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575 lines
21 KiB
HTML
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</style>
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<title>Asynchronous I/O in Windows for Unix Programmers</title>
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</head>
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<body>
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<h1>Asynchronous I/O in Windows for Unix Programmers</h1>
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<p>Ryan Dahl ryan@joyent.com
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<p>This document assumes you are familiar with how non-blocking socket I/O
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is done in Unix.
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<p>The syscall
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<a href="http://msdn.microsoft.com/en-us/library/ms740141(v=VS.85).aspx"><code>select</code>
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is available in Windows</a>
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but <code>select</code> processing is O(n) in the number of file descriptors
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unlike the modern constant-time multiplexers like epoll which makes select
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unacceptable for high-concurrency servers.
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This document will describe how high-concurrency programs are
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designed in Windows.
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<p>
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Instead of <a href="http://en.wikipedia.org/wiki/Epoll">epoll</a>
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or
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<a href="http://en.wikipedia.org/wiki/Kqueue">kqueue</a>,
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Windows has its own I/O multiplexer called
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<a href="http://msdn.microsoft.com/en-us/library/aa365198(VS.85).aspx">I/O completion ports</a>
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(IOCPs). IOCPs are the objects used to poll
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<a href="http://msdn.microsoft.com/en-us/library/ms686358(v=vs.85).aspx">overlapped I/O</a>
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for completion. IOCP polling is constant time (REF?).
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<p>
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The fundamental variation is that in a Unix you generally ask the kernel to
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wait for state change in a file descriptor's readability or writablity. With
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overlapped I/O and IOCPs the programmers waits for asynchronous function
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calls to complete.
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For example, instead of waiting for a socket to become writable and then
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using <a
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href="http://www.kernel.org/doc/man-pages/online/pages/man2/send.2.html"><code>send(2)</code></a>
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on it, as you commonly would do in a Unix, with overlapped I/O you
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would rather <a
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href="http://msdn.microsoft.com/en-us/library/ms742203(v=vs.85).aspx"><code>WSASend()</code></a>
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the data and then wait for it to have been sent.
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<p> Unix non-blocking I/O is not beautiful. A principle abstraction in Unix
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is the unified treatment of many things as files (or more precisely as file
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descriptors). <code>write(2)</code>, <code>read(2)</code>, and
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<code>close(2)</code> work with TCP sockets just as they do on regular
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files. Well—kind of. Synchronous operations work similarly on different
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types of file descriptors but once demands on performance drive you to world of
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<code>O_NONBLOCK</code> various types of file descriptors can act quite
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different for even the most basic operations. In particular,
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regular file system files do <i>not</i> support non-blocking operations.
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(Disturbingly no man page mentions this rather important fact.)
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For example, one cannot poll on a regular file FD for readability expecting
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it to indicate when it is safe to do a non-blocking read.
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Regular file are always readable and <code>read(2)</code> calls
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<i>always</i> have the possibility of blocking the calling thread for an
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unknown amount of time.
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<p>POSIX has defined <a
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href="http://pubs.opengroup.org/onlinepubs/007908799/xsh/aio.h.html">an
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asynchronous interface</a> for some operations but implementations for
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many Unixes have unclear status. On Linux the
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<code>aio_*</code> routines are implemented in userland in GNU libc using
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pthreads.
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<a
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href="http://www.kernel.org/doc/man-pages/online/pages/man2/io_submit.2.html"><code>io_submit(2)</code></a>
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does not have a GNU libc wrapper and has been reported <a
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href="http://voinici.ceata.org/~sana/blog/?p=248"> to be very slow and
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possibly blocking</a>. <a
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href="http://download.oracle.com/docs/cd/E19253-01/816-5171/aio-write-3rt/index.html">Solaris
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has real kernel AIO</a> but it's unclear what its performance
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characteristics are for socket I/O as opposed to disk I/O.
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Contemporary high-performance Unix socket programs use non-blocking
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file descriptors with a I/O multiplexer—not POSIX AIO.
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Common practice for accessing the disk asynchronously is still done using custom
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userland thread pools—not POSIX AIO.
