later_fd: Executes a function when a file descriptor is ready

In later: Utilities for Scheduling Functions to Execute Later with Event Loops

Executes a function when a file descriptor is ready

Description

Schedule an R function or formula to run after an indeterminate amount of time when file descriptors are ready for reading or writing, subject to an optional timeout.

Usage

later_fd(
  func,
  readfds = integer(),
  writefds = integer(),
  exceptfds = integer(),
  timeout = Inf,
  loop = current_loop()
)

Arguments

func	A function that takes a single argument, a logical vector that indicates which file descriptors are ready (a concatenation of readfds, writefds and exceptfds). This may be all FALSE if the timeout argument is non-Inf. File descriptors with error conditions pending are represented as NA, as are invalid file descriptors such as those already closed.
readfds	Integer vector of file descriptors, or Windows SOCKETs, to monitor for being ready to read.
writefds	Integer vector of file descriptors, or Windows SOCKETs, to monitor being ready to write.
exceptfds	Integer vector of file descriptors, or Windows SOCKETs, to monitor for error conditions pending.
timeout	Number of seconds to wait before giving up, and calling func with all FALSE. The default Inf implies waiting indefinitely. Specifying 0 will check once without blocking, and supplying a negative value defaults to a timeout of 1s.
loop	A handle to an event loop. Defaults to the currently-active loop.

Details

On the occasion the system-level poll (on Windows WSAPoll) returns an error, the callback will be made on a vector of all NAs. This is indistinguishable from a case where the poll succeeds but there are error conditions pending against each file descriptor.

If no file descriptors are supplied, the callback is scheduled for immediate execution and made on the empty logical vector logical(0).

Value

A function, which, if invoked, will cancel the callback. The function will return TRUE if the callback was successfully cancelled and FALSE if not (this occurs if the callback has executed or has been cancelled already).

Note

To avoid bugs due to reentrancy, by default, scheduled operations only run when there is no other R code present on the execution stack; i.e., when R is sitting at the top-level prompt. You can force past-due operations to run at a time of your choosing by calling run_now().

Error handling is not particularly well-defined and may change in the future. options(error=browser) should work and errors in func should generally not crash the R process, but not much else can be said about it at this point. If you must have specific behavior occur in the face of errors, put error handling logic inside of func.

Examples

# create nanonext sockets
s1 <- nanonext::socket(listen = "inproc://nano")
s2 <- nanonext::socket(dial = "inproc://nano")
fd1 <- nanonext::opt(s1, "recv-fd")
fd2 <- nanonext::opt(s2, "recv-fd")

# 1. timeout: prints FALSE, FALSE
later_fd(print, c(fd1, fd2), timeout = 0.1)
Sys.sleep(0.2)
run_now()

# 2. fd1 ready: prints TRUE, FALSE
later_fd(print, c(fd1, fd2), timeout = 1)
res <- nanonext::send(s2, "msg")
Sys.sleep(0.1)
run_now()

# 3. both ready: prints TRUE, TRUE
res <- nanonext::send(s1, "msg")
later_fd(print, c(fd1, fd2), timeout = 1)
Sys.sleep(0.1)
run_now()

# 4. fd2 ready: prints FALSE, TRUE
res <- nanonext::recv(s1)
later_fd(print, c(fd1, fd2), timeout = 1)
Sys.sleep(0.1)
run_now()

# 5. fds invalid: prints NA, NA
close(s2)
close(s1)
later_fd(print, c(fd1, fd2), timeout = 0)
Sys.sleep(0.1)
run_now()

later documentation built on April 3, 2025, 6:02 p.m.