Boost logo

Boost :

Subject: Re: [boost] [process] The Formal review preiod is nearing the end
From: Boris Schäling (boris_at_[hidden])
Date: 2016-11-06 14:59:22

Let me start by saying that I was the maintainer of [Boost.]Process from
2008 to 2015. [Boost.]Process was created in 2006, and I have seen each and
every draft of a C++ process management library in Boost since then. I'm
familiar with all the challenges faced throughout the years and the
shortcomings of the various solutions found. I participated in each and
every discussion on a C++ process management library on the Boost mailing
lists since 2008 and have heard the same arguments all over. :) I gave a
talk on [Boost.]Process at C++Now in 2011, gave another one at a C++ meetup
in Sydney this year and will give two more talks later this month at Meeting
C++ in Berlin and a C++ meetup in Amsterdam.

[Boost.]Process is a difficult library to work on, and I want to thank
Klemens that he dared to pick up the library and enter another review. I
have exchanged countless of emails with Klemens this year to make sure he
understands how we got to where we are. While Klemens made some changes to
the version I handed over to him and made his own steps forward, I helped
him avoiding making steps backwards.

The last [Boost.]Process version I worked on is known as version 0.5. The
documentation can be found at <>
(source code is at <>).

If you evaluate Klemens' version at
<> and have some
extra time on your hand, it would be extremely useful if you could compare
Klemens' version with [Boost.]Process 0.5. We went through a lot of
incremental improvements in the past, and it would be good to know whether
Klemens' version is yet another step in the right direction.

There were not only incremental improvements though but also tangible
results: We always wanted to support asynchronous I/O in [Boost.]Process.
This included waiting asynchronously for a child process to exit. So we
worked on a new I/O service object to support asynchronous waiting on
Windows. The I/O service object was added to Boost.Asio in 2012 (known as

A huge step forward was the introduction of executors and initializers - an
idea from Jeff Flinn in 2011. All [Boost.]Process versions since then are
based on executors and initializers. His idea changed the design of
[Boost.]Process quite a lot as it provides a generic solution to the problem
of how to make [Boost.]Process extensible. It is important to understand
Jeff's idea and the problem it solves as any other C++ process management
library has to solve the very same problem to be at least as good in that

It turns out that when creating a child process an infinite number of
configuration options can be set. The Windows and Linux functions being
called when a child process is created accept only a limited number of
arguments. If however, for example, the root directory of a child process on
Linux should be changed, the function chroot() has to be called. The
configuration option "root directory" is set through an additional function
call. It's easy to create a C++ process management library which just
supports a few cross-platform configuration options. It's much more
difficult to create a C++ process management library which allows a library
user to customize the process of creating child processes any way he likes.
Jeff's idea was to use objects as named parameters. If [Boost.]Process
provides a function which accepts any objects (or actually any types),
library users can create their own named parameters and pass them to that
function. They could for example create a named parameter which changes the
root directory for a child process.

Jeff's idea isn't necessarily the only solution to how to provide an
extensible interface. And of course one could argue that this is a
negligible problem a process management library doesn't need to solve. For
[Boost.]Process extensibility has always been one of the major goals though.
We want to avoid that developers start using [Boost.]Process only to find
out later that they can't do what they need to do because the library isn't
extensible enough. Ideally any alternative process management library states
if and how it solves this problem.

Here are some of the other not so obvious goals of [Boost.]Process:

* It should be entirely clear to a library user how resources are managed
(which resources could leak when and how, and how the library helps a user
to avoid leaking resources; and I'm not talking about resources you could
manage with a smart pointer or alike).

* It should be possible to do asynchronous I/O (this includes reading,
writing and waiting for child processes to exit).

* Similar concepts found across operating systems should be represented by
one concept (for example, standard streams), different concepts should be
represented by different concepts (for example, signals on Linux and object
handles on Windows - these are the OS services used for asynchronous

I'm not going to discuss these goals and the current solutions in detail
(this email is already too long; I will talk about resource management in
[Boost.]Process at Meeting C++ in case anyone wants to know more).

If you look at the history of this library, it's surprising how far we got.
As of today [Boost.]Process is still not where I believe it should be.
However I believe there is a greater chance that [Boost.]Process evolves
faster within the Boost community. After all there is considerable interest
in a process management library in Boost. That said I vote for including the
library in Boost because I hope there will be more momentum and a more
continuous interest from the Boost community. This again will hopefully lead
to better versions of [Boost.]Process faster than it did in the past 10
years. Whether this is a good enough reason to include the library in Boost
Antony should decide. :)


Boost list run by bdawes at, gregod at, cpdaniel at, john at