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<p>Windows IOCPs does support both sockets and regular file I/O which
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greatly simplifies the handling of disks. For example,
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<a href="http://msdn.microsoft.com/en-us/library/aa365468(v=VS.85).aspx"><code>ReadFileEx()</code></a>
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operates on both.
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As a first example let's look at how <code>ReadFile()</code> works.
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<pre>
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typedef void* HANDLE;
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BOOL ReadFile(HANDLE file,
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void* buffer,
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DWORD numberOfBytesToRead,
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DWORD* numberOfBytesRead,
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OVERLAPPED* overlapped);
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</pre>
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<p>
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The function has the possibility of executing the read synchronously
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or asynchronously. A synchronous operation is indicated by
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returning 0 and <a
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href="http://msdn.microsoft.com/en-us/library/ms741580(v=VS.85).aspx">WSAGetLastError()</code></a>
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returning <code>WSA_IO_PENDING</code>.
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When <code>ReadFile()</code> operates asynchronously the
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the user-supplied <a
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href="http://msdn.microsoft.com/en-us/library/ms741665(v=VS.85).aspx"><code>OVERLAPPED*</code></a>
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is a handle to the incomplete operation.
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<pre>
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typedef struct {
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unsigned long* Internal;
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unsigned long* InternalHigh;
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union {
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struct {
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WORD Offset;
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WORD OffsetHigh;
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};
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void* Pointer;
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};
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HANDLE hEvent;
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} OVERLAPPED;
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</pre>
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To poll on the completion of one of these functions,
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use an IOCP, <code>overlapped->hEvent</code>, and
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<a
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href="http://msdn.microsoft.com/en-us/library/aa364986(v=vs.85).aspx"><code>GetQueuedCompletionStatus()</code></a>.
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<h3>Simple TCP Connection Example</h3>
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<p>To demonstrate the use of <code>GetQueuedCompletionStatus()</code> an
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example of connecting to <code>localhost</code> at port 8000 is presented.
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<pre>
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char* buffer[200];
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WSABUF b = { buffer, 200 };
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size_t bytes_recvd;
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int r, total_events;
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OVERLAPPED overlapped;
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HANDLE port;
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port = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, NULL, 0);
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if (!port) {
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goto error;
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}
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r = WSARecv(socket, &b, 1, &bytes_recvd, NULL, &overlapped, NULL);
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CreateIoCompletionPort(port, &overlapped.hEvent,
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if (r == 0) {
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if (WSAGetLastError() == WSA_IO_PENDING) {
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/* Asynchronous */
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GetQueuedCompletionStatus()
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if (r == WAIT_TIMEOUT) {
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printf("Timeout\n");
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} else {
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}
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} else {
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/* Error */
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printf("Error %d\n", WSAGetLastError());
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}
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} else {
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/* Synchronous */
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printf("read %ld bytes from socket\n", bytes_recvd);
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}
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</pre>
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<h3>Previous Work</h3>
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<p> Writing code that can take advantage of the best worlds on across Unix operating
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systems and Windows is very difficult, requiring one to understand intricate
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APIs and undocumented details from many different operating systems. There
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are several projects which have made attempts to provide an abstraction
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layer but in the author's opinion, none are completely satisfactory.
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<p><b>Marc Lehmann's
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<a href="http://software.schmorp.de/pkg/libev.html">libev</a> and
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<a href="http://software.schmorp.de/pkg/libeio.html">libeio</a>.</b>
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libev is the perfect minimal abstraction of the Unix I/O multiplexers. It
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includes several helpful tools like <code>ev_async</code>, which is for
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asynchronous notification, but the main piece is the <code>ev_io</code>,
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which informs the user about the state of file descriptors. As mentioned
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before, in general it is not possible to get state changes for regular
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files—and even if it were the <code>write(2)</code> and
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<code>read(2)</code> calls do not guarantee that they won't block.
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Therefore libeio is provided for calling various disk-related
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syscalls in a managed thread pool. Unfortunately the abstraction layer
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which libev targets is not appropriate for IOCPs—libev works strictly
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with file descriptors and does not the concept of a <i>socket</i>.
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Furthermore users on Unix will be using libeio for file I/O which is not
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ideal for porting to Windows. On windows libev currently uses
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<code>select()</code>—which is limited to 64 file descriptors per
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thread.
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<p><b><a href="http://monkey.org/~provos/libevent/">libevent</a>.</b>
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Somewhat bulkier than libev with code for RPC, DNS, and HTTP included.
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Does not support file I/O.
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libev was
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created after Lehmann <a
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href="http://www.mail-archive.com/libevent-users@monkey.org/msg00753.html">evaluated
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libevent and rejected it</a>—it's interesting to read his reasons
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why. <a
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href="http://google-opensource.blogspot.com/2010/01/libevent-20x-like-libevent-14x-only.html">A
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major rewrite</a> was done for version 2 to support Windows IOCPs but <a
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href="http://www.mail-archive.com/libevent-users@monkey.org/msg01730.html">anecdotal
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evidence</a> suggests that it is still not working correctly.
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<p><b><a
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href="http://www.boost.org/doc/libs/1_43_0/doc/html/boost_asio.html">Boost
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ASIO</a>.</b> It basically does what you want on Windows and Unix for
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sockets. That is, epoll on Linux, kqueue on Macintosh, IOCPs on Windows.
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It does not support file I/O. In the author's opinion is it too large
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for a not extremely difficult problem (~300 files, ~12000 semicolons).
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<h2>File Types</h2>
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<p>
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Almost every socket operation that you're familiar with has an
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overlapped counter-part. The following section tries to pair Windows
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overlapped I/O syscalls with non-blocking Unix ones.
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<h3>TCP Sockets</h3>
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TCP Sockets are by far the most important stream to get right.
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Servers should expect to be handling tens of thousands of these
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per thread, concurrently. This is possible with overlapped I/O in Windows if
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one is careful to avoid Unix-ism like file descriptors. (Windows has a
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hard limit of 2048 open file descriptors—see
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<a
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href="http://msdn.microsoft.com/en-us/library/6e3b887c.aspx"><code>_setmaxstdio()</code></a>.)
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<dl>
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<dt><code>send(2)</code>, <code>write(2)</code></dt>
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<dd>Windows:
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<a href="http://msdn.microsoft.com/en-us/library/ms742203(v=vs.85).aspx"><code>WSASend()</code></a>,
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<a href="http://msdn.microsoft.com/en-us/library/aa365748(v=VS.85).aspx"><code>WriteFileEx()</code></a>
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</dd>
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<dt><code>recv(2)</code>, <code>read(2)</code></dt>
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<dd>
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Windows:
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<a href="http://msdn.microsoft.com/en-us/library/ms741688(v=VS.85).aspx"><code>WSARecv()</code></a>,
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<a href="http://msdn.microsoft.com/en-us/library/aa365468(v=VS.85).aspx"><code>ReadFileEx()</code></a>
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</dd>
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<dt><code>connect(2)</code></dt>
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<dd>
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Windows: <a href="http://msdn.microsoft.com/en-us/library/ms737606(VS.85).aspx"><code>ConnectEx()</code></a>
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<p>
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Non-blocking <code>connect()</code> is has difficult semantics in
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Unix. The proper way to connect to a remote host is this: call
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<code>connect(2)</code> while it returns
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<code>EINPROGRESS</code> poll on the file descriptor for writablity.
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Then use
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<pre>int error;
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socklen_t len = sizeof(int);
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getsockopt(fd, SOL_SOCKET, SO_ERROR, &error, &len);</pre>
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A zero <code>error</code> indicates that the connection succeeded.
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(Documented in <code>connect(2)</code> under <code>EINPROGRESS</code>
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on the Linux man page.)
