nrf52832作为从机，连接多主机的情况下，如何能感知出连接的主机的服务和特征呢？请给出具体的实现代码基于nrf52832软件库

在nrf52832的软件库中，可以使用SoftDevice API来实现BLE从机的连接和服务发现。具体实现代码如下：

首先，在初始化BLE从机时，需要开启SoftDevice和BLE从机模式：

// 初始化SoftDevice
ret_code_t err_code = nrf_sdh_enable_request();
APP_ERROR_CHECK(err_code);

// 初始化BLE从机
ble_stack_init();

// 开启BLE从机模式
ble_gap_adv_start(&m_adv_params, APP_BLE_CONN_CFG_TAG);

然后，在连接建立后，可以在BLE事件处理函数中处理发现服务和特征的事件。在连接建立后，会触发BLE_GAP_EVT_CONNECTED事件，可以在该事件处理函数中启动服务发现过程：

case BLE_GAP_EVT_CONNECTED:
{
    m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;

    // 开始服务发现
    err_code = sd_ble_gattc_primary_services_discover(m_conn_handle, 0x0001, NULL);
    APP_ERROR_CHECK(err_code);
    break;
}

在服务发现过程中，会触发BLE_GATTC_EVT_PRIM_SRVC_DISC_RSP事件和BLE_GATTC_EVT_CHAR_DISC_RSP事件。可以在这两个事件处理函数中获取服务和特征的UUID和句柄等信息：

case BLE_GATTC_EVT_PRIM_SRVC_DISC_RSP:
{
    ble_gattc_evt_prim_srvc_disc_rsp_t *p_response = &p_ble_evt->evt.gattc_evt.params.prim_srvc_disc_rsp;

    // 遍历所有发现的服务
    for (uint32_t i = 0; i < p_response->count; i++)
    {
        ble_uuid_t uuid = p_response->services[i].uuid;

        // 根据UUID判断服务类型
        if (ble_uuid_cmp(&uuid, &m_svc_uuid) == 0)
        {
            // 保存服务句柄
            m_svc_handle = p_response->services[i].handle_range.start_handle;
        }
    }
    break;
}

case BLE_GATTC_EVT_CHAR_DISC_RSP:
{
    ble_gattc_evt_char_disc_rsp_t *p_response = &p_ble_evt->evt.gattc_evt.params.char_disc_rsp;

    // 遍历所有发现的特征
    for (uint32_t i = 0; i < p_response->count; i++)
    {
        ble_uuid_t uuid = p_response->chars[i].uuid;

        // 根据UUID判断特征类型
        if (ble_uuid_cmp(&uuid, &m_char_uuid) == 0)
        {
            // 保存特征句柄
            m_char_handle = p_response->chars[i].handle_value;
        }
    }

    // 特征发现完成，可以进行读写操作等
    break;
}

完整的实现代码可以参考以下示例代码：

#include <stdint.h>
#include <string.h>
#include "nordic_common.h"
#include "nrf.h"
#include "app_error.h"
#include "ble.h"
#include "ble_advdata.h"
#include "ble_conn_params.h"
#include "ble_nus.h"
#include "nrf_sdh.h"
#include "nrf_sdh_ble.h"
#include "nrf_ble_gatt.h"
#include "nrf_ble_qwr.h"

#define DEVICE_NAME                     "My Device" /**< Name of device. Will be included in the advertising data. */
#define NUS_SERVICE_UUID_TYPE           BLE_UUID_TYPE_VENDOR_BEGIN /**< UUID type for the Nordic UART Service (vendor specific). */
#define APP_BLE_CONN_CFG_TAG            1 /**< A tag identifying the SoftDevice BLE configuration. */
#define APP_BLE_OBSERVER_PRIO           3 /**< Application's BLE observer priority. You shouldn't need to modify this value. */
#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(20, UNIT_1_25_MS) /**< Minimum acceptable connection interval (20 ms). */
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(75, UNIT_1_25_MS) /**< Maximum acceptable connection interval (75 ms). */
#define SLAVE_LATENCY                   0 /**< Slave latency. */
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(4000, UNIT_10_MS) /**< Connection supervisory timeout (4 seconds). */
#define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(5000) /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
#define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(30000) /**< Time between each call to sd_ble_gap_conn_param_update after the first (30 seconds). */
#define MAX_CONN_PARAMS_UPDATE_COUNT    3 /**< Number of attempts before giving up the connection parameter negotiation. */
#define BLE_NUS_MAX_DATA_LEN            (BLE_GATT_ATT_MTU_DEFAULT - 3) /**< Maximum length of data (in bytes) that can be transmitted to the peer by the Nordic UART service module. */

// UUID of the service and characteristic to discover
static ble_uuid_t m_svc_uuid =
{
    .uuid = BLE_UUID_NUS_SERVICE,
    .type = NUS_SERVICE_UUID_TYPE
};

static ble_uuid_t m_char_uuid =
{
    .uuid = BLE_UUID_NUS_TX_CHAR,
    .type = NUS_SERVICE_UUID_TYPE
};

// Service and characteristic handles
static uint16_t m_svc_handle = BLE_GATT_HANDLE_INVALID;
static uint16_t m_char_handle = BLE_GATT_HANDLE_INVALID;

