Let's communicate with external processes by executing the following steps:

The code is as follows:

import std.process, std.stdio;
auto info = pipeProcess("child_program");
scope(exit) wait(info.pid);
info.stdin.writeln("data to send to the process");
info.stdin.close();
foreach(line; stdout.byLine)
     writeln("Received ", line, " from child.");

The child program may be anything that writes to its stdout handle. For example, have a look at the following code:

import std.stdio;
void main() {
    writeln("Hello!");
    writeln("Second line.");
}

Calling the function from the preceding program will print the following:

Received Hello! from child.
Received Second line. from child.

Phobos' std.process module includes several cross-platform functions to communicate with other processes. It can run programs, spawn shells, and create pipes on standard handles. Once created, the resultant pipes are presented as std.stdio.File objects and can be written to (in the case of the stdin handle, you write to the child program's input) the input and read from (for stdout and stderr, reading the child process' output) the output like any other file.

You can also access the operating system or C functions yourself, bypassing the Phobos functions. This is necessary if you need asynchronous I/O with the process.

You may also wish to define an RPC protocol to use over your pipes, to call functions as well as pass data. Any network application protocol can be adapted to work with local processes and pipes. For example, there are D bindings to Apache Thrift that may be of use.