将上述代码封装成一个函数

好的，这里是我封装后的代码，函数的参数依次为 JSON 字符串指针、密钥、签名键名：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/hmac.h>
#include "cJSON.h"

char *sortJsonString(cJSON *json);

char *getSign(char *jsonString, char *key);

char *addSignToJson(char *jsonString, char *key, char *signKey);

char *sortJsonString(cJSON *json) {
    int count = cJSON_GetArraySize(json);
    char **keys = (char **) malloc(count * sizeof(char *));
    int i = 0;
    cJSON_ArrayForEach(json, json) {
        keys[i++] = strdup(json->string);
    }
    qsort(keys, count, sizeof(char *), strcmp);
    char *sortString = (char *) malloc(1);
    sortString[0] = '\0';
    for (int i = 0; i < count; i++) {
        cJSON *item = cJSON_GetObjectItemCaseSensitive(json, keys[i]);
        if (item->type == cJSON_Object) {
            char *subSortString = sortJsonString(item);
            sortString = (char *) realloc(sortString, strlen(sortString) + strlen(keys[i]) + strlen(subSortString) + 3);
            sprintf(sortString + strlen(sortString), "%s=%s&", keys[i], subSortString);
            free(subSortString);
        } else {
            sortString = (char *) realloc(sortString, strlen(sortString) + strlen(keys[i]) + strlen(item->valuestring) + 3);
            sprintf(sortString + strlen(sortString), "%s=%s&", keys[i], item->valuestring);
        }
    }
    sortString[strlen(sortString) - 1] = '\0';
    cJSON_ArrayForEach(json, json) {
        free(keys[--i]);
    }
    free(keys);
    return sortString;
}

char *getSign(char *jsonString, char *key) {
    char *sortString = sortJsonString(cJSON_Parse(jsonString));
    unsigned char *hmac = HMAC(EVP_sha256(), key, strlen(key), (const unsigned char *) sortString, strlen(sortString), NULL, NULL);
    char *sign = (char *) malloc(65);
    for (int i = 0; i < 32; i++) {
        sprintf(&sign[i * 2], "%02x", hmac[i]);
    }
    sign[64] = '\0';
    free(sortString);
    return sign;
}

char *addSignToJson(char *jsonString, char *key, char *signKey) {
    char *sign = getSign(jsonString, key);
    cJSON *json = cJSON_Parse(jsonString);
    cJSON_AddStringToObject(json, signKey, sign);
    char *result = cJSON_Print(json);
    free(sign);
    cJSON_Delete(json);
    return result;
}

`addSignToJson` 函数将前面的代码封装起来，并将签名键名作为参数传入。函数返回一个新的 JSON 字符串指针，需要注意在使用后释放内存。