新手谈C指针02__int *p=NULL、*p=NULL、*(int *)0x0018ff44.

最新推荐文章于 2023-08-02 16:57:31 发布
原创 最新推荐文章于 2023-08-02 16:57:31 发布 · 2.6k 阅读 · 1 ·
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#c #C #指针

这篇博客探讨了C语言中指针变量初始化为NULL（0）的含义，强调了NULL、0与ASCII码为0的空字符NUL之间的区别。作者解释了指针变量P存储地址0x00000000的情况，并指出编译器只能检查语法错误，不能确保程序在所有情况下都能正常运行。文章还讨论了尝试向未知地址写入内容的风险，并提醒读者注意计算机内存的访问限制。 
int  *p = NULL；

此语句什么意思？没错，就是定义了一个整型指针变量P，并进行了初始化。

那么，现在有个问题。它初始化了什么东西？NULL？

NULL又是什么？（这里需要注意）

NULL是被宏定义成 0 。即为 define   NULL  0 

很多系统下除了NULL之外，还有NUL（VC++6.0上提示说不认识NUL。error C2065: 'NUL' : undeclared identifier ）NUL是ASCII码表的第一个字符，表示的是空字符，其ASCII码值为0；虽然都为0，但表示的意思完全不一样。同样，NULL和0表示的意思也完全不一样。一定不要混淆。


既然我们知道NULL是0，就是说指针变量P初始化为0了。

在前一篇新手谈C指针01_指针?中我们已经说过，指针变量内任何数据都会被当作地址进行处理。所以，P 里存储的是地址0x00000000。而不是把 *p 里的内容初始化为0。

