Question

我正在使用Curl库创建一个简单的C代码，使用MSVC从URL下载文件。问题是如果在下载过程中连接断开，我的代码将冻结并且未完成的文件尚未从目录中删除。我想要的是如果下载失败，程序必须重试连接或删除未完成的文件，然后再试一次。我更喜欢使用C库而不是C ++库。这是我正在使用的代码：

//lib for curl
#include <curl/curl.h>
#define CURL_STATICLIB

bool downloader3(string url, string file_path) {
    CURL *curl;
    FILE *fp;
    CURLcode res;
    curl = curl_easy_init();
    if (curl) {
        fp = fopen(file_path.c_str(), "wb");
        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
        res = curl_easy_perform(curl);
        //always cleanup
        curl_easy_cleanup(curl);
        fclose(fp);
        double val;
        res = curl_easy_getinfo(curl, CURLINFO_SPEED_DOWNLOAD, &val);
        if ((CURLE_OK == res) && (val>0))
            printf("Average download speed: %0.3f kbyte/sec.\n", val / 1024);
        if ((res == CURLE_OK)) {
            printf("Download Successful!\r\n");
            return true;
        }
        else {
            printf("Downlaod Failed!\r\n");
            remove(file_path.c_str());  //remove the temp file
            return false;
        }
    }
}

EDIT --- 感谢RingØ回答。我修改了代码，但我正在寻找一个恢复功能，可以恢复不完整文件的下载。

bool downloader3(string url, string file_path) {
    CURL *curl;
    FILE *fp = NULL;
    CURLcode res;

    int status;
    int maxtries = 3;
    do {
        printf("Doing try # %d\r\n", maxtries);
        curl = curl_easy_init();
        if (curl) {
            fp = fopen(file_path.c_str(), "wb");
            curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
            curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
            curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
            curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10L); // 30 seconds 
            res = curl_easy_perform(curl);
            //always cleanup
            curl_easy_cleanup(curl);
            fclose(fp);
            if ((res == CURLE_OK)) {
                printf("Download Successful!\r\n");
                break;
                //return true;
                }
            }
    } while (--maxtries);
    if (maxtries) { // was OK
        //curl_easy_cleanup(curl);  // clean curl / delete file?
        //fclose(fp);
        return true;
    }
    else {
        printf("Download Failed!\r\n");
        printf("file path is: %s", file_path.c_str());
        Sleep(5000);
        status = remove(file_path.c_str());  //remove the unfinished file
        if (status == 0)
            printf("%s file deleted successfully.\n", file_path);
        else
        {
            printf("Unable to delete the file\n");
        }
        return false;
    }
}

Answer 1

您可以设置超时选项

curl_easy_setopt(curl, CURLOPT_TIMEOUT, 30L); // 30 seconds

如果操作未在30秒内完成，则触发超时。然后在while循环中检查结果值，例如

res = curl_easy_perform( ... );
if (res == CURLE_OK) {
    break;
}
// delete file
// keep retrying (add a counter if necessary)

另见the curl page。

循环示例

  int maxtries = 5;
  do {
     curl = curl_easy_init();
     if (curl) {
        ...
        res = curl_easy_perform( ... );
        if (res == CURLE_OK) {
           break;
        }

        // delete file, curl cleanup...
      }
  } while ( --maxtries );

  if (maxtries) { // was OK
     // clean curl / delete file?
  }

这不是理想的解决方案，正如您所说，下载可能需要更多或更少的时间。如果超时足够大，这（应该）可以防止永无止境的程序。

已知Curl库在连接不稳定时会出现一些问题 - 现在可能会有更好的东西，请尝试最新的稳定构建。

如果您在几天内没有得到更好的答案，请尝试添加＆＃34; Bounty＆＃34; 50个代表以吸引更多关注。

Answer 2

您正在寻找的是RESUME_FROM功能。要使用它，您必须知道要从哪个字节开始下载。在这个例子中，它是一个上传，但应该是相同的setopt技术。以下是curl网站的示例用法：

CURL *curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "ftp://example.com");

  /* resume upload at byte index 200 */
  curl_easy_setopt(curl, CURLOPT_RESUME_FROM, 200L);

  /* ask for upload */
  curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

  /* set total data amount to expect */
  curl_easy_setopt(curl, CURLOPT_INFILESIZE, size_of_file);

  /* Perform the request */
  curl_easy_perform(curl);
}

来源：https://curl.haxx.se/libcurl/c/CURLOPT_RESUME_FROM.html

未完成的下载与卷曲在C中

2 个答案: