intermediate.html

SDK FAQ集

                u_long host_addr = 0xAC104D58;                  // 172.16.77.88
                u_long net_mask = 0xFFFFE000;                   // 255.255.224.0
                u_long net_addr = host_addr & net_mask;         // 172.16.64.0
                u_long dir_bcast_addr = net_addr | (~net_mask); // 172.16.95.255
</pre>





<a name="nagle"></a>
<h5>3.8 - When should I turn off the <a href="glossary.html#Nagle">Nagle</a> algorithm?</h5>

<p>Generally, almost never.</p>

<p>Inexperienced Winsockers usually try disabling the Nagle algorithm when
they are trying to impose some kind of <a href="#packetscheme">packet
scheme</a> on a TCP data stream. That is, they want to be able to send,
say, two packets, one 40 bytes and the other 60, and have the receiver
get a 40-byte packet followed by a separate 60-byte packet. (With the
Nagle algorithm enabled, TCP will often coalesce these two packets
into a single 100 byte packet.) Unfortunately, this is futile, for the
following reasons:</p>

<ol>

<li>Even if the sender manages to send its packets individually,
the receiving TCP/IP stack may still coalesce the received packets into
a single packet. This can happen any time the sender can send data faster
than the receiver can deal with it.

<li>Turning off the Nagle algorithm in a client program will not
affect the way that the server sends packets, and vice versa.

<li>Routers and other intermediaries on the network can fragment
packets, and there is no guarantee of "proper" reassembly with stream
protocols.

<li>If the network stack runs out of buffers, it may fragment a packet,
queuing up as many bytes as it has buffer space for and discarding the
rest. 

<li>Winsock is not required to give you all the data it has queued
on a socket even if your <code>recv()</code> call gave Winsock enough
buffer space. It may require several calls to get all the data queued
on a socket.

</ol>

<p>Aside from these problems, disabling the Nagle algorithm almost always
causes a program's throughput to degrade. The only time you should disable
the algorithm is when some other consideration, such as packet timing,
is more important than throughput.</p>

<p>Often, programs that deal with real-time user input will disable
the Nagle algorithm to achieve the snappiest possible response, at
the expense of network bandwidth. Three examples are Telnet clients,
X Windows servers, and multiplayer network games. In all three cases,
it is more important that there be as little delay between packets as
possible than it is to conserve network bandwidth.</p>

<p>Note that this is a changable issue: back when network bandwidth was
truly scarce, Telnet clients usually <i>enabled</i> the Nagle algorithm
to save on bandwidth. A contributing factor was that user interfaces in
those days tended to be line oriented, whereas modern ones are real-time,
with each keystroke potentially changing the entire display. The moral
is, consider your application and resources carefully before deciding
to disable the Nagle algorithm.</p>

<p>For more on this topic, see the <a
href="articles/lame-list.html#item19">Lame List</a> and the FAQ article <a
href="articles/effective-tcp.html">How to Use TCP Effectively</a>.</p>





<a name="threadsafety"></a>
<h5>3.9 - Is Winsock thread-safe?</h5>

<p>The Winsock specification does not mandate that a Winsock
implementation be thread-safe, but it does <i>allow</i> an implementor
to create a thread-safe version of Winsock.</p>

<p><a href="reviews/wsnp.html">Bob Quinn</a> says, on this subject:</p>

<ul>

<li>"WinSock, <i>any</i> implementation, is thread safe if the WinSock
implementation developer makes it so (it doesn't just happen)."

<li>"I don't know of any implementations from Microsoft (or any
other vendors) that are not thread safe."

<li>"If a WinSock application developer creates a multi-threaded 
application that shares sockets among the threads, it is that 
developer's responsibility to synchronize activities between 
the threads."

</ul>

<p>By "synchronize activities", I believe Bob means that it may cause
problems if, for example, two threads repeatedly call <code>send()</code>
on the same socket. There is no guarantee in the Winsock specification
about how the data will be interleaved in this situation. Similarly, if
one thread calls <code>closesocket()</code> on a socket, it must somehow
signal other threads using that socket that the socket is now invalid.</p>

<p>I believe one thread calling <code>send()</code> and another thread
calling <code>recv()</code> on a single socket is safe, but I have not
tested this. Hard information, demonstration code and/or anecdotal
evidence either way would be appreciated.</p>

<p>Instead of multiple threads accessing a single socket, you may
want to consider setting up a pair of network I/O queues. Then, give
one thread sole ownership of the socket: this thread sends data from
one I/O queue and enqueues received data on the other. Then other
threads can access the queues (with suitable synchronization).</p>

<p>Applications that use some kind of non-synchronous socket typically
have some I/O queue already. Of particular interest in this case is
overlapped I/O or I/O completion ports, because these I/O strategies
are also thread-friendly. You can tell Winsock about several OVERLAPPED
blocks, and Winsock will finish sending one before it moves on to the
next. This means you can keep a chain of these OVERLAPPED blocks, each
perhaps added to the chain by a different thread. Each thread can also
call <code>WSASend()</code> on the block they added, making your main
loop simpler.</p>





<a name="threaddupdata"></a>
<h5>3.10 - If two threads in an application call <code>recv()</code> on a socket, will they each get the same data?</h5>

