Using a bi-directional pipe to read and write from 'cmd'

General Usage
1

Hi,

I’m using a bi-directional pipe for Julia 1.1.1 to communicate back and forth with an external process:

cvc4 = open(`cvc4 --incremental`, read=true, write=true)

The 2-way communication seems to work ok, except when I try to read a multi-line response from the process, and I don’t know a priori how many lines it consists of.

Let’s say the response is 3 lines. This works:

println(readline(cvc4.out))
println(readline(cvc4.out))
println(readline(cvc4.out))

But the obvious way of reading an unknown number of lines hangs:

println(readlines(cvc4.out))

I have tried various things, like readline() while !eof(cvc4.out), but nothing works.
Any help would be appreciated.

2

You could just call read(cvc4, String) and then split the lines yourself, via split(read(cvc4, String), '\n'), or perhaps you could also do:

while isreadable(cvc4)
  readline(cvc4)
end
3

Thanks. The first solution doesn’t work (hangs). The second one produces all the lines of the answer, and then hangs.
(Whether I run it from the REPL or from the command line).

4

I think you need at least one @async (see e.g., Collecting all output from shell commands - #7 by tkf), unless there is some kind of a marker from the external process that indicates the end of multi-line response. But, if you need to deal with multiple requests and responses, I don’t think there is a way to reliably relate a chunk of output lines to a response. So, using @async may not solve the actual problem.

5

You could use readuntil with an EOT character

6

I don’t know how @async works, and I don’t have control over the external process with which I’m communicating, so the EOT idea doesn’t work.

Anyhow, the solution turns out to be simple: Julia’s readavailable().
(Plus some processing of the result.)