// Connection handle
static uint16_t m_conn_handle = BLE_CONN_HANDLE_INVALID;

// BLE stack initialization
static void ble_stack_init(void)
{
    ret_code_t err_code;

    // Initialize the SoftDevice handler module
    err_code = nrf_sdh_enable_request();
    APP_ERROR_CHECK(err_code);

    // Configure the BLE stack using the default settings
    err_code = nrf_sdh_ble_default_cfg_set(APP_BLE_CONN_CFG_TAG, &NRF_BLE_GATT_DEFAULT_CFG);
    APP_ERROR_CHECK(err_code);

    // Enable BLE stack
    err_code = nrf_sdh_ble_enable(&APP_BLE_OBSERVER_PRIO);
    APP_ERROR_CHECK(err_code);
}

// Advertising initialization
static void advertising_init(void)
{
    ret_code_t err_code;

    // Declare and initialize a BLE advertising packet structure
    ble_advdata_t advdata;
    memset(&advdata, 0, sizeof(advdata));

    advdata.name_type             = BLE_ADVDATA_FULL_NAME;
    advdata.include_appearance    = true;
    advdata.flags                 = BLE_GAP_ADV_FLAGS_LE_ONLY_GENERAL_DISC_MODE;

    // Declare and initialize a BLE advertising parameters structure
    ble_gap_adv_params_t adv_params;
    memset(&adv_params, 0, sizeof(adv_params));

    adv_params.primary_phy        = BLE_GAP_PHY_1MBPS;
    adv_params.duration           = BLE_GAP_ADV_TIMEOUT_GENERAL_UNLIMITED;
    adv_params.properties.type    = BLE_GAP_ADV_TYPE_CONNECTABLE_SCANNABLE_UNDIRECTED;
    adv_params.p_peer_addr        = NULL;
    adv_params.filter_policy      = BLE_GAP_ADV_FP_ANY;
    adv_params.interval           = MSEC_TO_UNITS(1000, UNIT_0_625_MS);

    // Start advertising
    err_code = sd_ble_gap_adv_set_configure(&adv_handle, &advdata, &adv_params);
    APP_ERROR_CHECK(err_code);

    err_code = sd_ble_gap_adv_start(adv_handle, APP_BLE_CONN_CFG_TAG);
    APP_ERROR_CHECK(err_code);
}

// BLE event handler
static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
    ret_code_t err_code;

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_CONNECTED:
        {
            m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;

            // Start service discovery
            err_code = sd_ble_gattc_primary_services_discover(m_conn_handle, 0x0001, NULL);
            APP_ERROR_CHECK(err_code);
            break;
        }

        case BLE_GAP_EVT_DISCONNECTED:
        {
            m_conn_handle = BLE_CONN_HANDLE_INVALID;
            break;
        }

        case BLE_GATTC_EVT_PRIM_SRVC_DISC_RSP:
        {
            ble_gattc_evt_prim_srvc_disc_rsp_t *p_response = &p_ble_evt->evt.gattc_evt.params.prim_srvc_disc_rsp;

            // Traverse all discovered services
            for (uint32_t i = 0; i < p_response->count; i++)
            {
                ble_uuid_t uuid = p_response->services[i].uuid;

                // Determine the service type based on the UUID
                if (ble_uuid_cmp(&uuid, &m_svc_uuid) == 0)
                {
                    // Save the service handle
                    m_svc_handle = p_response->services[i].handle_range.start_handle;
                }
            }
            break;
        }

        case BLE_GATTC_EVT_CHAR_DISC_RSP:
        {
            ble_gattc_evt_char_disc_rsp_t *p_response = &p_ble_evt->evt.gattc_evt.params.char_disc_rsp;

            // Traverse all discovered characteristics
            for (uint32_t i = 0; i < p_response->count; i++)
            {
                ble_uuid_t uuid = p_response->chars[i].uuid;

                // Determine the characteristic type based on the UUID
                if (ble_uuid_cmp(&uuid, &m_char_uuid) == 0)
                {
                    // Save the characteristic handle
                    m_char_handle = p_response->chars[i].handle_value;
                }
            }

            // Characteristic discovery completed, can perform read/write operations, etc.
            break;
        }

        default:
            // No implementation needed.
            break;
    }
}

// BLE stack initialization
static void ble_init(void)
{
    ret_code_t err_code;

    // Initialize BLE stack
    ble_stack_init();

    // Initialize GATT module
    err_code = nrf_ble_gatt_init(&m_gatt, NULL);
    APP_ERROR_CHECK(err_code);

    // Initialize QWR module
    err_code = nrf_ble_qwr_init(&m_qwr, &m_qwr_mem);
    APP_ERROR_CHECK(err_code);

    // Register BLE event handler
    err_code = nrf_ble_gatt_cbs_register(&m_gatt, ble_evt_handler, NULL);
    APP_ERROR_CHECK(err_code);

    // Set the MTU size
    err_code = sd_ble_gattc_exchange_mtu_request(m_conn_handle, BLE_GATT_ATT_MTU_DEFAULT);
    APP_ERROR_CHECK(err_code);

    // Initialize advertising
    advertising_init();
}

int main(void)
{
    // Initialize BLE
    ble_init();

    while (true)
    {
        // Do nothing
    }
}