c 通過url獲取json是一種常見的操作，它主要是通過訪問指定的url，獲取網頁上的json數據，以便進行相應的數據處理。

#include <stdio.h>
#include <stdlib.h>
#include <curl/curl.h>
#include <jansson.h>
static size_t callback(void *ptr, size_t size, size_t nmemb, void *stream)
{
size_t written = fwrite(ptr, size, nmemb, (FILE *)stream);
return written;
}
int main(int argc, char *argv[])
{
CURL *curl;
FILE *fp;
CURLcode res;
char *url = "http://localhost:8080/books";
char outfilename[FILENAME_MAX] = "/tmp/books.json";
char *json_string;
json_error_t error;
json_t *json_data, *json_element;
curl = curl_easy_init();
if (curl)
{
fp = fopen(outfilename,"wb");
curl_easy_setopt(curl, CURLOPT_URL, url);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, callback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
fclose(fp);
}
fp = fopen(outfilename, "r");
fseek(fp, 0, SEEK_END);
long fsize = ftell(fp);
fseek(fp, 0, SEEK_SET);
json_string = malloc(fsize + 1);
fread(json_string, fsize, 1, fp);
fclose(fp);
json_data = json_loads(json_string, 0, &error);
if(!json_data)
{
fprintf(stderr, "error: on line %d: %s\n", error.line, error.text);
exit(1);
}
size_t index;
json_array_foreach(json_data, index, json_element)
{
json_t *id, *name;
if(!json_object_get(json_element, "id") || !json_object_get(json_element, "name"))
{
continue;
}
id = json_object_get(json_element, "id");
name = json_object_get(json_element, "name");
if(!json_is_number(id) || !json_is_string(name))
{
continue;
}
printf("id: %ld, name: %s\n", json_integer_value(id), json_string_value(name));
}
json_decref(json_data);
free(json_string);
return 0;
}

以上是一個通過url獲取json數據并進行處理的簡單示例代碼，主要利用了curl庫和jansson庫進行數據獲取和解析。其中，通過curl_easy_setopt設置相應的選項，如URL、回調函數等，實現從指定URL獲取數據。然后，利用jansson庫中的json_loads函數對獲取到的json字符串進行解析，并進行相應的數據處理。