Solaris System Management Agent Developer's Guide

Modifying the `demo_module_7` Code

The following procedure lists the steps you should follow to enable your module to be dynamically updated. The procedure uses examples from the demo_module_7.c code to illustrate each step. The code contains modifications to code templates that were produced by using mib2c on a MIB group in SDK-DEMO1–MIB.txt.

The demo_module_7 example registers new instances as contexts that represent files. Subsequent snmpget requests to these contexts retrieve the size of a specified file.

How to Enable Dynamic Update of a Multi-Instance Module

Define two objects in the MIB for the module:

A string with read-write MAX-ACCESS that, when set, registers the specified string as a context name.
A string with read-write MAX-ACCESS that, when set, unregisters the specified string context name.

For example, the following objects, which are defined in the SDK-DEMO1-MIB.txt file, register and unregister a context string that is set with an snmpset request:

 me1createContext OBJECT-TYPE
    SYNTAX      OCTET STRING (SIZE(0..1024))
    MAX-ACCESS  read-write
    STATUS      current
    DESCRIPTION
                "String which when set, registers a context."
    ::= { me1MultiGroup 2 }
    
  me1removeContext OBJECT-TYPE
    SYNTAX      OCTET STRING (SIZE(0..1024))
    MAX-ACCESS  read-write
    STATUS      current
    DESCRIPTION
                "String which when set, unregisters a context."
    ::= { me1MultiGroup 3 }

In the module, declare the location within the MIB tree where the OIDs for the context objects should be registered.

For example, the following code declares the OIDs for context strings:

// Registers a context
   static oid me1createContext_oid[] =
  { 1,3,6,1,4,1,42,2,2,4,4,6,1,2,0 }; 

   // Unregisters a context
   static oid me1removeContext_oid[] =
  { 1,3,6,1,4,1,42,2,2,4,4,6,1,3,0 };

In the module, register both OIDs of the context objects with the SMA.

The following code shows an example:

    // Create a read-write registration handler named filesize, 
    // which calls the set_createContext function to service snmp requests
    // for the me1createContext_oid object.  The OID_LENGTH argument
    // calculates the length of the me1createContext_oid.
    myreg1 = netsnmp_create_handler_registration
                                       ("filesize",
         set_createContext,
         me1createContext_oid,
         OID_LENGTH(me1createContext_oid),
         HANDLER_CAN_RWRITE);
         
     
    // Create a read-write registration handler named filesize, 
    // which calls the set_removeContext function to service snmp requests
    // for the me1removeContext_oid object.  The OID_LENGTH argument
    // calculates the length of the me1removeContext_oid.
    myreg1 = netsnmp_create_handler_registration
        ("filesize",
          set_removeContext,
          me1removeContext_oid,
          OID_LENGTH(me1removeContext_oid),
          HANDLER_CAN_RWRITE);

In the set_createContext() function handler code, extract the context name string from the SNMP message. Register the string as a new context.

The following code shows an example:

int
set_createContext(netsnmp_mib_handler *handler,
      netsnmp_handler_registration *reginfo,
      netsnmp_agent_request_info *reqinfo,
      netsnmp_request_info *requests)
{

// This handler handles set requests on the m1createContext_oid.
// The handler extracts the string from the snmp set request and
// uses it to register a new context for the me1filesize_oid.
//
// For detailed info. on net-snmp set processing, 
// see http://www.net-snmp.org/tutorial-5/toolkit/mib_module/index.html 
// The agent calls each SNMP mode in sequence. We include a case
// statement with only a break statement for each snmp set mode the
// the agent handles.  In this example, we implement only the
// snmp set action mode.  The case statement 
// transfers control to the default: case when no other condition 
// is satisfied. 
netsnmp_handler_registration *myreg;
char *context_names[256];

switch(reqinfo->mode) {

case MODE_SET_RESERVE1:
            break;
case MODE_SET_RESERVE2:
            break;
case MODE_SET_FREE:
            break;
case MODE_SET_ACTION:

