最近在做一个项目，需要通过NL80211和驱动打交道。
以前没用过，走了不少弯路，因此做个笔记，方便自己，当然如果能够帮助别人更好。
无线网卡驱动主要有两个“标准”（这样的形容并不准确！）：
- WEXT（Wireless Extension）：使用WEXT的工具通过ioctl和驱动通信，典型工具ifconfig等;
- NL80211（Netlink 80211）：使用NL80211的工具通过一个特殊的socket和驱动打通信，典型工具包括IW、iwconfig等。
这里的NL80211仅仅是netlink工具在无线驱动方面的一个应用，其实它的应用很广泛，也很基础。Netlink提供了一种通信方式，通信双方可以是用户态或内核态，关于这方面的介绍，直接看libnl（netlink的一个实现）的网站（http://www.infradead.org/~tgr/libnl/），讲解的很详细。目前使用netlink主要是通过libnl来做，当然也可以自己拼凑和解析消息。
这里提供一个我编译的libnl动态库，使用的是Google提供的ANDROID-NDK-R7，理论上可用于android 3.2以上的系统。http://115.com/file/e789tu6y#libnltest.so
下面是我使用的一些代码，是安卓的JNI接口，用于在连接上AP后获取信号强度。主要参考了iw和wpa_supplicant的相关实现。
int GetSignalPower(const unsigned char *addr, char *signal) // 消息函数
{
struct nl_sock *l_nl_handle = NULL;
struct nl_cache *l_nl_cache = NULL;
struct genl_family *l_nl_family = NULL;
struct nl_cb *l_nl_cb = NULL;
int ret = 0;
int err = 1;
struct nl_msg *l_msg = NULL;
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Enter GetSignalPower”);
if(signal == NULL)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to get signal power, bad params.”);
return -1;
}
*signal = 0;
//char signal = 0;
//unsigned char addr[6] = {0×00, 0x1D, 0x0F, 0x3F, 0×43, 0x3E};
l_nl_cb = nl_cb_alloc(NL_CB_DEFAULT); // 创建回调
if(l_nl_cb == NULL)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to alloc cb.”);
printf(“Failed to alloc cb.\n”);
ret = -1;
goto error_catch;
}
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to alloc callback.”);
l_nl_handle = nl_socket_alloc_cb(l_nl_cb); // 根据回调创建netlink socket
if(l_nl_handle == NULL)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to alloc a netlink socket.”);
printf(“Failed to alloc a netlink socket.\n”);
ret = -1;
goto error_catch;
}
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to alloc socket.”);ret = genl_connect(l_nl_handle); // 连接内核
if(ret != 0)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to connect to generic socket, error code is %d.\n”, ret);
printf(“Failed to connect to generic socket, error code is %d.\n”, ret);
goto error_catch;
}__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to connect generic socket.”);
ret = genl_ctrl_alloc_cache(l_nl_handle, &l_nl_cache); // 创建cache，我也不清楚这个操作必要性
if(ret < 0)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to alloc cache.”);
printf(“Failed to alloc cache.\n”);
goto error_catch;
}
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to genl_ctrl_alloc_cache.”);
l_nl_family = genl_ctrl_search_by_name(l_nl_cache, “nl80211″); // 查找NL80211簇
if (l_nl_family == NULL)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “nl80211: ‘nl80211′ generic netlink not found.”);
printf(“nl80211: ‘nl80211′ generic netlink not found.\n”);
ret = -1;
goto error_catch;
}
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to genl_ctrl_search_by_name.”);l_msg = nlmsg_alloc(); // 创建netlink message
if (l_msg == NULL)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to alloc message.”);
printf(“Failed to alloc message.\n”);
ret = -1;
goto error_catch;
}
__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Succeed to nlmsg_alloc.”);
genlmsg_put(l_msg, 0, 0, genl_family_get_id(l_nl_family), 0,
0, NL80211_CMD_GET_STATION, 0); // 填充消息，这里的“NL80211_CMD_GET_STATION”是一条NL80211的命令，具体作用参考nl80211.h
nla_put(l_msg, NL80211_ATTR_MAC, ETH_ALEN, addr); // 填充消息，这里的MAC地址是AP的MAC
nla_put_u32(l_msg, NL80211_ATTR_IFINDEX, if_nametoindex(“wlan0″)); // 填充消息，很明显，这里指定了要查询的网卡nl_cb_set(l_nl_cb, NL_CB_FINISH, NL_CB_CUSTOM, finish_handler, &err); // 设置回调，这里的“NL_CB_FINISH”等是libnl定义的一些回调类型，Google一下就知道用处和用法
nl_cb_set(l_nl_cb, NL_CB_ACK, NL_CB_CUSTOM, ack_handler, &err);