其实，在上一篇博文也提到了。我们可以把 int * 看成一个整体，这时候变量就是P，而不是

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= 1, style_cnt = 4, h_layout = 0, w_layout = 0, is_deleting = 0}, ta = 0x57, popup = 0x3d72d0 <work_mem_int+1422508>, header = 0x1, year_label = 0x717, month_label = 0x3de, calendar = 0x739, time_cont = 0x3e8, hms_selector = 0x717, footer = 0x3de, year_picker = 0x739, month_picker = 0x3e8, hms_picker = 0x0, show_label = 0x0, icon = 0x0, showed_date = {year = 0, month = 0 '\000', day = 0 '\000'}, selected_date = {year = 812, month = 0 '\000', day = 0 '\000'}, origin_date = {year = 0, month = 0 '\000', day = 0 '\000'}, clicked_date = {year = 0, month = 0 '\000', day = 0 '\000'}, date_format = 1079, hour_format = (TP_HOUR_FORMAT_12 | unknown: 2), hour = 224 '\340', minute = 253 '\375', second = 77 'M', origin_hour = 224 '\340', origin_minute = 253 '\375', origin_second = 77 'M', am_pm = 77 'M', content_cont = 0x3b, year_grid_btn = {0x430384 <work_mem_int+1787232>, 0x27be24 <work_mem_int>, 0x0, 0x0, 0x27d914 <work_mem_int+6896>, 0x38, 0x0, 0x0, 0x175b44ff, 0x0, 0xffffff <work_mem_int+14172635>, 0x0}, month_grid_btn = {0xff000000, 0x4dfd44 <work_mem_int+2506528>, 0x57, 0x3c3ce0 <work_mem_int+1343164>, 0x1, 0x48f, 0x3de, 0x48f, 0x3e8, 0x48f, 0x3de, 0x48f}, is_calendar = 232} (gdb) p tp_timebar $11 = (tp_lv_timebar_t *) 0x460324 <work_mem_int+1983744> (gdb) p g_playback_timebar_obj $12 = (lv_obj_t *) 0x460324 <work_mem_int+1983744> (gdb) p (tp_lv_timebar_t *) 0x460324 $13 = (tp_lv_timebar_t *) 0x460324 <work_mem_int+1983744> (gdb) p *(tp_lv_timebar_t *) 0x460324 $14 = {obj = {class_p = 0x23fd60 <tp_lv_timebar_class>, parent = 0x420dd8 <work_mem_int+1724340>, spec_attr = 0x3daba8 <work_mem_int+1437060>, styles = 0x3c6ff4 <work_mem_int+1356240>, style_main_prop_is_set = 65551, style_other_prop_is_set = 0, user_data = 0x606f50 <work_mem_int+3715372>, id = 0x971, coords = {x1 = 280, y1 = 864, x2 = 1919, y2 = 1079}, flags = 64358, state = 0, layout_inv = 0, readjust_scroll_after_layout = 0, scr_layout_inv = 0, skip_trans = 0, style_cnt = 1, h_layout = 0, w_layout = 0, is_deleting = 0}, ctrl_obj = 0x3dab6c <work_mem_int+1437000>, chn_name_obj = 0x3cc31c <work_mem_int+1377528>, video_info_obj = 0x3cc224 <work_mem_int+1377280>, bottom_ctrl_obj = 0x3e2ee8 <work_mem_int+1470660>, play_obj = 0x3d56a8 <work_mem_int+1415300>, single_frame_obj = 0x3d57e0 <work_mem_int+1415612>, speed_obj = 0x4449bc <work_mem_int+1870744>, smartspeed_obj = 0x4b7560 <work_mem_int+2340668>, cut_obj = 0x4810d4 <work_mem_int+2118320>, export_obj = 0x4f30d4 <work_mem_int+2585264>, export_label_obj = 0x52bcf8 <work_mem_int+2817748>, scale_obj = 0x42b620 <work_mem_int+1767420>, people_obj = 0x46771c <work_mem_int+2013432>, vehicle_obj = 0x46788c <work_mem_int+2013800>, pet_obj = 0x4679bc <work_mem_int+2014104>, cut_begin_obj = 0x0, cut_end_obj = 0x0, b30_obj = 0x42a540 <work_mem_int+1763100>, calendar_obj = 0x4dfd4c <work_mem_int+2506536>, f30_obj = 0x3d4f8c <work_mem_int+1413480>, hq_obj = 0x467acc <work_mem_int+2014376>, audio_obj = 0x42a260 <work_mem_int+1762364>, 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0x478364 <work_mem_int+2082112>, 0x47843c <work_mem_int+2082328>, 0x478538 <work_mem_int+2082580>, 0x478638 <work_mem_int+2082836>, 0x47873c <work_mem_int+2083096>, 0x478838 <work_mem_int+2083348>}, hover_time_obj = 0x462d38 <work_mem_int+1994516>, chn_label = {0x3cc3c8 <work_mem_int+1377700>, 0x3cc500 <work_mem_int+1378012>, 0x3cc5f8 <work_mem_int+1378260>, 0x42905c <work_mem_int+1757752>}, scrollbar_obj = 0x4def7c <work_mem_int+2503000>, show_scrollbar = 1, chn_num = 8, show_chn_idx = 0, expend = 1, mode = 0, stop_update_time = 0, cur_chn = 0, full_show = 0, export_slice_num = 0, cut_setting_time = 0, left_cut_bar_hoverd = false, right_cut_bar_hoverd = false, left_cut_bar_area = {x1 = 0, y1 = 0, x2 = 0, y2 = 0}, right_cut_bar_area = {x1 = 0, y1 = 0, x2 = 0, y2 = 0}} (gdb) p tp_timebar $15 = (tp_lv_timebar_t *) 0x460324 <work_mem_int+1983744> (gdb) p tp_timebar->ctrl_obj $16 = (lv_obj_t *) 0x3dab6c <work_mem_int+1437000> (gdb) p *(lv_obj_t *) A syntax error in expression, near `'. (gdb) p *(lv_obj_t *) A syntax error in expression, near `'. (gdb) p *(lv_obj_t *)0x460324 $17 = {class_p = 0x23fd60 <tp_lv_timebar_class>, parent = 0x420dd8 <work_mem_int+1724340>, spec_attr = 0x3daba8 <work_mem_int+1437060>, styles = 0x3c6ff4 <work_mem_int+1356240>, style_main_prop_is_set = 65551, style_other_prop_is_set = 0, user_data = 0x606f50 <work_mem_int+3715372>, id = 0x971, coords = {x1 = 280, y1 = 864, x2 = 1919, y2 = 1079}, flags = 64358, state = 0, layout_inv = 0, readjust_scroll_after_layout = 0, scr_layout_inv = 0, skip_trans = 0, style_cnt = 1, h_layout = 0, w_layout = 0, is_deleting = 0} (gdb) p *(lv_obj_t *) 0x3dab6c $18 = {class_p = 0x21e488 <lv_obj_class>, parent = 0x460324 <work_mem_int+1983744>, spec_attr = 0x3dac98 <work_mem_int+1437300>, styles = 0x3dabe8 <work_mem_int+1437124>, style_main_prop_is_set = 232015, style_other_prop_is_set = 4111, user_data = 0x0, id = 0x1c42aa <tp_lv_muti_dropdown_set_select+18>, coords = {x1 = 280, y1 = 864, x2 = 1903, y2 = 918}, flags = 64358, state = 0, layout_inv = 0, readjust_scroll_after_layout = 0, scr_layout_inv = 0, skip_trans = 0, style_cnt = 4, h_layout = 1, w_layout = 1, is_deleting = 0} (gdb) f 5 #5 0x001514b4 in tp_lv_timebar_event_cb (class_p=<optimized out>, e=<optimized out>) at PlaybackBar.c:2268 2268 update_time_str_label(obj); (gdb) tp_timebar->chn_num Undefined command: "tp_timebar->chn_num". 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#include "nvs_flash.h" #include "esp_bt.h" #include "esp_bt_main.h" #include "esp_gap_ble_api.h" #include "esp_gatts_api.h" #include "esp_netif.h" #include "esp_eth.h" #include "esp_event.h" #include "esp_log.h" #define TAG "ETHERNET" #define GATTS_SERVICE_UUID_TEST_A 0x00FF #define GATTS_CHAR_UUID_TEST_A 0xFF01 #define GATTS_NUM_HANDLE_TEST_A 4 // 全局错误标志 static bool global_error_flag = false; // 错误处理函数 void handle_error(const char *error_msg, esp_err_t err) { ESP_LOGE(TAG, "%s, error code = %x", error_msg, err); global_error_flag = true; } // 蓝牙天线与主控芯片的封装 static uint16_t service_handle, char_handle; static esp_gatts_attr_db_t test_gatt_db[GATTS_NUM_HANDLE_TEST_A] = { // 服务声明 [0] = { .attr_control = {.auto_rsp = ESP_GATT_AUTO_RSP}, .attr_uuid = {.len = ESP_UUID_LEN_16, .uuid = {.uuid16 = ESP_GATT_UUID_PRI_SERVICE}}, .perm = ESP_GATT_PERM_READ, .property = 0, .p_value = (uint8_t *)&GATTS_SERVICE_UUID_TEST_A, .value_len = 2, }, // 特征声明 [1] = { .attr_control = {.auto_rsp = ESP_GATT_AUTO_RSP}, .attr_uuid = {.len = ESP_UUID_LEN_16, .uuid = {.uuid16 = ESP_GATT_UUID_CHAR_DECLARE}}, .perm = ESP_GATT_PERM_READ, .property = ESP_GATT_CHAR_PROP_BIT_READ | ESP_GATT_CHAR_PROP_BIT_WRITE, .p_value = (uint8_t *)&ESP_GATT_CHAR_PROP_BIT_READ | ESP_GATT_CHAR_PROP_BIT_WRITE, .value_len = 1, }, // 特征值 [2] = { .attr_control = {.auto_rsp = ESP_GATT_AUTO_RSP}, .attr_uuid = {.len = ESP_UUID_LEN_16, .uuid = {.uuid16 = GATTS_CHAR_UUID_TEST_A}}, .perm = ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE, .property = ESP_GATT_CHAR_PROP_BIT_READ | ESP_GATT_CHAR_PROP_BIT_WRITE, .p_value = NULL, .value_len = 0, }, // 客户端特征配置描述符 [3] = { .attr_control = {.auto_rsp = ESP_GATT_AUTO_RSP}, .attr_uuid = {.len = ESP_UUID_LEN_16, .uuid = {.uuid16 = ESP_GATT_UUID_CHAR_CLIENT_CONFIG}}, .perm = ESP_GATT_PERM_READ | ESP_GATT_PERM_WRITE, .property = 0, .p_value = NULL, .value_len = 2, }, }; static void gatts_event_handler(esp_gatts_cb_event_t event, esp_gatt_if_t gatts_if, esp_ble_gatts_cb_param_t *param) { switch (event) { case ESP_GATTS_REG_EVT: { esp_err_t set_dev_name_ret = esp_ble_gap_set_device_name("ESP32_BLE_Device"); if (set_dev_name_ret) { handle_error("set device name failed", set_dev_name_ret); } esp_ble_adv_params_t adv_params = { .adv_int_min = 0x20, .adv_int_max = 0x40, .adv_type = ADV_TYPE_IND, .own_addr_type = BLE_ADDR_TYPE_PUBLIC, .channel_map = ADV_CHNL_ALL, .adv_filter_policy = ADV_FILTER_ALLOW_SCAN_ANY_CON_ANY, }; // 优化蓝牙广播数据，添加更多有用信息 esp_ble_adv_data_t adv_data = { .set_scan_rsp = false, .include_name = true, .include_txpower = true, .min_interval = 0x0006, .max_interval = 0x0010, .appearance = 0x00, .manufacturer_len = 0, .p_manufacturer_data = NULL, .service_data_len = 0, .p_service_data = NULL, .service_uuid_len = sizeof(GATTS_SERVICE_UUID_TEST_A), .p_service_uuid = (uint8_t *)&GATTS_SERVICE_UUID_TEST_A, .flag = (ESP_BLE_ADV_FLAG_GEN_DISC | ESP_BLE_ADV_FLAG_BREDR_NOT_SPT), }; esp_err_t ret = esp_ble_gap_config_adv_data(&adv_data); if (ret) { handle_error("config adv data failed", ret); } ret = esp_ble_gap_set_adv_params(&adv_params); if (ret) { handle_error("set adv params failed", ret); } esp_err_t create_attr_ret = esp_ble_gatts_create_attr_tab(test_gatt_db, gatts_if, GATTS_NUM_HANDLE_TEST_A, 0); if (create_attr_ret) { handle_error("create attr table failed", create_attr_ret); esp_ble_gap_stop_adv(); return; } break; } case ESP_GATTS_CREATE_EVT: { service_handle = param->create.service_handle; break; } case ESP_GATTS_ADD_CHAR_EVT: { char_handle = param->add_char.attr_handle; break; } default: break; } } // 主控芯片与网口的通信 class EthernetManager { public: EthernetManager() { const int max_retries = 3; int retry_count = 0; esp_err_t err; do { err = esp_netif_init(); if (err) { handle_error("esp_netif_init failed", err); retry_count++; vTaskDelay(pdMS_TO_TICKS(1000)); continue; } err = esp_event_loop_create_default(); if (err) { handle_error("esp_event_loop_create_default failed", err); retry_count++; vTaskDelay(pdMS_TO_TICKS(1000)); continue; } esp_netif_config_t cfg = ESP_NETIF_DEFAULT_ETH(); esp_netif_create_default_eth(&cfg); eth_mac_config_t mac_config = ETH_MAC_DEFAULT_CONFIG(); eth_phy_config_t phy_config = ETH_PHY_DEFAULT_CONFIG(); phy_config.phy_addr = 0; phy_config.reset_gpio_num = -1; mac = esp_eth_mac_new_esp32(&mac_config); phy = esp_eth_phy_new_lan8720(&phy_config); esp_eth_config_t eth_config = ETH_DEFAULT_CONFIG(mac, phy); err = esp_eth_driver_install(&eth_config, &eth_handle); if (err) { handle_error("Failed to install Ethernet driver", err); retry_count++; vTaskDelay(pdMS_TO_TICKS(1000)); continue; } err = esp_event_handler_register(ETH_EVENT, ESP_EVENT_ANY_ID, &eth_event_handler, this); if (err) { handle_error("esp_event_handler_register for ETH_EVENT failed", err); } err = esp_event_handler_register(IP_EVENT, IP_EVENT_ETH_GOT_IP, &ip_event_handler, this); if (err) { handle_error("esp_event_handler_register for IP_EVENT failed", err); } break; } while (retry_count < max_retries); if (retry_count >= max_retries) { handle_error("Ethernet initialization failed after multiple retries", ESP_FAIL); } } ~EthernetManager() { esp_eth_stop(eth_handle); esp_eth_driver_uninstall(eth_handle); esp_eth_phy_del(phy); esp_eth_mac_del(mac); } void start() { esp_err_t err = esp_eth_start(eth_handle); if (err) { handle_error("Failed to start Ethernet", err); } } private: static void eth_event_handler(void *arg, esp_event_base_t event_base, int32_t event_id, void *event_data) { EthernetManager *manager = static_cast<EthernetManager*>(arg); switch (event_id) { case ETHERNET_EVENT_CONNECTED: ESP_LOGI(TAG, "Ethernet Link Up"); break; case ETHERNET_EVENT_DISCONNECTED: ESP_LOGI(TAG, "Ethernet Link Down"); break; default: break; } } static void ip_event_handler(void *arg, esp_event_base_t event_base, int32_t event_id, void *event_data) { ip_event_got_ip_t *event = (ip_event_got_ip_t *) event_data; const esp_netif_ip_info_t *ip_info = &event->ip_info; ESP_LOGI(TAG, "Ethernet Got IP Address"); ESP_LOGI(TAG, "~~~~~~~~~~~"); ESP_LOGI(TAG, "ETHIP:" IPSTR, IP2STR(&ip_info->ip)); ESP_LOGI(TAG, "ETHMASK:" IPSTR, IP2STR(&ip_info->netmask)); ESP_LOGI(TAG, "ETHGW:" IPSTR, IP2STR(&ip_info->gw)); ESP_LOGI(TAG, "~~~~~~~~~~~"); } esp_eth_mac_t *mac; esp_eth_phy_t *phy; esp_eth_handle_t eth_handle; }; // 主程序运行 void app_main() { esp_err_t ret; // 初始化 NVS ret = nvs_flash_init(); if (ret == ESP_ERR_NVS_NO_FREE_PAGES || ret == ESP_ERR_NVS_NEW_VERSION_FOUND) { esp_err_t erase_ret = nvs_flash_erase(); if (erase_ret) { handle_error("nvs flash erase failed", erase_ret); } ret = nvs_flash_init(); } if (ret) { handle_error("nvs flash init failed", ret); return; } // 初始化蓝牙控制器 esp_bt_controller_mem_release(ESP_BT_MODE_CLASSIC_BT); esp_bt_controller_config_t bt_cfg = BT_CONTROLLER_INIT_CONFIG_DEFAULT(); ret = esp_bt_controller_init(&bt_cfg); if (ret) { handle_error("initialize controller failed", ret); return; } // 启用蓝牙控制器 ret = esp_bt_controller_enable(ESP_BT_MODE_BLE); if (ret) { handle_error("enable controller