如何在libcurl中发送表单字段的值？(在命令行curl中使用-F选项的那个)

#include <stdio.h>
#include <curl/curl.h>

int main(void) {
    CURL *curl;
    CURLcode res;
    
    curl_global_init(CURL_GLOBAL_ALL);
    curl = curl_easy_init();
    
    if(curl) {
        // 设置请求的URL地址
        curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/upload");
        
        // 创建表单字段数组
        struct curl_httppost *formpost = NULL;
        struct curl_httppost *lastptr = NULL;
        
        // 添加表单字段
        curl_formadd(&formpost, &lastptr,
                     CURLFORM_COPYNAME, "username",
                     CURLFORM_COPYCONTENTS, "john",
                     CURLFORM_END);
        curl_formadd(&formpost, &lastptr,
                     CURLFORM_COPYNAME, "password",
                     CURLFORM_COPYCONTENTS, "secret",
                     CURLFORM_END);
        
        // 设置表单字段数组
        curl_easy_setopt(curl, CURLOPT_POSTFIELDS, formpost);
        
        // 发送请求
        res = curl_easy_perform(curl);
        
        // 释放资源
        curl_easy_cleanup(curl);
        curl_formfree(formpost);
    }
    
    curl_global_cleanup();
    return 0;
}