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</dd>
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<dt><code>accept(2)</code></dt>
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<dd>
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Windows: <a href="http://msdn.microsoft.com/en-us/library/ms737524(v=VS.85).aspx"><code>AcceptEx()</code></a>
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</dd>
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<dt><code>sendfile(2)</code></dt>
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<dd>
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Windows: <a href="http://msdn.microsoft.com/en-us/library/ms740565(v=VS.85).aspx"><code>TransmitFile()</code></a>
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<p> The exact API of <code>sendfile(2)</code> on Unix has not been agreed
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on yet. Each operating system does it slightly different. All
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<code>sendfile(2)</code> implementations (except possibly FreeBSD?) are blocking
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even on non-blocking sockets.
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<ul>
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<li><a href="http://www.kernel.org/doc/man-pages/online/pages/man2/sendfile.2.html">Linux <code>sendfile(2)</code></a>
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<li><a href="http://www.freebsd.org/cgi/man.cgi?query=sendfile&sektion=2">FreeBSD <code>sendfile(2)</code></a>
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<li><a href="http://www.manpagez.com/man/2/sendfile/">Darwin <code>sendfile(2)</code></a>
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</ul>
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Marc Lehmann has written <a
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href="https://github.com/joyent/node/blob/2c185a9dfd3be8e718858b946333c433c375c295/deps/libeio/eio.c#L954-1080">a
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portable version in libeio</a>.
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</dd>
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<dt><code>shutdown(2)</code>, graceful close, half-duplex connections</dt>
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<dd>
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<a
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href="http://msdn.microsoft.com/en-us/library/ms738547(v=VS.85).aspx">Graceful
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Shutdown, Linger Options, and Socket Closure</a>
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<br/>
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<a
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href="http://msdn.microsoft.com/en-us/library/ms737757(VS.85).aspx"><code>DisconnectEx()</code></a>
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</dd>
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<dt><code>close(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/ms737582(v=VS.85).aspx"><code>closesocket()</code></a>
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</dd>
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The following are nearly same in Windows overlapped and Unix
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non-blocking sockets. The only difference is that the Unix variants
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take integer file descriptors while Windows uses <code>SOCKET</code>.
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<ul>
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<li><a href="http://msdn.microsoft.com/en-us/library/ms740496(v=VS.85).aspx"><code>sockaddr</code></a>
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<li><a href="http://msdn.microsoft.com/en-us/library/ms737550(v=VS.85).aspx"><code>bind()</code></a>
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<li><a href="http://msdn.microsoft.com/en-us/library/ms738543(v=VS.85).aspx"><code>getsockname()</code></a>
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</ul>
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<h3>Named Pipes</h3>
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Windows has "named pipes" which are more or less the same as <a
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href="http://www.kernel.org/doc/man-pages/online/pages/man7/unix.7.html"><code>AF_Unix</code>
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domain sockets</a>. <code>AF_Unix</code> sockets exist in the file system
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often looking like
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<pre>/tmp/<i>pipename</i></pre>
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Windows named pipes have a path, but they are not directly part of the file
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system; instead they look like
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<pre>\\.\pipe\<i>pipename</i></pre>
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<dl>
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<dt><code>socket(AF_Unix, SOCK_STREAM, 0), bind(2), listen(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/aa365150(VS.85).aspx"><code>CreateNamedPipe()</code></a>
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<p>Use <code>FILE_FLAG_OVERLAPPED</code>, <code>PIPE_TYPE_BYTE</code>,
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<code>PIPE_NOWAIT</code>.
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</dd>
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<dt><code>send(2)</code>, <code>write(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/aa365748(v=VS.85).aspx"><code>WriteFileEx()</code></a>
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</dd>
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<dt><code>recv(2)</code>, <code>read(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/aa365468(v=VS.85).aspx"><code>ReadFileEx()</code></a>
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</dd>
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<dt><code>connect(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/aa365150(VS.85).aspx"><code>CreateNamedPipe()</code></a>
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</dd>
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<dt><code>accept(2)</code></dt>
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<dd>
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<a href="http://msdn.microsoft.com/en-us/library/aa365146(v=VS.85).aspx"><code>ConnectNamedPipe()</code></a>
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</dd>
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</dl>
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Examples:
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<ul>
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<li><a
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href="http://msdn.microsoft.com/en-us/library/aa365601(v=VS.85).aspx">Named
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Pipe Server Using Completion Routines</a>
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<li><a
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href="http://msdn.microsoft.com/en-us/library/aa365603(v=VS.85).aspx">Named
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Pipe Server Using Overlapped I/O</a>
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</ul>
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<h3>Regular Files</h3>
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<p>
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In Unix file system files are not able to use non-blocking I/O. There are
|
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some operating systems that have asynchronous I/O but it is not standard and
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at least on Linux is done with pthreads in GNU libc. For this reason
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applications designed to be portable across different Unixes must manage a
|
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thread pool for issuing file I/O syscalls.
