本文主要是介绍cJSON库的安装与使用,希望对大家解决编程问题提供一定的参考价值,需要的开发者们随着小编来一起学习吧!
原文链接:https://blog.csdn.net/woay2008/article/details/94367652
介绍
cJSON 库是C语言中的最常用的 JSON 库。github 地址是 https://github.com/DaveGamble/cJSON 。
安装
环境是 Ubuntu 16.04。需要先安装cmake。
cJSON 库安装步骤如下:
git clone https://github.com/DaveGamble/cJSON.git
cd cJSON/
mkdir build
cd build/
cmake ..
make
make install
执行完上述命令后,cJSON.h 头文件会安装在 /usr/local/include/cjson 目录下。libcjson.so 库文件会安装在 /usr/local/lib 目录下。还需要将/usr/local/lib目录添加到 /etc/ld.so.conf 文件中,然后执行 /sbin/ldconfig,否则程序在运行时会报 error while loading shared libraries: libcjson.so.1: cannot open shared object file: No such file or directory 错误。
Data Structure
cJSON对象结构的数据类型:
/* The cJSON structure: */
typedef struct cJSON
{struct cJSON *next;struct cJSON *prev;struct cJSON *child;int type;char *valuestring;/* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */int valueint;double valuedouble;char *string;
} cJSON;
使用
还是使用显示器支持的分辨率的例子来说明如何使用 cJSON 库生成和解析JSON如下的格式:
{"name": "Awesome 4K","resolutions": [{"width": 1280,"height": 720},{"width": 1920,"height": 1080},{"width": 3840,"height": 2160}]
}
使用 cJSON 库时,程序只需包含<cjson/cJSON.h>
头文件。而在编译时,需要添加 -lcjson
选项。
生成JSON格式
生成 JSON 对象有两种方式:
- 一是调用 cJSON_CreateXXX 创建相应类型的值(sJSON结构),然后调用 cJSON_AddItemToObject 将值加入到对象中。
- 二是直接调用 cJSON_AddXXXToObject 创建并添加值到对象中 。
生成 JSON 数组只有一种方式,就是先调用 cJSON_CreateXXX 创建相应类型的值,然后调用 cJSON_AddItemToArray 将值加入到数组中。
最后都要调用 cJSON_Print 将对象或数组转化成 JSON 格式的字符串,调用 cJSON_Delete 释放对象。
生成 JSON 对象的第一种示例代码如下:
//NOTE: Returns a heap allocated string, you are required to free it after use.
char* create_monitor(void)
{const unsigned int resolution_numbers[3][2] = {{1280, 720},{1920, 1080},{3840, 2160}};char *string = NULL;cJSON *name = NULL;cJSON *resolutions = NULL;cJSON *resolution = NULL;cJSON *width = NULL;cJSON *height = NULL;size_t index = 0;/* 创建一个对象 */cJSON *monitor = cJSON_CreateObject();if (monitor == NULL){goto end;}/* 创建一个字符串 */name = cJSON_CreateString("Awesome 4K");if (name == NULL){goto end;}/* 将字符串添加到对象中 *//* after creation was successful, immediately add it to the monitor,* thereby transfering ownership of the pointer to it */cJSON_AddItemToObject(monitor, "name", name);/* 创建一个数组 */resolutions = cJSON_CreateArray();if (resolutions == NULL){goto end;}/* 将数组添加到对象中 */cJSON_AddItemToObject(monitor, "resolutions", resolutions);for (index = 0; index < (sizeof(resolution_numbers) / (2 * sizeof(int))); ++index){resolution = cJSON_CreateObject();if (resolution == NULL){goto end;}/* 将对象添加到数组中 */cJSON_AddItemToArray(resolutions, resolution);/* 创建一个数字 */width = cJSON_CreateNumber(resolution_numbers[index][0]);if (width == NULL){goto end;}/* 将数字添加到对象中 */cJSON_AddItemToObject(resolution, "width", width);height = cJSON_CreateNumber(resolution_numbers[index][1]);if (height == NULL){goto end;}cJSON_AddItemToObject(resolution, "height", height);}/* 转换成 JSON 格式的字符串 */string = cJSON_Print(monitor);if (string == NULL){fprintf(stderr, "Failed to print monitor.\n");}end:/* 释放对象 */cJSON_Delete(monitor);return string;
}
生成 JSON 对象的第二种示例代码如下:
//NOTE: Returns a heap allocated string, you are required to free it after use.
