How to call an interactive D process from Mathematica?

Question

I'm trying to establish an interactive session with another process using Mathematicas' StartProcess. It is basically the same scenario as in this question, only that I'm calling a D program instead of a Fortran one.

Take for instance a minimal D program that reads from Standard Input and writes to Standard Output interactively (notice the infinite loop):

// D
void main(string[] argv) {
    while(true) {
        auto name = readln().chomp;
        writefln("Hello %s!", name);
    }
}

When I run this program from the command prompt, it behaves as expected. If I would want to run it from Mathematica, this is supposed to work:

(* Mathematica *)
dhello = StartProcess["dhello.exe"];
WriteLine[dhello, "Wulfrick"]; 
ReadLine[dhello]

but it doesn't. The call to ReadLine[] blocks, as if waiting for the Process to finish. I initially thought it may be a problem in Mathematica, but I tried calling a C# program instead and it worked! Take for instance:

// C#
static void Main(string[] args) {
    while (true) {
        var name = Console.ReadLine();
        Console.WriteLine($"Hello {name}!");
    }
}

Now on the Mathematica side, doing:

(* Mathematica *)
cshello = StartProcess["cshello.exe"];
WriteLine[cshello, "Wulfrick"]; 
ReadLine[cshello]

Works as expected, printing the output as soon as I call ReadLine[] and maintaining interactivity. So it looks like the problem is in the D side really. Also that's why I decided to post here and not on mathematica.stackexchange.

I would really like to make this work with the D program. Any input is much appreciated.

System:

• Windows 10 x64
• Mathematica 11.2
• DMD v2.078.2

Answer 1

Standard output may not have been flushed.

Try:

import std.stdio : stdout;
stdout.flush();

At the end of your while loop.