windows下使用net-snmp實現agent擴充套件(二)
剛剛實現了int型的get命令,可能更多的情況下,我們更需要的是字串型別的。在實現int型的時候,用到了netsnmp_register_int_instance這個函式,很自然想到如果是string型的,用類似的netsnmp_register_string_instance,或者netsnmp_register_char_instance不就行了?很可惜的是:net-snmp並沒有提供這兩個函式。通過查詢,在net-snmp5.7.1/agent/mibgroup/examples下面,有個watched.c檔案,這裡提供了對字串型別的操作。實際上,net-snmp將string型歸到了scalar型別裡,做了統一的處理。好,看程式碼:
watched.h檔案:
#ifndef EXAMPLES_WATCHED_H #define EXAMPLES_WATCHED_H #ifdef __cplusplus extern "C" { #endif void init_watched(void); #ifdef __cplusplus } #endif #endif /* EXAMPLES_WATCHED_H */
watched.c檔案:
/* * start by including the appropriate header files */ #include <net-snmp/net-snmp-config.h> #include <net-snmp/net-snmp-includes.h> #include <net-snmp/agent/net-snmp-agent-includes.h> void init_watched_string(void); void init_watched(void) { init_watched_string(); } void init_watched_string(void) { /* * the storage for our string. It must be static or allocated. * we use static here for simplicity. */ static char my_string[256] = "welcome to vcsky.net!"; /* * the OID we want to register our string at. This should be a * fully qualified instance. In our case, it's a scalar at: * NET-SNMP-EXAMPLES-MIB::netSnmpExampleString.0 (note the trailing * 0 which is required for any instantiation of any scalar object) */ oid my_registration_oid[] = { 1, 3, 6, 1, 4, 1, 8072, 2, 1, 3, 0 }; /* * variables needed for registration */ <a name="_a0"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__handler__registration__s.html">netsnmp_handler_registration</a> *reginfo; static <a name="_a1"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__watcher__info__s.html">netsnmp_watcher_info</a> watcher_info; int watcher_flags; /* * a debugging statement. Run the agent with -Dexample_string_instance * to see the output of this debugging statement. */ DEBUGMSGTL(("example_string_instance", "Initalizing example string instance. Default value = %s\n", my_string)); /* * If we wanted a callback when the value was retrieved or set * (even though the details of doing this are handled for you), * you could change the NULL pointer below to a valid handler * function. * * Change RWRITE to RONLY for a read-only string. */ reginfo = <a name="a2"></a><a href="http://www.net-snmp.org/dev/agent/group__handler.html#ga537fac61fff7e112e121e5f629eded65">netsnmp_create_handler_registration</a>("my example string", NULL, my_registration_oid, OID_LENGTH(my_registration_oid), HANDLER_CAN_RWRITE); /* * the three options for a string watcher are: * fixed size string (length never changes) * variable size (length can be 0 - MAX, for some MAX) * c string (length can be 0 - MAX-1 for some max, \0 is not a valid * character in the string, the length is provided by strlen) * * we'll use a variable length string. */ watcher_flags =WATCHER_MAX_SIZE; /* * create the watcher info for our string. */ netsnmp_init_watcher_info6(&watcher_info, my_string, strlen(my_string), ASN_OCTET_STR, watcher_flags, sizeof(my_string), NULL); /* * the line below registers our "my_string" variable above as * accessible and makes it writable. */ <a name="a4"></a><a href="http://www.net-snmp.org/dev/agent/group__watcher.html#ga32250edcf6f39dd7376a20699e1c7cd0">netsnmp_register_watched_instance</a>(reginfo, &watcher_info); DEBUGMSGTL(("example_string_instance", "Done initalizing example string instance\n")); }
