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From: Edward Diener (eddielee_at_[hidden])
Date: 2005-03-18 18:03:50

In .NET the asynchronous call occurs on a delegate ( Boost equivalent is a
boost::function<> ) running on a separate thread. The delegate is passed a
structure which allows the asynchronous callback to complete. Thus to
asynchronously accept a connection, one calls aSocket.BeginAccept passing
the delegate as well as user-defined data, including the original socket, in
a structure, and when the delegate on the thread accepts a connection, it
calls aSocket.EndAccept(theStructure) to get back a socket for the
connection. All .NET asynchronous socket calls work similiarly, and the use
of threads and delegates seems to me a strong combination and works very

Obviously Boost has a thread library, although I have not used it, and it
has a callback mechanism in boost::function<> which I have used extensively,
so designing an asynchronous I/O library around these two concepts seems
like a good design to me.

"Boris" <boris_at_[hidden]> wrote in message
> While trying to find a reasonable interface for a socket class to support
> asynchronous I/O I was examining various implementations. Scott Woods had
> thought loudly about an asynchronous library which could be the platform
> a network library and maybe others. I like this idea but look at this
> 1) Signal-Driven I/O
> SIGIO is sent to a process whenever something happens with the socket. As
> there are too many things happening with a TCP socket the signal is
> generated too often and "signal-driven I/O is next to useless with a TCP
> socket" (W. Richard Stevens in his network bible). As there is just one
> signal handler per signal it shouldn't be used in a library anyway.
> 2) Threads
> There are different solutions possible eg. every thread calling a blocking
> function or one thread blocking while the other threads handle requests.
> However these are just variations of the blocking I/O model and
> with the disadvantage of increased complexity because of threads.
> 3) AIO
> AIO is part of the Single UNIX Specification and an official standard for
> asynchronous input and output. The AIO API is based on file descriptors
> defines a few functions inluding aio_read() and aio_write(). I/O is
> asynchronous because the process is notified when an input or output
> operation is completed - there is either a signal sent to the process or a
> callback function is executed (as a thread). Further more it is possible
> poll a file descriptor if the process doesn't want to wait for the signal
> the callback function.
> Now to the disadvantages: Linux 2.6 doesn't support AIO with sockets (see
> There are some AIO patches for
> Linux 2.4 but nothing official. Even if AIO support was complete there are
> no aio_connect() and aio_accept() functions. I wonder if connections have
> be built up synchronously (I guess so as AIO wasn't designed only for
> sockets anyway).
> 4) I/O completion ports
> This I/O model is based on a concept called port. A port is similar to a
> multiplexor and waits for several input or output operations to complete.
> There can be several ports each multiplexing on several file descriptors.
> Windows XP, Windows 2000 Pro and Windows NT 3.5 support I/O completion
> ports. However the application has to call a blocking function for a port
> be notified when an I/O operation is complete.
> 4) Event completion framework
> The event completion framework behaves like I/O completion ports. However
> the process can be notified of different types of events not only of the
> completion of I/O operations. Solaris 10 supports these events: AIO,
> file descriptor, user-defined and waking up threads (see
> OpenBSD and NetBSD support: read, write, AIO, file descriptor, process IDs
> (exit, fork etc. of other processes), signal, timer and network (see
> The framework of *BSD is called kqueue and not compatible to Solaris.
> I don't know what other operating systems like Mac OS X support. There is
> kqueue() is their man pages (see
> but I don't know if it is supported. AIO doesn't seem to be supported.
> Microsoft operating systems provide WaitForMultipleObjects() since Windows
> 95 which supports these objects: change notification, console input,
> job, memory resource notification, mutex, process, semaphore, thread and
> waitable timer (see
> Then I had a look at ACE as I wondered how they support asynchronous I/O
> with their Proactor class on different operating systems: They don't. They
> use this preprocessor directive in the beginning of Proactor.h:
> #if ((defined (ACE_WIN32) && !defined (ACE_HAS_WINCE)) || (defined
> // This only works on Win32 platforms and on Unix platforms supporting
> aio calls.
> Someone wanted to port a Windows applications to Linux which was using the
> Proactor class. The ACE people told him he should say good-bye to Proactor
> (see
> Last but not least Linux 2.5 introduced a new API called epoll (see
> It doesn't support
> asynchronous I/O but tries to get rid of all the disadvantages select()
> poll() have. The Linux world seems to pursue the traditional multiplexing
> approach trying to improve it.
> Now who has an idea how the asynchronous C++ library should look like? ;)

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