<p>No. Winsock does not duplicate data among threads.</p>

<p>Note that if you <i>do</i> call <code>recv()</code> at the same time
on a single socket from two different threads, havoc may result. See
<a href="#threadsafety">the previous question</a> for more info.</p>




<a name="threadnotification"></a>
<h5>3.11 - Is there any way for two threads to be notified when something happens on a socket?</h5>

<p>No. If two threads call <code>WSAAsyncSelect()</code> on
a single socket, only the thread that made the last call to
<code>WSAAsyncSelect()</code> will receive further notification
messages. Similarly, if two threads call <code>WSAEventSelect()</code>
on a socket, only the event object used in the last call will
be signaled when an event occurs on that socket. You also can't
call <code>WSAAsyncSelect()</code> on a socket in one thread and
<code>WSAEventSelect()</code> on that same socket in another thread,
because the calls are mutually exclusive for any single socket. Finally,
you cannot reliably call <code>select()</code> on a single socket from
two threads and get the same notifications in each, because one thread
could clear or cause an event, which would change the events that the
other thread sees.</p>





<a name="modemconn"></a>
<h5>3.12 - How do I detect if the modem is connected?</h5>

<p>Although this is not a Winsock question <i>per se</i>, it is sometimes
useful for a Winsock program to only do its thing if the computer is
already connected to the Internet. Before implementing this, however,
keep in mind that many computers are not connected to the Internet through
a modem. Often, they are connected to a LAN, and that LAN is somehow <a
href="glossary.html#gateway">gatewayed</a> to the Internet. For these
situations, the following call will fool your program into believing that
the user either has no Internet connection, or that it is never up.</p>

<p>Having said that, the Remote Access Service (RAS) API gives an
application access to the dial-up networking subsystem. In particular,
the <code>RasEnumConnections()</code> call lets you easily get a list
of the connected modems.</p>





<a name="username"></a>
<h5>3.13 - How can I get the local user name?</h5>

<p>There are a few ways. The easiest is to use the Win32 call
<code>GetUserName()</code>. <a href="examples/username.html">[C++
Example]</a>.</p>

<p>The other way is shown in the Microsoft Knowledge Base article
<a href="http://support.microsoft.com/support/kb/articles/q155/6/98.asp">Q155698</a>. It is much more complex, and it shows two completely
different methods, one for Windows 9x/Windows 3.1 and one for Windows
NT. Unless you need Windows 3.1 support or the LAN Manager domain name
(as opposed to the DNS domain name), I suggest you give this article
a miss.</p>





<a name="dunbug"></a>
<h5>3.14 - Windows 9x's Dial Up Networking keeps popping up an automatic dial window, even when it isn't necessary. Can I make it stop?</h5>

<p>On some Windows 9x systems with more than one network interface,
Dial Up Networking sometimes pops up an automatic-dial window even when
it is obviously not required. An example of such a setup is a machine
on a LAN that also has a modem for connecting to the Internet.</p>

<p>The most common trigger for the DUN dial window is a Winsock program
calling the <code>gethostbyname()</code> function, which initiates a
DNS lookup. Even if the name is that of a LAN machine and there's a DNS
server on the LAN, DUN will still try to bring up the Internet link to
try that first.</p>

<p>If you try messing with the DNS configuration of a <a
href="glossary.html#multi-homed">multihomed</a> Win9x box, it's clear
that the network subsystem just isn't designed to support a local DNS
server in addition to a remote one. The best solution, then, is to just
use straight IP addresses, and write your programs to recognize an IP
address, so they don't have to call <code>gethostbyname()</code>.</p>

<p>I've heard that DUN 1.3 and/or the Winsock 2 updates fix this
problem, but other reports say it doesn't help.</p>



		</td>
	</tr>
</table>


<!--  ---- Document Footer ----  -->

<hr noshade size=1 color=#404040>

<table cellpadding=5 cellspacing=0 border=0 width=95% align=center> 
	<tr>
		<td align=left>
		    <a href="newbie.html">&lt;&lt; Information for New Winsockers</a>
		</td>

		<td align=right>
		    <a href="advanced.html">Advanced Winsock Issues &gt;&gt;</a>
		</td>
	</tr>

	<tr>
		<td align=left>
			<i>Last modified on 29 April 2000 at 15:52 UTC-7</i>
		</td>

		<td align=right>
			<font size=-1>Please send corrections to <a href="mailto:[e-mail]">[e-mail]</a>.</font>
		</td>
	</tr>	
</table>	

<table cellpadding=5 cellspacing=0 border=0 width=95% align=center> 
	<tr>
		<td align=left width=33%>
			<font size=-1>
				<a href="index.html"><b>&lt;</b> Go to the main FAQ page</a>
			</font>
		</td>

		<td width=33%>
			<font size=-1>
			<center>
				<a href="http://www.cyberport.com/~tangent/programming"><b>&lt;&lt;</b> Go to my Programming pages</a>
			</center>
			</font>
		</td>

		<td align=right width=33%>
			<font size=-1>
				<a href="http://www.cyberport.com/~tangent/"><b>&lt;&lt;&lt;</b> Go to my Home Page</a>
			</font>
		</td>
	</tr>	
</table>	


</body>
</html>