在使用C語言開發Web應用時，可能需要獲取從URL傳過來的JSON數據。可以使用以下代碼實現：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>
struct MemoryStruct {
char *memory;
size_t size;
};
static size_t WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp) {
size_t realsize = size * nmemb;
struct MemoryStruct *mem = (struct MemoryStruct *)userp;
mem->memory = realloc(mem->memory, mem->size + realsize + 1);
if (mem->memory == NULL) {
/* out of memory! */
printf("not enough memory (realloc returned NULL)\n");
return 0;
}
memcpy(&(mem->memory[mem->size]), contents, realsize);
mem->size += realsize;
mem->memory[mem->size] = '\0';
return realsize;
}
int main(void) {
CURL *curl_handle;
CURLcode res;
struct MemoryStruct chunk;
chunk.memory = malloc(1);
chunk.size = 0;
curl_global_init(CURL_GLOBAL_ALL);
curl_handle = curl_easy_init();
curl_easy_setopt(curl_handle, CURLOPT_URL, "https://example.com/json_data");
curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
res = curl_easy_perform(curl_handle);
if (res != CURLE_OK) {
fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
} else {
printf("%lu bytes retrieved\n", (unsigned long)chunk.size);
printf("JSON Data:\n%s\n", chunk.memory);
}
curl_easy_cleanup(curl_handle);
if (chunk.memory) {
free(chunk.memory);
}
curl_global_cleanup();
return 0;
}

在上述代碼中，我們使用C語言的curl庫獲取URL傳過來的JSON數據。我們定義了一個結構體MemoryStruct來表示數據，使用realloc來動態分配內存存儲數據，使用curl_easy_setopt函數設置URL和回調函數、用戶數據等信息，最終通過curl_easy_perform函數執行請求。如果請求成功，就可以在回調函數中獲取JSON數據。