C：如何訪問由Net-SNMP返回的值GET

Question

對於樸素的問題，Iam對Net-SNMP新增了一些道歉。我曾嘗試使用Net-SNMP網站中給出的this simple SNMP demo app。

該代碼執行SNMP-GET和操縱響應，以檢查是否返回值是一個ASN_OCTET_STRING，如果是，則訪問使用vars->val.string並分配給一個字符指針sp的字符串。

但Iam無法知道如何訪問此值，如果該類型不是ASN_OCTET_STRING以外的任何其他類型。例如，如何獲取此值，並將其分配給變量（如果其類型爲'ASN_INTEGER'或'ASN_OBJECT_ID'）。

#include <net-snmp/net-snmp-config.h> 
#include <net-snmp/net-snmp-includes.h> 
#include <string.h> 
#define DEMO_USE_SNMP_VERSION_3 

#ifdef DEMO_USE_SNMP_VERSION_3 
const char *our_v3_passphrase = "MD5Password"; 
#endif 

int main(int argc, char ** argv) 
{ 
    netsnmp_session session, *ss; 
    netsnmp_pdu *pdu; 
    netsnmp_pdu *response; 

    oid anOID[MAX_OID_LEN]; 
    size_t anOID_len; 

    netsnmp_variable_list *vars; 
    int status; 
    int count=1; 

    init_snmp("snmpdemoapp"); 

    snmp_sess_init(&session);     
    session.peername = strdup("localhost:161"); 


#ifdef DEMO_USE_SNMP_VERSION_3 

    session.version=SNMP_VERSION_3; 

    session.securityName = strdup("user2"); 
    session.securityNameLen = strlen(session.securityName); 

    session.securityLevel = SNMP_SEC_LEVEL_AUTHNOPRIV; 

    session.securityAuthProto = usmHMACMD5AuthProtocol; 
    session.securityAuthProtoLen = sizeof(usmHMACMD5AuthProtocol)/sizeof(oid); 
    session.securityAuthKeyLen = USM_AUTH_KU_LEN; 

    if (generate_Ku(session.securityAuthProto, 
        session.securityAuthProtoLen, 
        (u_char *) our_v3_passphrase, strlen(our_v3_passphrase), 
        session.securityAuthKey, 
        &session.securityAuthKeyLen) != SNMPERR_SUCCESS) { 
     snmp_perror(argv[0]); 
     snmp_log(LOG_ERR, 
       "Error generating Ku from authentication pass phrase. \n"); 
     exit(1); 
    } 

#else /* we'll use the insecure (but simplier) SNMPv1 */ 

    session.version = SNMP_VERSION_1; 

    session.community = "demopublic"; 
    session.community_len = strlen(session.community); 

#endif /* SNMPv1 */ 

    SOCK_STARTUP; 
    ss = snmp_open(&session);      
    if (!ss) { 
     snmp_sess_perror("ack", &session); 
     SOCK_CLEANUP; 
     exit(1); 
    } 

    pdu = snmp_pdu_create(SNMP_MSG_GET); 
    anOID_len = MAX_OID_LEN; 
    if (!snmp_parse_oid("ip.21.1.8.xx.xx.xx.xx", anOID, &anOID_len)) { 
     snmp_perror("ip.21.1.8.xx.xx.xx.xx"); 
     SOCK_CLEANUP; 
     exit(1); 
    } 

    snmp_add_null_var(pdu, anOID, anOID_len); 

    status = snmp_synch_response(ss, pdu, &response); 

    if (status == STAT_SUCCESS && response->errstat == SNMP_ERR_NOERROR) { 

     for(vars = response->variables; vars; vars = vars->next_variable) 
     print_variable(vars->name, vars->name_length, vars); 

     /* manipuate the information ourselves */ 
     for(vars = response->variables; vars; vars = vars->next_variable) { 



     if (vars->type == ASN_OCTET_STR) { 
     char *sp = (char *)malloc(1 + vars->val_len); 
     memcpy(sp, vars->val.string, vars->val_len); 
     sp[vars->val_len] = '\0'; 
      printf("value #%d is a string: %s\n", count++, sp); //Here sp now has the string - But this doesnt work when the string is for eg."HOST-RESOURCES-MIB::hrSWInstalledDate.1953 = STRING: 0-1-1,0:0:0.0" 
     free(sp); 
    } 


     else if(vars->type == ASN_INTEGER) { 
      printf("value is an Integer\n"); 
      int ObjVal; 
      // How do I get the Integer value and assign it to 'ObjVal' 
     } 


     else if(vars->type == ASN_OBJECT_ID) { 
      printf("value is an OID\n"); 
      // How do I get the OID and assign it to some variable 
     } 


     else if(vars->type == ASN_TIMETICKS) { 
      printf("value is in Timeticks\n"); 
      // How do I get the Timeticks and assign it to some variable for further processing 
     } 



     } 
    } else { 
     if (status == STAT_SUCCESS) 
     fprintf(stderr, "Error in packet\nReason: %s\n", 
       snmp_errstring(response->errstat)); 
     else if (status == STAT_TIMEOUT) 
     fprintf(stderr, "Timeout: No response from %s.\n", 
       session.peername); 
     else 
     snmp_sess_perror("snmpdemoapp", ss); 

    } 

    if (response) 
     snmp_free_pdu(response); 
    snmp_close(ss); 

    SOCK_CLEANUP; 
    return (0); 
} 

試過vars->val.integer或vars->val.object_id，但不列入包含值。我在這裏錯過了什麼？

我的另一個問題是，即使是ASN_OCTET_STRING型的，當GET答覆是這樣的，

HOST-RESOURCES-MIB::hrSWInstalledDate.1953 = STRING: 0-1-1,0:0:0.0 

然後vars->val.string可是沒有"0-1-1,0:0:0.0"爲字符串。

基本上我的問題是如何將值存儲在response結構中，從中我可以檢索這些值？

在此先感謝！

P.S：Makefile來自Net-SNMP網站的鏈接。

編輯1： 對於整數，我可以使用*vars->val->string閱讀，如immibis所指出的。有關如何訪問其他數據類型的任何想法？

Answer 1

正如你可以/usr/include/net-snmp/types.h文件中看到或您的系統類似，net-snmpvars->val具有以下聯合類型：

typedef union { 
    long   *integer; 
    u_char   *string; 
    oid   *objid; 
    u_char   *bitstring; 
    struct counter64 *counter64; 
#ifdef NETSNMP_WITH_OPAQUE_SPECIAL_TYPES 
    float   *floatVal; 
    double   *doubleVal; 
    /* 
    * t_union *unionVal; 
    */ 
#endif       /* NETSNMP_WITH_OPAQUE_SPECIAL_TYPES */ 
} netsnmp_vardata; 

也*vars有val_len場，其中存儲的數據的長度。 所以你可以訪問整數作爲*vars->val.integer，字符串作爲指針u_char vars->val.string與vars->val_len字符，oid作爲指針oidvars->val.objid與vars->val_len/sizeof(oid) oid元素等。