nl_cb_err(l_nl_cb, NL_CB_CUSTOM, error_handler, &err);
nl_cb_set(l_nl_cb, NL_CB_VALID, NL_CB_CUSTOM, get_sta_handler, signal);ret = nl_send_auto_complete(l_nl_handle, l_msg); // 发送消息
if (ret < 0)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to send msg.”);
printf(“Failed to send msg.\n”);
ret = -1;
goto error_catch;
}do{
nl_recvmsgs(l_nl_handle, l_nl_cb); // 注意！这里要重复读取直到我们的回调函数被调用
sleep(1);
}while(err>0);error_catch: // 一些释放工作
if(l_nl_family != NULL)
{
genl_family_put(l_nl_family);
l_nl_family = NULL;
}
if(l_nl_cache != NULL)
{
nl_cache_free(l_nl_cache);
l_nl_cache = NULL;
}
if(l_nl_handle != NULL)
{
nl_socket_free(l_nl_handle);
l_nl_handle = NULL;
}
if(l_nl_cb != NULL)
{
nl_cb_put(l_nl_cb);
l_nl_cb = NULL;
}
if(l_msg != NULL)
{
nlmsg_free(l_msg);
l_msg = NULL;
}__android_log_print(ANDROID_LOG_DEBUG, NL80211_LOG, “Exit GetSignalPower”);
if(*signal == 0)
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “signal 0.”);
return -1;
}
else
{
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “signal %d.”, *signal);
return 0;
}
}static int error_handler(struct sockaddr_nl *nla, struct nlmsgerr *err,
void *arg) // 错误回调函数
{
int *ret = (int *)arg;
*ret = err->error;
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “nl80211 Error handle!”);
printf(“Error handle\n”);
return NL_SKIP;
}static int finish_handler(struct nl_msg *msg, void *arg) // 结束回调函数
{
int *ret = (int *)arg;
*ret = 0;
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “nl80211 Finish handle!”);
printf(“Finish!\n”);
return NL_SKIP;
}static int ack_handler(struct nl_msg *msg, void *arg) // 确认回调函数
{
int *ret = (int *)arg;
*ret = 0;
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “nl80211 ACK handle!”);
printf(“ACK!\n”);
return NL_STOP;
}static int get_sta_handler(struct nl_msg *msg, void *arg) // 这里是我们真正读取消息的地方！！
{
struct nlattr *tb[NL80211_ATTR_MAX + 1];
struct nlattr *tb_nested[NL80211_STA_INFO_MAX + 1];
struct genlmsghdr *gnlh = (struct genlmsghdr *)nlmsg_data(nlmsg_hdr(msg));
struct _ap_status *data = (struct _ap_status *)arg;
char signal;if( 0 != nla_parse(tb, NL80211_ATTR_MAX, genlmsg_attrdata(gnlh, 0),
genlmsg_attrlen(gnlh, 0), NULL)) // 解析消息，第一次解析，这里的“NL80211_ATTR_MAX”等的用处和用法参考nl80211.h
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to parse nl attrs.!”);
printf(“Failed to parse nl attrs.\n”);
return NL_SKIP;
}
else
{
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “Succeeded to parse nl attrs!”);
printf(“Succeeded to parse nl attrs.\n”);
}if (tb[NL80211_ATTR_STA_INFO])
{
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “We have NL80211_ATTR_STA_INFO!”);
printf(“We have NL80211_ATTR_STA_INFO\n”);
if(nla_parse_nested(tb_nested, NL80211_STA_INFO_MAX, tb[NL80211_ATTR_STA_INFO], NULL) != 0) // 解析消息，第二次解析
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “Failed to parse nl attrs nested.”);printf(“Failed to parse nl attrs nested.\n”);
return NL_SKIP;
}
else
{
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “Succeeded to parse attrs nested.”);
printf(“Succeeded to parse attrs nested.\n”);
}
if(tb_nested[NL80211_STA_INFO_SIGNAL])
{
signal = nla_get_u8(tb_nested[NL80211_STA_INFO_SIGNAL]);
if(arg != NULL)
{
(*(char *)arg) = signal;
}
__android_log_print(ANDROID_LOG_INFO, NL80211_LOG, “ap signal is %d\n”, *(char *)arg);
printf(“ap signal is %d\n”, signal);
}
// if(tb_nested[NL80211_STA_INFO_SIGNAL_AVG])
// {
// signal = nla_get_u8(tb_nested[NL80211_STA_INFO_SIGNAL_AVG]);
// printf(“ap avg signal is %d\n”, signal);
// }
}
else
{
__android_log_print(ANDROID_LOG_ERROR, NL80211_LOG, “No NL80211_ATTR_STA_INFO.”);
printf(“No NL80211_ATTR_STA_INFO\n”);
}return NL_SKIP;
}