char *create_monitor_with_helpers(void)
{const unsigned int resolution_numbers[3][2] = {{1280, 720},{1920, 1080},{3840, 2160}};char *string = NULL;cJSON *resolutions = NULL;size_t index = 0;cJSON *monitor = cJSON_CreateObject();/* 将字符串添加到对象中 */if (cJSON_AddStringToObject(monitor, "name", "Awesome 4K") == NULL){goto end;}/* 将数组添加到对象中,返回一个数组 */resolutions = cJSON_AddArrayToObject(monitor, "resolutions");if (resolutions == NULL){goto end;}for (index = 0; index < (sizeof(resolution_numbers) / (2 * sizeof(int))); ++index){cJSON *resolution = cJSON_CreateObject();/* 将数字添加到对象中 */if (cJSON_AddNumberToObject(resolution, "width", resolution_numbers[index][0]) == NULL){goto end;}if(cJSON_AddNumberToObject(resolution, "height", resolution_numbers[index][1]) == NULL){goto end;}cJSON_AddItemToArray(resolutions, resolution);}string = cJSON_Print(monitor);if (string == NULL) {fprintf(stderr, "Failed to print monitor.\n");}end:cJSON_Delete(monitor);return string;
}
解析JSON格式
解析 JSON 格式,首先要调用 cJSON_Parse 生成用于解析的 cJSON 结构,然后调用 cJSON_GetObjectItemCaseSensitive或 cJSON_GetObjectItem 获取对应名字的值,用 cJSON_IsXXX 判断值的类型是否正确,然后用 结构中的 valuestring 或 valuedouble 等成员获取值。实例函数代码如下,这个函数判断显示器是否支持 1920x1080 分辨率:
/* return 1 if the monitor supports full hd, 0 otherwise */
int supports_full_hd(const char *const monitor)
{const cJSON *resolution = NULL;const cJSON *resolutions = NULL;const cJSON *name = NULL;int status = 0;/* 创建一个用于解析的 cJSON 结构 */cJSON *monitor_json = cJSON_Parse(monitor);if (monitor_json == NULL){const char *error_ptr = cJSON_GetErrorPtr();if (error_ptr != NULL){fprintf(stderr, "Error before: %s\n", error_ptr);}status = 0;goto end;}/* 获取名为“name”的值 */name = cJSON_GetObjectItemCaseSensitive(monitor_json, "name");if (cJSON_IsString(name) && (name->valuestring != NULL)){printf("Checking monitor \"%s\".\n", name->valuestring);}/* 获取名为“resolutions“的值,它是一个数组 */resolutions = cJSON_GetObjectItemCaseSensitive(monitor_json, "resolutions");/* 遍历数组 */cJSON_ArrayForEach(resolution, resolutions){cJSON *width = cJSON_GetObjectItemCaseSensitive(resolution, "width");cJSON *height = cJSON_GetObjectItemCaseSensitive(resolution, "height");if (!cJSON_IsNumber(width) || !cJSON_IsNumber(height)){status = 0;goto end;}if ((width->valuedouble == 1920) && (height->valuedouble == 1080)){status = 1;goto end;}}end:cJSON_Delete(monitor_json);return status;
}
main函数
代码如下:
int main(int argc, char **argv)
{char *monitor = create_monitor_with_helpers();printf("%s\n", monitor);if (supports_full_hd(monitor)) {printf("It support full HD.\n");} free(monitor);
}
程序运行结果如下:
{"name": "Awesome 4K","resolutions": [{"width": 1280,"height": 720}, {"width": 1920,"height": 1080}, {"width": 3840,"height": 2160}]
}
Checking monitor "Awesome 4K".
It support full HD.
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