linux c 解析生成json（jansson安装和使用）

Jansson是用于编码，解码和生成JSON数据的C语言库。它的特性：

• 简单直观的API和数据模型
• 全面的文档
• 不依赖其他库
• 全面支持Unicode(UTF-8)
• 大量测试套件

 

0.下载jansson  https://github.com/akheron/jansson/releases

1.安装jansson

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./configure make make install

2.编程

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包含头文件: #include   编译连接时加库 -ljansson              gcc –o source source.c –ljansson   如果执行程序出现：error while loading shared libraries: libjansson.so.4: cannot open shared object file: No such file or directory             找不到libjansson.so.4这个库时用下面方法解决   　　　　   //查找libjansson.so.4所在位置             Whereis libjansson   显示：libjansson: /usr/local/lib/libjansson.a /usr/local/lib/libjansson.la /usr/local/lib/libjansson.so   自己的libjansson.so所在位置为:/usr/local/lib/libjansson.so   　　　　   //确定libjansson.so是否真的存在             cd /usr/local/lib   　　　　   //如果存在，执行下面语句，创建一个libjansson.so的符号链接到/usr/lib目录下             ln -s /usr/local/lib/libjansson.so /usr/lib/libjansson.so.4   　　　　   //重新加载库             ldconfig    常用接口函数：   json_t *json_string(const char *value)           返回一个json string的数据类型，转换成这个库可以识别的格式。错误返回NULL，必须是UTF-8格式的。   　　     Return value: New reference.   　　     Returns a new JSON string, or NULL on error. value must be a valid UTF-8 encoded Unicode string.   json_t *json_string_nocheck(const char *value)           与json_string()相同，不检查value的有效性   　　    Return value: New reference.   　　    Like json_string(), but doesn’t check that value is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).   const char *json_string_value(const json_t *string)          返回json string中的字符串,是c语言的字符串。   　　    Returns the associated value of string as a null terminated UTF-8 encoded string,   or NULL if string is not a JSON string.   　　    The retuned value is read-only and must not be modified or freed by the user. It is valid as long as string exists, i.e. as long as its reference count has not dropped to zero.   int json_string_set(const json_t *string, const char *value)          设置string对应的值，如果value是无效的UTF-8值，设置失败。   　　　  Sets the associated value of string to value. value must be a valid UTF-8 encoded Unicode string. Returns 0 on success and -1 on error.   int json_string_set_nocheck(const json_t *string, const char *value)          与json_string_set()相同，只是不检查value是否是有效的UTF-8类型   Like json_string_set(), but doesn’t check that value is valid UTF-8. Use this function only if you are certain that this really is the case (e.g. you have already checked it by other means).

我主要是使用字符串的形式，参考的jansson-2.6/test/suites/api/test_object.c代码

下面的程序功能是：输出一个json格式的字符串，gcc 的时候别忘了加 -ljansson

需要特别注意的地方时jansson数组的处理，在需要循环的加入数组的时候需要使用 json_deep_copy()函数。如下例子：

json_t copy;
json_t object;
json_t array;

json_object_set_new (object, "test", json_string("testvalue1"));

copy = json_deep_copy(object);
json_array_append(arr, copy);

json_object_set_new (object, "test", json_string("testvalue2"));

copy = json_deep_copy(object);
json_array_append(arr, copy);

下面的是处理字符串

#include 
#include 

int event_jansson()
{
        json_t *objectmsg;
        char *result;

        objectmsg = json_object();

        json_object_set_new (objectmsg,   "inc", json_string("value-incID"));
        json_object_set_new (objectmsg,   "src", json_string("a"));
        json_object_set_new (objectmsg,   "des", json_string("b"));
        json_object_set_new (objectmsg,   "protocol", json_string("c"));
        json_object_set_new (objectmsg,   "policy", json_string("d"));
        json_object_set_new (objectmsg,   "snapshot", json_string("e"));
        json_object_set_new (objectmsg,   "name", json_string("f"));
        json_object_set_new (objectmsg,   "Type", json_string("g"));
        json_object_set_new (objectmsg,   "Path", json_string("h"));
        json_object_set_new (objectmsg,   "domain", json_string("i"));