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|
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<p>
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The situation is better in Windows: true overlapped I/O is available when
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reading or writing a stream of data to a file.
|
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|
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<dl>
|
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|
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<dt><code>write(2)</code></dt>
|
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<dd> Windows:
|
|
<a href="http://msdn.microsoft.com/en-us/library/aa365748(v=VS.85).aspx"><code>WriteFileEx()</code></a>
|
|
|
|
<p>Solaris's event completion ports has true in-kernel async writes with <a
|
|
href="http://download.oracle.com/docs/cd/E19253-01/816-5171/aio-write-3rt/index.html">aio_write(3RT)</a>
|
|
</dd>
|
|
|
|
<dt><code>read(2)</code></dt>
|
|
<dd> Windows:
|
|
<a href="http://msdn.microsoft.com/en-us/library/aa365468(v=VS.85).aspx"><code>ReadFileEx()</code></a>
|
|
|
|
<p>Solaris's event completion ports has true in-kernel async reads with <a
|
|
href="http://download.oracle.com/docs/cd/E19253-01/816-5171/aio-read-3rt/index.html">aio_read(3RT)</a>
|
|
</dd>
|
|
|
|
</dl>
|
|
|
|
<h3>Console/TTY</h3>
|
|
|
|
<p>It is (usually?) possible to poll a Unix TTY file descriptor for
|
|
readability or writablity just like a TCP socket—this is very helpful
|
|
and nice. In Windows the situation is worse, not only is it a completely
|
|
different API but there are not overlapped versions to read and write to the
|
|
TTY. Polling for readability can be accomplished by waiting in another
|
|
thread with <a
|
|
href="http://msdn.microsoft.com/en-us/library/ms685061(VS.85).aspx"><code>RegisterWaitForSingleObject()</code></a>.
|
|
|
|
<dl>
|
|
|
|
<dt><code>read(2)</code></dt>
|
|
<dd>
|
|
<a
|
|
href="http://msdn.microsoft.com/en-us/library/ms684958(v=VS.85).aspx"><code>ReadConsole()</code></a>
|
|
and
|
|
<a
|
|
href="http://msdn.microsoft.com/en-us/library/ms684961(v=VS.85).aspx"><code>ReadConsoleInput()</code></a>
|
|
do not support overlapped I/O and there are no overlapped
|
|
counter-parts. One strategy to get around this is
|
|
<pre><a href="http://msdn.microsoft.com/en-us/library/ms685061(VS.85).aspx">RegisterWaitForSingleObject</a>(&tty_wait_handle, tty_handle,
|
|
tty_want_poll, NULL, INFINITE, WT_EXECUTEINWAITTHREAD |
|
|
WT_EXECUTEONLYONCE)</pre>
|
|
which will execute <code>tty_want_poll()</code> in a different thread.
|
|
You can use this to notify the calling thread that
|
|
<code>ReadConsoleInput()</code> will not block.
|
|
</dd>
|
|
|
|
|
|
<dt><code>write(2)</code></dt>
|
|
<dd>
|
|
<a href="http://msdn.microsoft.com/en-us/library/ms687401(v=VS.85).aspx"><code>WriteConsole()</code></a>
|
|
is also blocking but this is probably acceptable.
