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：