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Subject: Re: [boost] Boost.Process 0.5: Another update/potential candidate for an official library
From: Boris Schaeling (boris_at_[hidden])
Date: 2012-11-14 19:09:18

On Tue, 13 Nov 2012 20:57:36 +0100, Vicente J. Botet Escriba
<vicente.botet_at_[hidden]> wrote:

> [...]Could you then update the documentation so that there are no such
> dependencies on the underlying platform?

There are notes referring to boost/process/mitigate.hpp wherever #ifdefs
are used in the tutorial (the asynchronous waiting example excluded). The
reason why I didn't use the helpers from that header file in the tutorial
directly: They don't work necessarily as expected in all cases like the
rest of the library.

For instance, while boost::process::pipe_end works for the examples in the
tutorial it's two different types on POSIX and Windows. And Boost.Process
makes no guarantee that these two types behave consistently on POSIX and
Windows (especially not as the types are from another Boost library). So
I'm trying to make it clear that there is a different quality of service:
Most features are fully supported, 100% cross-platform and safe but a few
aren't. And you use these few on your own risk. This isn't done to make
cross-platform code harder to write but to help users to make a conscious
decision and avoid shooting in their own leg.

It is of course possible to rearrange the documentation. For instance, all
examples with asynchronous operations are in that sense problematic. They
could be removed from the tutorial and put somewhere else in the
documentation. But I'm not sure whether that makes a difference in the end.

> [...]Yes, I see that Boost.Asio has not reached to provide a platform
> independent abstraction (I'm wondering how this feature appear in the
> proposal to the standard C++ committe). Any way, could you explain why
> do you need to use a non portable feature?

To wait asynchronously on POSIX I use boost::asio::signal_set to register
a signal handler for SIGCHLD. This is a global setting and must be done
before a child process is created (actually exits). To wait asynchronously
on Windows I use boost::asio::windows::object_handle to wait on a child
process handle. This is a per-child setting and can only be done after the
child process has been created (as only then we have the handle to wait
on). For a cross-platform solution we would need to merge these two
concepts somehow: A global setting before child processes are created on
POSIX and a per-child setting after child processes are created on
Windows. Here POSIX and Windows are unfortunately totally different.

> [...]You could define a implementation defined native_handle typedef
> struct child {
> typedef platform-specific-type native_handle_type;
> // construct/copy/destruct
> explicit child(native_handle);
> native_handle_type native_handle();
> };

Would be fine with me. I wish the problems we have to deal with around
Boost.Process were all like this. ;)


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