failed", ret); return; } // 初始化蓝牙协议栈 ret = esp_bluedroid_init(); if (ret) { handle_error("init bluetooth failed", ret); return; } ret = esp_bluedroid_enable(); if (ret) { handle_error("enable bluetooth failed", ret); return; } esp_ble_gatts_register_callback(gatts_event_handler); esp_ble_gatts_app_register(0); EthernetManager ethManager; ethManager.start(); while (1) { // 主循环可以添加其他任务 if (global_error_flag) { // 可以在这里添加错误处理逻辑，如重启等 } vTaskDelay(pdMS_TO_TICKS(1000)); } } 不涉及wifi只涉及蓝牙连接
09-26
- 另一个问题是，在特征值（`test_gatt_db[2]`）中，`p_value`设置为`NULL`，`value_len`为0。这意味着特征值初始为空，需要后续通过`esp_ble_gatts_set_attr_value`设置，或者在读取时由应用程序提供。但在当前...
c语言中int*p=null和int *p =" ";的区别,c语言中 int *p = NULL 和 *p = NULL 有什么区别...
weixin_32630003的博客
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1. int *p = NULL; 代表定义一个指向整型变量的指针p，然后p的值设为NULL,也就是设为0；用另一种方式说，就是对一个刚定义的指向整型变量的指针，赋初始值，让其指向0地址。2. *p = NULL; 代表对一个由指针p指向的变量(什么类型，不知道)，赋值为0，是将那个变量赋值为0。我们可以先看下面的代码：int *p = NULL;这时候我们可以通过编译器查看p的值为0x00000...
c语言中int*p=null和int *p =" ";的区别,c语言中 int *p = NULL 和 *p = NULL 有什么区别
weixin_33584986的博客
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1. int *p = NULL; 代表定义一个指向整型变量的指针p，然后p的值设为NULL,也就是设为0；用另一种方式说，就是对一个刚定义的指向整型变量的指针，赋初始值，让其指向0地址。2. *p = NULL; 代表对一个由指针p指向的变量(什么类型，不知道)，赋值为0，是将那个变量赋值为0。我们可以先看下面的代码：int *p = NULL;这时候我们可以通过编译器查看p的值为0x00000...
int *p=NULL 和 *p=NULL有什么区别
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　　很多初学者搜无法分清这两者之间的区别。先看下面的代码： int *p = NULL; 　　这时候我们可以通过调试器查看p的值为0x00000000。这句代码的意思是：定义一个指针变量p，其指向的内存里面保存的是int类型的数据；在定义变量p的同时，把p的值设置为0x00000000，而不是把*p的值设置为0x00000000。这个过程叫做初始化，是在编译的时候进行的。 ...
int *p=NULL和*p= NULL有什么区别
zsjalive的专栏
08-27 2265
很多初学者都无法分清这两者之间的区别。我们先看下面的代码： int *p = NULL; 这时候我们可以通过编译器查看p 的值为0×00000000。这句代码的意思是：定义一个指针变量p，其指向的内存里面保存的是int 类型的数据；在定义变量p 的同时把p 的值设置为0×00000000，而不是把*p 的值设置为0×00000000。这个过程叫做初始化，是在编译的时候进行的。 明白了什么是初
c语言指针int *p = NULL 和*p = NULL
weixin_30507481的博客
03-22 351
其中int*p = NULL的意思是定义一个指针变量p其指向的变量存储的是整形的数据：在定义时把p的值付成0x00000000而不是把*p的值附成0x00000000，这个过程叫做初始化，是在编译的时候进行的。 接下来看以下两行代码： int *p; p = NULL;这就不一样了这里只是知道定义了一个指针p，但是p的值未知，也就是说p保存的有可能是一个非法的地址，编译器会报错。 编写代码...
*(int *)0x108000=0x80是什么意思？
qq_15063463的博客
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0x108000是一个地址 (int*)把此地址强转为int型的地址 再解引用 则把由此地址开始的连续4字节作为int类型来进行存储 memcpy((u8 *)&dist, data, sizeof(float)); &为取地址符号
如何理解*(int *)0x12ff7c = 0x100;？
魏波
07-13 2365
如何理解*(int *)0x12ff7c = 0x100;？
指针(上）
cognietiv的博客
01-26 202
1.指针的定义和使用： 指针的定义：一个地址 如何正确定义？ 语法：数据类型+*指针变量 指针的使用：操纵内存空间 通过解引用（* 变量名）找到指针指向的内存。 注意：(1)定义中的“*”是指针的象征，此后的“*”是解引用，即取内存中的数据。 (2)int *p=&a（&是取地址符号） &p=a（&是引用符号，即变量的别名）。 (3)数据类型注意要一致。 2.指针所占内存空间 规定：32位操作系统，不管什么数据类型，指针所占的内存空间永远是4个字节(x86
指针综合篇
weixin_34221073的博客
05-04 351
一、指针1、指针的概念：用来保存地址的“变量”叫做指针，可以理解成指针是地址的一个别名。例：定义一个×××指针2、“指针的内容”，“指针所指向的内容”，“指针变量的地址” （1）、指针的内容： 指针变量p里面存放的是a的地址，也就是0x0018ff44. （2）、指针所指向的内容： 指针变量p里面存放的地址（0x18ff44）这块空间所对应的值，也就是10，我...
524-C++入门(const，指针，引用)
LINZEYU666的博客
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const，一级指针，引用的结合 写一句代码，在内存的0x0018ff44处写一个4字节的10 int *p = (int*)0x0018ff44; 对于指针变量，我们也可以定义引用变量 看下面这个表达式对错？ 把引用变量转换为指针 ...
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