一些重要的全局变量                                                                        
 struct net_proto_family *                                                
   net_families                           struct net_prototo              
  +------+                                  protocols []                  
  | 0    |                                +--------------------------+    
  |------|    struct net_proto_family     |         .                |    
  | 1    |      inet_family_ops           |         .                |    
  |------|------>+---------------+        |         .                |    
  | 2    |       |   2           |        |------------------------- |    
  |------|       |---------------|        |"PACKET"                  |    
  | .    |       |inet_creat()   |        |---  ----- ---  --  --  --|    
  |      |       |---------------|        |packet_proto_init()       |    
  | .    |       |     .         |        |--------------------------|    
  |      |       |     .         |        |        .                 |    
  | .    |       |     .         |        |        .                 |    
  |      |       |     .         |        |        .                 |    
  | .    |       |               |        |        .                 |    
  |      |       +---------------+        |--------------------------|    
  | .    |                                |                          |    
  |      |      struct net_proto_family   |"INET"                    |    
  |------|        packet_family_ops       | ---   ---  ---   --  --- |    
  |17    |------>+---------------+        |inet_proto_init()         |    
  |------|       | 17            |        |--------------------------|    
  | .    |       |---------------|        |          .               |    
  | .    |       |packet_create()|        |          .               |    
  | .    |       |---------------|        |          .               |    
  |------|       |      .        |        |--------------------------|    
  |32    |       |      .        |        |NULL                      |    
  +------+       |      .        |        |-- ---  --  --  --- --  --|    
                 |      .        |        |NULL                      |    
                 +---------------+        +--------------------------+