[Rd] Detect a terminated pipe

Simon Urbanek simon.urbanek at r-project.org
Fri Mar 14 15:54:10 CET 2014

On Mar 14, 2014, at 8:09 AM, Kirill Müller <kirill.mueller at ivt.baug.ethz.ch> wrote:

> Hi
> Is there a way to detect that the process that corresponds to a pipe has ended? On my system (Ubuntu 13.04), I see
> > p <- pipe("true", "w"); Sys.sleep(1); system("ps -elf | grep true | grep -v grep"); isOpen(p)
> [1] TRUE
> The "true" process has long ended (as the filtered ps system call emits no output), still R believes that the pipe p is open.

As far as R is concerned, the connection is open. In addition, pipes exist even without the process - you can close one end of a pipe and it will still exist (that’s what makes pipes useful, actually, because you can choose to close arbitrary combination of the R/W ends). Detecting that the other end of the pipe has closed is generally done by sending/receiving data to/from the end of interest - i.e. reading from a pipe that has closed the write end on the other side will yield 0 bytes read. Writing to a pipe that has closed the read end on the other side will yield SIGPIPE error (note that for text connections you have to call flush() to send the buffer):

> p=pipe("true","r")
> readLines(p)
> close(p)

> p=pipe("true","w")
> writeLines("", p)
> flush(p)
Error in flush.connection(p) : ignoring SIGPIPE signal
> close(p)


> Thanks for your input.
> Best regards
> Kirill
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