield(id)
ipcvalue(id)
ipcreset(id, n = 1)
}
\arguments{
\item{id}{character(1) identifier string for mutex or
counter. \code{ipcid()} ensures that the identifier is universally
unique.}
\item{n}{integer(1) value from which \code{ipcyield()} will
increment.}
}
\value{
Locks:
\code{ipclock()} creates a named lock, returning \code{TRUE}
on success.
\code{trylock()} returns \code{TRUE} if the lock is
obtained, \code{FALSE} otherwise.
\code{ipcunlock()} returns \code{TRUE} on success,
\code{FALSE} (e.g., because there is nothing to unlock)
otherwise.
\code{ipclocked()} returns \code{TRUE} when \code{id} is locked, and
\code{FALSE} otherwise.
Counters:
\code{ipcyield()} returns an integer(1) value representing the next
number in sequence. The first value returned is 1.
\code{ipcvalue()} returns the value to be returned by the next call to
\code{ipcyield()}, without incrementing the counter. If the counter is
no longer available, \code{ipcyield()} returns \code{NA}.
\code{ipcreset()} returns \code{n}, invisibly.
Utilities:
\code{ipcid()} returns a character(1) unique identifier, with
\code{id} (if not missing) prepended.
\code{ipcremove()} returns (invisibly) \code{TRUE} if external
resources were released or \code{FALSE} if not (e.g., because the
resources has already been released).
}
\examples{
ipcid()
## Locks
id <- ipcid()
ipclock(id)
ipctrylock(id)
ipcunlock(id)
ipctrylock(id)
ipclocked(id)
ipcremove(id)
id <- ipcid()
system.time({
## about 1s, .2s for each process instead of .2s if no lock
result <- bplapply(1:2, function(i, id) {
BiocParallel::ipclock(id)
Sys.sleep(.2)
time <- Sys.time()
BiocParallel::ipcunlock(id)
time
}, id)
})
ipcremove(id)
diff(sort(unlist(result, use.names=FALSE)))
## Counters
id <- ipcid()
ipcyield(id)
ipcyield(id)
ipcvalue(id)
ipcyield(id)
ipcreset(id, 10)
ipcvalue(id)
ipcyield(id)
ipcremove(id)
id <- ipcid()
result <- bplapply(1:2, function(i, id) {
BiocParallel::ipcyield(id)
}, id)
ipcremove(id)
sort(unlist(result, use.names=FALSE))
}
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