How to retrieve OID value using Net SNMP in C? - c

Question

I am new to this. I am trying to use Net-SNMP's snmpget file to retrieve an OID and store it in a variable. Where can I specify the IP address of the device I am querying? Where can I specify what OID I am looking for? And how can I store the value of the OID?
I already know the IP address of my device and the OID I am looking for.
Here is the code Net-SNMP provides for snmpget:
#include <net-snmp/net-snmp-config.h>
#if HAVE_STDLIB_H
#include <stdlib.h>
#endif
#if HAVE_UNISTD_H
#include <unistd.h>
#endif
#if HAVE_STRING_H
#include <string.h>
#else
#include <strings.h>
#endif
#include <sys/types.h>
#if HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#include <stdio.h>
#include <ctype.h>
#if TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# if HAVE_SYS_TIME_H
# include <sys/time.h>
# else
# include <time.h>
# endif
#endif
#if HAVE_SYS_SELECT_H
#include <sys/select.h>
#endif
#if HAVE_NETDB_H
#include <netdb.h>
#endif
#if HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif
#include <net-snmp/utilities.h>
#include <net-snmp/net-snmp-includes.h>
#define NETSNMP_DS_APP_DONT_FIX_PDUS 0
static void
optProc(int argc, char *const *argv, int opt)
{
switch (opt) {
case 'C':
while (*optarg) {
switch (*optarg++) {
case 'f':
netsnmp_ds_toggle_boolean(NETSNMP_DS_APPLICATION_ID,
NETSNMP_DS_APP_DONT_FIX_PDUS);
break;
default:
fprintf(stderr, "Unknown flag passed to -C: %c\n",
optarg[-1]);
exit(1);
}
}
break;
}
}
void
usage(void)
{
fprintf(stderr, "USAGE: snmpget ");
snmp_parse_args_usage(stderr);
fprintf(stderr, " OID [OID]...\n\n");
snmp_parse_args_descriptions(stderr);
fprintf(stderr,
" -C APPOPTS\t\tSet various application specific behaviours:\n");
fprintf(stderr,
"\t\t\t f: do not fix errors and retry the request\n");
}
int
main(int argc, char *argv[])
{
netsnmp_session session, *ss;
netsnmp_pdu *pdu;
netsnmp_pdu *response;
netsnmp_variable_list *vars;
int arg;
int count;
int current_name = 0;
char *names[SNMP_MAX_CMDLINE_OIDS];
oid name[MAX_OID_LEN];
size_t name_length;
int status;
int failures = 0;
int exitval = 0;
/*
* get the common command line arguments
*/
switch (arg = snmp_parse_args(argc, argv, &session, "C:", optProc)) {
case NETSNMP_PARSE_ARGS_ERROR:
exit(1);
case NETSNMP_PARSE_ARGS_SUCCESS_EXIT:
exit(0);
case NETSNMP_PARSE_ARGS_ERROR_USAGE:
usage();
exit(1);
default:
break;
}
if (arg >= argc) {
fprintf(stderr, "Missing object name\n");
usage();
exit(1);
}
if ((argc - arg) > SNMP_MAX_CMDLINE_OIDS) {
fprintf(stderr, "Too many object identifiers specified. ");
fprintf(stderr, "Only %d allowed in one request.\n", SNMP_MAX_CMDLINE_OIDS);
usage();
exit(1);
}
/*
* get the object names
*/
for (; arg < argc; arg++)
names[current_name++] = argv[arg];
SOCK_STARTUP;
/*
* Open an SNMP session.
*/
ss = snmp_open(&session);
if (ss == NULL) {
/*
* diagnose snmp_open errors with the input netsnmp_session pointer
*/
snmp_sess_perror("snmpget", &session);
SOCK_CLEANUP;
exit(1);
}
/*
* Create PDU for GET request and add object names to request.
*/
pdu = snmp_pdu_create(SNMP_MSG_GET);
for (count = 0; count < current_name; count++) {
name_length = MAX_OID_LEN;
if (!snmp_parse_oid(names[count], name, &name_length)) {
snmp_perror(names[count]);
failures++;
} else
snmp_add_null_var(pdu, name, name_length);
}
if (failures) {
snmp_close(ss);
SOCK_CLEANUP;
exit(1);
}
/*
* Perform the request.
*
* If the Get Request fails, note the OID that caused the error,
* "fix" the PDU (removing the error-prone OID) and retry.
*/
retry:
status = snmp_synch_response(ss, pdu, &response);
if (status == STAT_SUCCESS) {
if (response->errstat == SNMP_ERR_NOERROR) {
for (vars = response->variables; vars;
vars = vars->next_variable)
print_variable(vars->name, vars->name_length, vars);
} else {
fprintf(stderr, "Error in packet\nReason: %s\n",
snmp_errstring(response->errstat));
if (response->errindex != 0) {
fprintf(stderr, "Failed object: ");
for (count = 1, vars = response->variables;
vars && count != response->errindex;
vars = vars->next_variable, count++)
/*EMPTY*/;
if (vars) {
fprint_objid(stderr, vars->name, vars->name_length);
}
fprintf(stderr, "\n");
}
exitval = 2;
/*
* retry if the errored variable was successfully removed
*/
if (!netsnmp_ds_get_boolean(NETSNMP_DS_APPLICATION_ID,
NETSNMP_DS_APP_DONT_FIX_PDUS)) {
pdu = snmp_fix_pdu(response, SNMP_MSG_GET);
snmp_free_pdu(response);
response = NULL;
if (pdu != NULL) {
goto retry;
}
}
} /* endif -- SNMP_ERR_NOERROR */
} else if (status == STAT_TIMEOUT) {
fprintf(stderr, "Timeout: No Response from %s.\n",
session.peername);
exitval = 1;
} else { /* status == STAT_ERROR */
snmp_sess_perror("snmpget", ss);
exitval = 1;
} /* endif -- STAT_SUCCESS */
if (response)
snmp_free_pdu(response);
snmp_close(ss);
SOCK_CLEANUP;
return exitval;
} /* end main() */