編譯執行(注意在入口函式中將init_nstAgentSubagentObject替換為init_watched)。 開啟cmd,測試一下:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 返回結果:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “welcome to vcsky.net!” snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “havenzhao” 返回結果:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “havenzhao” OK,沒問題! 接下來,我們想改變my_string這個變數的值,這個值需要從程式的其它地方獲得,這樣才能起到監測的作用。先簡單改變一下試試看,能否正確執行: 修改下watched.c檔案,新增一行為my_string改變值的語句,標紅的程式碼即是。
#include <net-snmp/net-snmp-config.h> #include <net-snmp/net-snmp-includes.h> #include <net-snmp/agent/net-snmp-agent-includes.h>
static char my_string[256] = “welcome to vcsky.net!”; //作為全域性變數
/*change_string_value()模擬在其它程式碼中改變要監測的my_string值的函式,此函式可以放在定時器中,或者其它能夠動態改變my_string值的程式碼段裡,以保證獲取的my_string值是變化的。別忘了在標頭檔案watched.h裡宣告*/
void change_string_value() { strcpy(my_string, "haven zhao"); } void init_watched_string(void); void init_watched(void) { init_watched_string(); } void init_watched_string(void) { oid my_registration_oid[] = { 1, 3, 6, 1, 4, 1, 8072, 2, 1, 3, 0 }; <a name="_a0"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__handler__registration__s.html">netsnmp_handler_registration</a> *reginfo; static <a name="_a1"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__watcher__info__s.html">netsnmp_watcher_info</a> watcher_info; int watcher_flags; DEBUGMSGTL(("example_string_instance", "Initalizing example string instance. Default value = %s\n", my_string)); reginfo = <a name="a2"></a><a href="http://www.net-snmp.org/dev/agent/group__handler.html#ga537fac61fff7e112e121e5f629eded65">netsnmp_create_handler_registration</a>("my example string", NULL, my_registration_oid, OID_LENGTH(my_registration_oid), HANDLER_CAN_RWRITE);
watcher_flags = WATCHER_MAX_SIZE;
netsnmp_init_watcher_info6(&watcher_info, my_string, strlen(my_string), ASN_OCTET_STR, watcher_flags, sizeof(my_string), NULL);
DEBUGMSGTL((“example_string_instance”, “Done initalizing example string instance\n”)); } 為了方便,我們把change_string_value()函式放在入口函式檔案(example-demon.c)的死迴圈裡: while(keep_running) {
/* if you use select(), see snmp_select_info() in snmp_api(3) */ /* — OR — */ change_string_value(); //放在這裡了 agent_check_and_process(1); /* 0 == don’t block */ }
再次編譯執行,看看有沒有達到效果:
輸入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = Hex-STRING: 68 61 76 65 6E 20 7A 68 61 6F 00 76
出現亂碼,沒有成功!
再試試set命令:snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “haven”
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven”
可見set成功!try again,再輸入get命令:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven”
這次卻成功了。
難道要先set,再get,才行?什麼情況?再試一次!
輸入:snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “vcsky.net haven zhao”
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “vcsky.net haven zhao”
輸入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = Hex-STRING: 68 61 76 65 6E 20 7A 68 61 6F 00 61
照樣失敗!
經過反覆測試,好像和字串的長度有關係。改變後的字串與初始化時的字串長度不一致時,總是達不到我們想要的效果。是的,問題就出在這裡!
解決方案:
看這行程式碼:watcher_flags = WATCHER_MAX_SIZE;
就在這個小小的引數上,例子中的WATCHER_MAX_SIZE:variable size (length can be 0 – MAX, for some MAX),If set then the variable data_size_p points to is supposed to hold the current size of the watched object and will be updated on writes.
而我們則需要這個:c string (length can be 0 – MAX-1 for some max, \0 is not a valid character in the string, the length is provided by strlen),也就是這個:WATCHER_SIZE_STRLEN
好,馬上替換watcher_flags = WATCHER_MAX_SIZE;為watcher_flags = WATCHER_SIZE_STRLEN;
編譯執行,測試:
輸入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0
返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven zhao”
OK!到此為止,我們終於前進了一步,由get/set一個int型變數,到get/set一個字串型變數,而且這個字串變數,可由外部的程式碼對其進行設定,從而實現了監測變化的字串變數的目的。
這只是單個的變數,可能要監測的變數不只一個,那怎麼辦呢?未完待續: