Because the subagent cannot know in advance which kind of packet arrives from the agent, await a DPI packet (forever), then parse the packet, check the packet type, and process the request based on the DPI packet type. A call to pDPIpacket, which expects as parameter a pointer to the encoded or serialized DPI packet, returns a pointer to a DPI parse tree. The pointer points to an snmp_dpi_hdr structure which looks as follows:

struct snmp_dpi_hdr {
  unsigned char  proto_major;
  unsigned char  proto_version;
  unsigned char  proto_release;
  unsigned short packet_id;
  unsigned char  packet_type;
  union {
     snmp_dpi_reg_packet      *reg_p;
     snmp_dpi_ureg_packet     *ureg_p;
     snmp_dpi_get_packet      *get_p;
     snmp_dpi_next_packet     *next_p;
     snmp_dpi_next_packet     *bulk_p;
     snmp_dpi_set_packet      *set_p;
     snmp_dpi_resp_packet     *resp_p;
     snmp_dpi_trap_packet     *trap_p;
     snmp_dpi_open_packet     *open_p;
     snmp_dpi_close_packet    *close_p;
     unsigned char            *any_p;
  } data_u;
};
typedef struct snmp_dpi_hdr    snmp_dpi_hdr;
#define snmp_dpi_hdr_NULL_p    ((snmp_dpi_hdr *)0)

With the DPI parse tree, you decide how to process the DPI packet. The following code example demonstrates the high-level process of a DPI subagent.

main(int argc, char
*argv[], char *envp][{}[][])
{
       unsigned char *packet_p;
       int            i     = 0;
       int            rc    = 0;
#ifndef DPI_VERY_MINIMAL_SUBAGENT         /* with VERY minimal agent
*/
       int            debug = 0;
#endif /* ndef DPI_VERY_MINIMAL_SUBAGENT */
       unsigned long  length;
       snmp_dpi_hdr  *hdr_p;
       char          *hostname_p = NULL;                      /* @L1C*/
       char          *community_p = SNMP_COMMUNITY;
       char          *cmd_p       = "";
       char           hostname[MAX_HOSTNAME_LEN+1];            /*
@L1A*/
 
       if (argc >= 1) cmd_p = argv[0];
 
       for (i=1; i < argc; i++) {
          if (strcmp(argv[i],"-h") == 0) {
             if (i+1 >= argc) {
                printf("Need hostname\n\n");
                usage(cmd_p);
             } /* endif */
             hostname_p = argv[++i];
#ifndef DPI_VERY_MINIMAL_SUBAGENT         /* with VERY minimal agent
*/
          } else if (strcmp(argv[i],"-c") == 0) {
             if (i+1 >= argc) {
                printf("Need community name\n\n");
                usage(cmd_p);
             } /* endif */
             community_p = argv[++i];
#ifdef INCLUDE_UNIX_DOMAIN_FOR_DPI
          } else if (strcmp(argv[i],"-unix") == 0) {
             unix_sock = 1;
#endif /* def INCLUDE_UNIX_DOMAIN_FOR_DPI */
          } else if (strcmp(argv[i],"-ireg") == 0) {
             instance_level = 1;
          } else if (strcmp(argv[i],"-d") == 0) {
             if (i+1 >= argc) {
                debug = 1;
                continue;
             }
             if ((strlen(argv[i+1]) == 1) && isdigit(*argv[i+1]))
{
                i++;
                debug = atoi(argv[i]);
             } else {
                debug = 1;
             } /* endif */
#endif /* ndef DPI_VERY_MINIMAL_SUBAGENT */
          } else {
             usage(cmd_p);
          } /* endif */
       } /* endfor */
 
#ifndef DPI_VERY_MINIMAL_SUBAGENT
       if (debug) {
          printf("\n%s - %s\n",__FILE__, VERSION);
          DPIdebug(debug);                /* turn on DPI dubugging
  */
          timeout += 6;                   /* longer timeout please
  */
       } /* endif */
#endif /* ndef DPI_VERY_MINIMAL_SUBAGENT */
 
       if (hostname_p == NULL) {          /* -h not specified.  Try
to
                                             obtain local host name
                                                                 @L1A*/
         if (gethostname(hostname, MAX_HOSTNAME_LEN) != 0) {
           printf("\ngethostname failed. "
                  "Restart with -h parameter.\n\n");          /* @L1A*/
           exit(1);                                           /* @L1A*/
         }
         else {                           /* gethostname worked  @L1A*/
           hostname_p = hostname;                             /* @L1A*/
         }                                                    /* @L1A*/
       }                                  /* -h not specified    @L1A*/
 
       /* first init value2_p, our dpiSimpleString (DisplayString)
  */
       /* since we treat it as display string keep terminating NULL
 */
       value2_p = (char *) malloc(strlen("Initial String")+1);
       if (value2_p) {
          memcpy(value2_p,"Initial String",strlen("Initial String")+1);
          value2_len = strlen("Initial String")+1;
       } /* endif */
 
       do_connect_and_open(hostname_p,
                            community_p); /* connect and DPI-OPEN
   */
 
       do_register();                     /* register our subtree
   */
 
       do_trap();                         /* issue a trap as sample
 */
 
       while (rc == 0) {                  /* do forever          
   */
         rc = DPIawait_packet_from_agent( /* wait for a DPI packet
  */
                 handle,                  /* on this connection  
   */
                 -1,                      /* wait forever        
   */
                 &packet_p,               /* receives ptr to packet
 */
                 &length);                /* receives packet length
 */
 
         if (rc != DPI_RC_OK) exit(1);    /* If it failed, exit  
   */
 
         hdr_p = pDPIpacket(packet_p);    /* parse DPI packet    
   */
         if (hdr_p == snmp_dpi_hdr_NULL_p)/* If we fail to parse it
 */
            exit(1);                      /* then exit           
   */
 
         switch(hdr_p->packet_type) {     /* handle by DPI type  
   */
         case SNMP_DPI_GET:
           rc = do_get(hdr_p,
                       hdr_p->data_u.get_p);
           break;
         case SNMP_DPI_GETNEXT:
           rc = do_next(hdr_p,
                        hdr_p->data_u.next_p);
           break;
         case SNMP_DPI_SET:
         case SNMP_DPI_COMMIT:
         case SNMP_DPI_UNDO:
           rc = do_set(hdr_p,
                       hdr_p->data_u.set_p);
           break;
         case SNMP_DPI_CLOSE:
           rc = do_close(hdr_p,
                         hdr_p->data_u.close_p);
           break;
         case SNMP_DPI_UNREGISTER:
           rc = do_unreg(hdr_p,
                         hdr_p->data_u.ureg_p);
           break;
         default:
           printf("Unexpected DPI packet type %d\n",
                  hdr_p->packet_type);
           rc = -1;
         } /* endswitch */
         if (rc) exit(1);
       } /* endwhile */
 
       return(0);
} /* end of main() */