使用C++ curl下载文件时始终得到0KB空文件，求排查问题

问题描述 #

尝试用C++结合Curl下载文件，代码如下，但本该下载2000KB的文件实际得到0KB空文件：

#define CURL_STATICLIB

#include <iostream>
#include <string>
#include <curl/curl.h>

int main() {
    CURL *curl;
    FILE *fp;
    CURLcode res;
    std::string url = "https://rtk3dsf-lb01-1291588867.us-east-1.elb.amazonaws.com/api/rpgdownload?args=%7B%22ver%22:%22l.0.0.1%22,%22sid%22:28555,%22region%22:%22EUR%22,%22lang%22:%22en%22,%22token%22:%22Fqbt-EuVHsrOVeLwiVIrmXS.ck3Ph8MGH03DjKp5Q7uqYFL9CNAr9WVeyxgnHvYH6wE6RnvAtUkUMmas2N0Gj.Q**%22%7D";
    char outfilename[FILENAME_MAX] = "game.dat";
    curl = curl_easy_init();
    if (curl) {
        fp = fopen(outfilename,"wb");
        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fp);
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
        fclose(fp);
    }
    return 0;
}

错误点分析 #

• CURLOPT_WRITEFUNCTION参数误用：这个选项要求传入符合size_t (*)(char*, size_t, size_t, void*)签名的函数指针，但你直接传了FILE*类型的fp，导致Curl无法正确处理数据写入逻辑。如果要直接写入文件，只需设置CURLOPT_WRITEDATA为文件指针，CURLOPT_WRITEFUNCTION可以留空使用默认实现（默认适配fwrite）。
• 缺少Curl全局初始化：调用curl_easy_init()前必须执行curl_global_init(CURL_GLOBAL_ALL)完成全局初始化，否则会出现未定义行为。
• 无关键步骤错误检查：没有判断fopen是否成功（比如权限不足、路径无效会导致文件打开失败），也没检查curl_easy_perform的返回值，无法定位请求是否成功。

修正后的代码 #

#define CURL_STATICLIB

#include <iostream>
#include <string>
#include <curl/curl.h>

int main() {
    // 全局初始化Curl
    curl_global_init(CURL_GLOBAL_ALL);

    CURL *curl = nullptr;
    FILE *fp = nullptr;
    CURLcode res;
    std::string url = "https://rtk3dsf-lb01-1291588867.us-east-1.elb.amazonaws.com/api/rpgdownload?args=%7B%22ver%22:%22l.0.0.1%22,%22sid%22:28555,%22region%22:%22EUR%22,%22lang%22:%22en%22,%22token%22:%22Fqbt-EuVHsrOVeLwiVIrmXS.ck3Ph8MGH03DjKp5Q7uqYFL9CNAr9WVeyxgnHvYH6wE6RnvAtUkUMmas2N0Gj.Q**%22%7D";
    char outfilename[FILENAME_MAX] = "game.dat";

    curl = curl_easy_init();
    if (curl) {
        // 检查文件是否成功打开
        fp = fopen(outfilename, "wb");
        if (!fp) {
            std::cerr << "无法打开文件: " << outfilename << std::endl;
            curl_easy_cleanup(curl);
            curl_global_cleanup();
            return 1;
        }

        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        // 设置写入目标的文件指针
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);

        // 执行请求并检查结果
        res = curl_easy_perform(curl);
        if (res != CURLE_OK) {
            std::cerr << "Curl请求失败: " << curl_easy_strerror(res) << std::endl;
        }

        curl_easy_cleanup(curl);
        fclose(fp);
    } else {
        std::cerr << "无法初始化Curl" << std::endl;
    }

    // 全局清理
    curl_global_cleanup();
    return 0;
}

内容的提问来源于stack exchange，提问作者Myth