        result = json_dumps(objectmsg, JSON_PRESERVE_ORDER);

        printf("result=%s\n", result);

        free(result);
        json_decref(objectmsg);

     　return 0;
}

int main()
{
     　　event_jansson();
        return 0;
}

下面的程序是从一个json文件中取出内容。

tmp.json文件中的内容是：

{"name1": "value1", "name2": "value2", "name3": "value3", "name4": "value4"}

 vim test.c

#include 
#include 
#include 

/* 关键字个数 */
#define COUNTMAX 256
 
/* 关键字长度 */
#define LENMAX 256

struct policymsg
{
    int size;
    char keyword[COUNTMAX][LENMAX];
    char keycount[COUNTMAX][LENMAX];
};

/* 判断janson的类型 */
int my_json_type(json_t * value)
{
    if (json_is_object(value))
    {
        printf("json_is_object");
        return JSON_OBJECT;
    }

    if (json_is_array(value))
    {
        printf("json_is_array");
        return JSON_ARRAY;
    }

    if (json_is_string(value))
    {
        printf("json_is_string");
        return JSON_STRING;
    }

    if (json_is_integer(value))
    {
        printf("json_is_integer");
        return JSON_INTEGER;
    }

    if (json_is_real(value))
    {
        printf("json_is_real");
        return JSON_REAL;
    }

    if (json_is_number(value))
    {
        printf("json_is_number");
    }

    if (json_is_boolean(value))
    {
        printf("json_is_boolean");
    }

    if (json_is_null(value))
    {
        printf("json_is_null");
        return JSON_NULL;
    }

    if (json_is_true(value))
    {
        printf("json_boolean(1)");
        return JSON_TRUE;
    }

    if (json_is_false(value))
    {
        printf("json_boolean(0)");
        return JSON_FALSE;
    }
}


struct policymsg get_policy_json(char *jsonfile)
{
    struct policymsg policyinfo;

    int i, size;
    void *iter;
    json_t *object;
    json_t *iter_values;
    json_error_t error;

    object = json_object();

    object = json_load_file (jsonfile, 0, &error);
    policyinfo.size = json_object_size (object);
#if 0

    //size = json_object_size (object);
    //printf("size=%d", size);

    /* 取出object中的值 */

    //struct policy iter_get_value(json_t *object)
    char *result;
    result = json_dumps(object, JSON_PRESERVE_ORDER);
    printf("result=%s", result);

    /* 判断读取的jansson类型 */
    printf("判断是什么类型");
    my_json_type(object);
    printf("result_size = %d",  strlen(result));
#endif


    iter = json_object_iter(object);
    i = 0;
    while (1)
    {
        strcpy (policyinfo.keyword[i], json_object_iter_key(iter));
        iter_values = json_object_iter_value(iter);
        strcpy (policyinfo.keycount[i], json_string_value(iter_values));

        //printf("values[%d]=%s", i,json_string_value(iter_values[i]));

        if ((iter = json_object_iter_next(object, iter)) == NULL)
        {
            //printf("iterate end");
            break;
        }

        i++;
    }

#if 0
    iter = json_object_iter_at(object,   "b");
    if (iter)
    {
        iter_keys[i] = json_object_iter_key(iter);
        iter_values[i] = json_object_iter_value(iter);
        printf("values[%d]=%s", i, json_string_value(iter_values[i]));
    }
#endif

    json_decref(object);
    return policyinfo;
}

#if 1
int main()
{
    //result = "{\"objectmsg\": \"{\"name1\": \"value1\", \"name2\": \"value2\", \"name3\": \"value3\", \"name4\": \"value4\"}\", \"name5\": \"value6\"}";
    //{"name1": "value1", "name2": "value2", "name3": "value3", "name4": "value4"}
    char *str =   "./tmp.json";
    struct policymsg policyinfo;

    policyinfo = get_policy_json(str);
    int i = 0;

    while ( i < policyinfo.size)
    {
        printf ("keyword = %s", policyinfo.keyword[i]);
        printf ("value = %s", policyinfo.keycount[i]);
        i++;
    }
}
#endif

编译：

gcc test.c -ljansson