    // You must allocate memory for this variable because
    // the unregister_mib function frees it.
    filename = malloc(requests->requestvb->val_len + 1);
    snprintf(filename, sizeof(filename), "%s",  (u_char *)
                requests->requestvb->val.string);
                
    // Create a registration handler for the me1filesize_oid 
    // object in the new context name specified by 
    // the snmp set on the me1createContext OID. 
    myreg = netsnmp_create_handler_registration 
	         ("test",
             get_test,
             me1filesize_oid,
             OID_LENGTH(me1filesize_oid),
             HANDLER_CAN_RONLY);
    myreg->contextName=filename;
    break;
case MODE_SET_COMMIT:
            break;
case MODE_SET_UNDO:
            break;

default:
    /* we should never get here, so this is a really bad error */
    DEBUGMSGTL(("filesize", "default CALLED\n"));
}
    return SNMP_ERR_NOERROR;
}

In the set_removeContext handler code, extract the context name string from the SNMP message. Unregister the context.

The following code shows an example:

// This handler handles set requests on the m1removeContext_oid     
int
set_removeContext(netsnmp_mib_handler *handler,
      netsnmp_handler_registration *reginfo,
      netsnmp_agent_request_info *reqinfo,
      netsnmp_request_info *requests)
{

    static int PRIORITY = 0;
    static int SUB_ID = 0;
    static int RANGE_UBOUND = 0;

    switch(reqinfo->mode) {

        case MODE_SET_RESERVE1:
            break;
        case MODE_SET_RESERVE2:
            break;
        case MODE_SET_ACTION:

       snprintf(filename, sizeof(filename), "%s\n",  (u_char *)
             requests->requestvb->val.string);
  unregister_mib_context(me1filesize_oid, OID_LENGTH(me1filesize_oid), 
       PRIORITY, SUB_ID, RANGE_UBOUND,
       filename);
       break;

           case MODE_SET_COMMIT:
               break;
           case MODE_SET_FREE:
               break;
           case MODE_SET_UNDO:
               break;

    default:
        /* we should never get here, so this is a really bad error */
        DEBUGMSGTL(("filesize", "set_removeContext CALLED\n"));
    }
  return SNMP_ERR_NOERROR;
}

In the handler code for a new context, get the context string from the reginfo->contextName variable.

 /* This handler is called to handle snmp get requests for 
       the me1filesize_oid for a specified context name. */
   
int
get_test(netsnmp_mib_handler *handler,
      netsnmp_handler_registration *reginfo,
      netsnmp_agent_request_info *reqinfo,
      netsnmp_request_info *requests)
{

/* We are never called for a GETNEXT if it's registered as an
   "instance", as it's "magically" handled for us.  */

/* An instance handler also only hands us one request at a time, so
   we don't need to loop over a list of requests; we'll only get one. */

struct stat buf;
static int fd = 0;

switch(reqinfo->mode) {

case MODE_GET:

   if (strcmp(reginfo->contextName, filename) == 0) 

     // An open() for reading only returns without delay.
     if ((fd = open(filename, O_NONBLOCK | O_RDONLY)) == -1)
        DEBUGMSGTL(("filesize", "ERROR\n")); 

     if (fstat(fd, &buf) == -1)
       DEBUGMSGTL(("filesize", "ERROR\n")); 
     else
       DEBUGMSGTL(("filesize", "FILE SIZE IN BYTES = %d:\n", buf.st_size));
       
     snmp_set_var_typed_value(requests->requestvb, ASN_INTEGER, 
     (u_char *) &buf.st_size /* XXX: a pointer to the scalar's data */, 
     sizeof(buf.st_size) /* XXX: the length of the data in bytes */);

        break;

   default:
    /* we should never get here, so this is a really bad error */
       return SNMP_ERR_GENERR;
}

return SNMP_ERR_NOERROR;
}

Modifying the demo_module_7 Code

How to Enable Dynamic Update of a Multi-Instance Module

Modifying the `demo_module_7` Code