doc/html/ref/net-common-dhcp.html

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  12 >DHCP</TITLE
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  84 ><A
  85 NAME="NET-COMMON-DHCP">DHCP</H1
  86 ><P
  87 >This API publishes a routine to maintain DHCP state, and a
  88 semaphore that is signalled when a lease requires attention: this
  89 is your clue to call the aforementioned routine.</P
  90 ><P
  91 >The intent with this API is that a simple DHCP client thread,
  92 which maintains the state of the interfaces, can go as follows:
  93 (after <TT
  94 CLASS="FUNCTION"
  95 >init_all_network_interfaces()</TT
  96 > is
  97 called from elsewhere)</P
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 104 ><PRE
 105 CLASS="PROGRAMLISTING"
 106 >while ( 1 ) {
 107         while ( 1 ) {
 108             cyg_semaphore_wait( &amp;dhcp_needs_attention );
 109             if ( ! dhcp_bind() ) // a lease expired
 110                 break; // If we need to re-bind
 111         }
 112         dhcp_halt(); // tear everything down
 113         init_all_network_interfaces(); // re-initialize
 114 }</PRE
 115 ></TD
 116 ></TR
 117 ></TABLE
 118 ><P
 119 >and if the application does not want to suffer the overhead
 120 of a separate thread and its stack for this, this functionality
 121 can be placed in the app&#8217;s server loop in an obvious fashion.
 122  That is the goal of breaking out these internal elements.  For example,
 123 some server might be arranged to poll DHCP from time to time like
 124 this:</P
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 131 ><PRE
 132 CLASS="PROGRAMLISTING"
 133 >while ( 1 ) {
 134     init_all_network_interfaces();
 135     open-my-listen-sockets();
 136     while ( 1 ) {
 137        serve-one-request();
 138        // sleeps if no connections, but not forever;
 139        // so this loop is polled a few times a minute...
 140        if ( cyg_semaphore_trywait( &amp;dhcp_needs_attention )) {
 141              if ( ! dhcp_bind() ) {
 142                  close-my-listen-sockets();
 143                  dhcp_halt();
 144                  break;
 145              }
 146        }
 147     }
 148 }</PRE
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 152 ><P
 153 >If the configuration option CYGOPT_NET_DHCP_DHCP_THREAD
 154 is defined, then eCos provides a thread as described initially.
 155 Independent of this option, initialization of the interfaces still
 156 occurs in <TT
 157 CLASS="FUNCTION"
 158 >init_all_network_interfaces()</TT
 159 > and
 160 your startup code can call that.  It will start the DHCP management
 161 thread if configured.  If a lease fails to be renewed, the management
 162 thread will shut down all interfaces and attempt to initialize all
 163 the interfaces again from scratch.  This may cause chaos in the
 164 app, which is why managing the DHCP state in an application aware
 165 thread is actually better, just far less convenient for testing.</P
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