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<html>
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<head><title>lwIP - A Lightweight TCP/IP Stack - OS vs non-OS</title></head>
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<body bgcolor="white">
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<table width="640" border="0" cellpadding="0"
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cellspacing="0">
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<tr><td>
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<center><h1>lwIP - A Lightweight TCP/IP Stack</h1></center>
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</td></tr></table>
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<table width="640" border="0" cellpadding="0"
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cellspacing="0">
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<tr><td align="center">
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<hr>
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<a href="index.html">Introduction</a>
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<a href="news.html">News</a>
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<a href="documentation.html">Documentation</a>
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<a href="mailinglist.html">Mailing list</a>
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<a href="changelog.html">Changelog</a>
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<a href="download.html">Download</a>
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<a href="links.html">Links</a>
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<hr>
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</td></tr>
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<tr>
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<td><center><h2>OS vs non-OS</h2></center>
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</td></tr></table>
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<table width="640" border="0" cellpadding="0"
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cellspacing="0"><tr>
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<td width="50">
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</td>
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<td bgcolor="white" width="540">
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<h2>Using lwIP with or without an operating system</h2>
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<p align="justify">
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There has been a few questions about how lwIP can be used in a
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standalone environment (i.e., an environment without a multi-threaded
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operating system) lately. The purpose of this document is to describe
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how lwIP is designed to be used with and without a multi-threaded
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operating system.
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</p>
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<center><img src="os.png"></center>
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<h3>The lwIP single-threaded core</h3>
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<p align="justify">
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The core of lwIP consists of the actual implementations of the IP,
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ICMP, UDP, and TCP protocols, as well as the support functions such as
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buffer and memory management. The core components are the only ones
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that are needed when lwIP is to be run in a single-threaded (non-OS)
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environment.
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</p>
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<p align="justify">
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The core components can be viewed as a software library which has the
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following interface:
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</p>
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<ul>
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<li><tt>ip_input(pbuf, netif)</tt>: Takes an IP packet and the incoming network
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interface as arguments and does the TCP/IP processing for the packet.
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<li><tt>tcp_tmr()</tt>: Should be called every 100 ms. Does all TCP
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timer processing such as doing retransmissions.
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</ul>
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<p align="justify">
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Because none of the core functions ever needs to block when run in a
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single-threaded environment, the <tt>sys_arch</tt> (the operating
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system abstraction layer) does not need to implement locking
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semaphores or mailboxes. In future versions of lwIP, the dependancy of
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the <tt>sys_arch</tt> will be removed in the single-threaded case.
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</p>
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<p align="justify">
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A simple main loop for a single-threaded system might look
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like this:
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</p>
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<pre>
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while(1) {
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if(poll_driver(netif) == PACKET_READY) {
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pbuf = get_packet(netif);
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ip_input(pbuf, netif);
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}
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if(clock() - last_time == 100 * CLOCK_MS) {
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tcp_tmr();
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last_time = clock();
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}
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}
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</pre>
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<h3>lwIP in a multi-threaded system</h3>
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<p align="justify">
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lwIP is designed to be able to be run in a multi-threaded system with
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applications running in concurrent threads. The model used in this
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case is that all TCP/IP processing is done in a single thread. The
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application thread communicate with the TCP/IP thread using the
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sequential API.
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</p>
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<center><img src="threads.png"></center>
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<p align="justify">
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The inter-thread communication is implemented in the two files
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<tt>api_lib.c</tt> and <tt>api_msg.c</tt>. The former contains the
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functions used by the application programs and the latter implements
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the TCP/IP stack interface. A third file, <tt>tcpip.c</tt>, handles
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incoming packets and timer events as described in the previous
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section.
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</p>
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<p align="justify">
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When run in a multi-threaded environment, incoming packets are handled
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by the function <tt>tcpip_input()</tt>, which takes the same arguments
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as the <tt>ip_input()</tt> function. The difference between the two
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functions is that the <tt>tcpip_input()</tt> function does not process
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the incoming packet immediately. It merely puts the packet on a queue
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which later is drained by the TCP/IP thread.
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</p>
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<p align="justify">
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When being run in a multi-threaded system, timer events are taken care
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of internally in <tt>tcpip.c</tt>.
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</p>
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<p align="right">
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<font size="-1"><i>
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$Date: 2002/10/19 13:00:03 $
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</i></font>
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</p>
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</td>
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<td width="50">
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</td>
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</tr></table>
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<table width="640" border="0" cellpadding="0"
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cellspacing="0">
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<tr><td align="center">
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<hr>
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<a href="index.html">Introduction</a>
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<a href="news.html">News</a>
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<a href="documentation.html">Documentation</a>
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<a href="mailinglist.html">Mailing list</a>
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<a href="changelog.html">Changelog</a>
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|
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<a href="download.html">Download</a>
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|
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<a href="links.html">Links</a>
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<hr>
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</td></tr><tr>
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<td>
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<div align="right">
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<a href="http://www.sics.se/~adam/">Adam Dunkels</a></div>
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</td>
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</tr>
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</table>
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</body>
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</html>
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