[Rd] pipe(): input to, and output from, a single process

[Ivan Krylov]

On Fri, 13 Mar 2020 20:26:43 +0300
Greg Minshall <minshall using umich.edu> wrote:

> my sense from pipe and looking at the sources (sys-unix.c) is that is
> not possible.  is that true?  are there any thoughts of providing
> such a facility?

Pipes (including those created by popen(3), which R pipe() uses
internally) are uni-directional data channels. While it could be
possible to open two pipes for both stdin and stdout of the child
process, doing so correctly is complicated because of differences in
buffering provided by the runtime: when stdin/stdout is not a terminal,
buffering mode may be set to block-oriented instead of line-oriented,
resulting in both parent and child being dead-locked, waiting to fill
the buffer instead of returning from the blocking call after the first
newline. (Hence the -l flag to sed mentioned by Gábor Csárdi, which
avoids this problem for sed).

Programs designed to first read stdin until end-of-file, then process
the input and print results on the stdout are usually safe to use in
this way, but others may be not. Software specifically designed to
control other software (e.g. Expect [*]) gets around this limitation by
running the child processes inside pseudo-terminals and/or running in
event-driven manner, being ready to service the child process whether
it wants to read its stdin or write to stdout.

Since sed has its -l flag, it should be possible to safely drive it
line-by-line with the help of processx, but not via pipe().

-- 
Best regards,
Ivan

[*] https://core.tcl-lang.org/expect/index