|
|
</dd>
|
|
|
|
|
|
<dt><a
|
|
href="http://www.kernel.org/doc/man-pages/online/pages/man3/tcsetattr.3.html"><code>tcsetattr(3)</code></a></dt>
|
|
<dd>
|
|
<a href="http://msdn.microsoft.com/en-us/library/ms686033(VS.85).aspx"><code>SetConsoleMode()</code></a>
|
|
</dd>
|
|
|
|
|
|
</dl>
|
|
|
|
|
|
|
|
<h2>Assorted Links</h2>
|
|
<p>
|
|
tips
|
|
<ul>
|
|
<li> overlapped = non-blocking.
|
|
<li> There is no overlapped <a href="http://msdn.microsoft.com/en-us/library/ms738518(VS.85).aspx"><code>GetAddrInfoEx()</code></a> function. It seems Asynchronous Procedure Calls must be used instead.
|
|
<li> <a href=http://msdn.microsoft.com/en-us/library/ms740673(VS.85).aspx"><code>Windows Sockets 2</code></a>
|
|
</ul>
|
|
|
|
<p>
|
|
IOCP:
|
|
<ul>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms686358(v=vs.85).aspx">Synchronization and Overlapped Input and Output</a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms741665(v=VS.85).aspx"><code>OVERLAPPED</code> Structure</a>
|
|
<ul>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms683209(v=VS.85).aspx"><code>GetOverlappedResult()</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms683244(v=VS.85).aspx"><code>HasOverlappedIoCompleted()</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/aa363792(v=vs.85).aspx"><code>CancelIoEx()</code></a>
|
|
— cancels an overlapped operation.
|
|
</ul>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms742203(v=vs.85).aspx"><code>WSASend()</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms741688(v=VS.85).aspx"><code>WSARecv()</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms737606(VS.85).aspx"><code>ConnectEx()</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms740565(v=VS.85).aspx"><code>TransmitFile()</code></a>
|
|
— an async <code>sendfile()</code> for windows.
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms741565(v=VS.85).aspx"><code>WSADuplicateSocket()</code></a>
|
|
— describes how to share a socket between two processes.
|
|
<li id="setmaxstdio"><a href="http://msdn.microsoft.com/en-us/library/6e3b887c.aspx"><code>_setmaxstdio()</code></a>
|
|
— something like setting the maximum number of file decriptors
|
|
and <a
|
|
href="http://www.kernel.org/doc/man-pages/online/pages/man2/setrlimit.2.html"><code>setrlimit(3)</code></a>
|
|
AKA <code>ulimit -n</code>. Note the file descriptor limit on windows is
|
|
2048.
|
|
</ul>
|
|
|
|
<p>
|
|
APC:
|
|
<ul>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms681951(v=vs.85).aspx">Asynchronous Procedure Calls</a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/ms682016"><code>DNSQuery()</code></a>
|
|
— General purpose DNS query function like <code>res_query()</code> on Unix.
|
|
</ul>
|
|
|
|
|
|
Pipes:
|
|
<ul>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/aa365781(v=VS.85).aspx"><code>Pipe functions</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/aa365150(VS.85).aspx"><code>CreateNamedPipe</code></a>
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/aa365144(v=VS.85).aspx"><code>CallNamedPipe</code></a>
|
|
— like <code>accept</code> is for Unix pipes.
|
|
<li><a href="http://msdn.microsoft.com/en-us/library/aa365146(v=VS.85).aspx"><code>ConnectNamedPipe</code></a>
|
|
</ul>
|
|
|
|
<code>WaitForMultipleObjectsEx</code> is pronounced "wait for multiple object sex".
|
|
|
|
Also useful:
|
|
<a
|
|
href="http://msdn.microsoft.com/en-us/library/xw1ew2f8(v=vs.80).aspx">Introduction
|
|
to Visual C++ for Unix Users</a>
|
|
|
|
<br><br>
|
|
<a
|
|
href="http://ebookbrowse.com/network-programming-for-microsoft-windows-2nd-edition-2002-pdf-d73663829">Network
|
|
Programming For Microsoft Windows 2nd Edition 2002</a>. Juicy details on
|
|
page 119.
|
|
|
|
</body></html>
|