libcurl中的进度显示类似于读取数据，需要设置回调函数和回调存储数据的指针到url中，CURLOPT_PROGRESSFUNCTION传入回调函数，CURLOPT_PROGRESSDATA传入存储数据的指针。
具体实例如下：

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

#define STOP_DOWNLOAD_AFTER_THIS_MANY_BYTES         6000
#define MINIMAL_PROGRESS_FUNCTIONALITY_INTERVAL     3

struct myprogress {
  double lastruntime;
  CURL *curl;
};

/* this is how the CURLOPT_XFERINFOFUNCTION callback works */
static int xferinfo(void *p,curl_off_t dltotal,curl_off_t dlnow,curl_off_t ultotal,curl_off_t ulnow)
{
  struct myprogress *myp = (struct myprogress *)p;
  CURL *curl = myp->curl;
  double curtime = 0;

  curl_easy_getinfo(curl,CURLINFO_TOTAL_TIME,&curtime);

  /* under certain circumstances it may be desirable for certain functionality
     to only run every N seconds,in order to do this the transaction time can
     be used */
  if((curtime - myp->lastruntime) >= MINIMAL_PROGRESS_FUNCTIONALITY_INTERVAL) {
    myp->lastruntime = curtime;
    fprintf(stderr,"TOTAL TIME: %f \r\n",curtime);
  }

  fprintf(stderr,"UP: %" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T
          "  DOWN: %" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T
          "\r\n",ulnow,ultotal,dlnow,dltotal);

  if(dlnow > STOP_DOWNLOAD_AFTER_THIS_MANY_BYTES)
    return 1;
  return 0;
}
//旧版本是参数类型double，新版本换成了curl_off_t
/* for libcurl older than 7.32.0 (CURLOPT_PROGRESSFUNCTION) */
static int older_progress(void *p,double dltotal,double dlnow,double ultotal,double ulnow)
{
  return xferinfo(p,(curl_off_t)dltotal,(curl_off_t)dlnow,(curl_off_t)ultotal,(curl_off_t)ulnow);
}


int main(void)
{
  CURL *curl;
  CURLcode res = CURLE_OK;
  struct myprogress prog;

  curl = curl_easy_init();
  if(curl) {
    prog.lastruntime = 0;
    prog.curl = curl;

    curl_easy_setopt(curl,CURLOPT_URL,"http://example.com/");

    curl_easy_setopt(curl,CURLOPT_PROGRESSFUNCTION,older_progress);//回调函数
    /* pass the struct pointer into the progress function */
    curl_easy_setopt(curl,CURLOPT_PROGRESSDATA,&prog);//存储数据结构体指针

#if LIBCURL_VERSION_NUM >= 0x072000
    /* xferinfo was introduced in 7.32.0,no earlier libcurl versions will
       compile as they won't have the symbols around.

       If built with a newer libcurl,but running with an older libcurl:
       curl_easy_setopt() will fail in run-time trying to set the new
       callback,making the older callback get used.

       New libcurls will prefer the new callback and instead use that one even
       if both callbacks are set. */

    curl_easy_setopt(curl,CURLOPT_XFERINFOFUNCTION,xferinfo);
    /* pass the struct pointer into the xferinfo function,note that this is
       an alias to CURLOPT_PROGRESSDATA */
    curl_easy_setopt(curl,CURLOPT_XFERINFODATA,&prog);
#endif

    curl_easy_setopt(curl,CURLOPT_NOPROGRESS,0L);
    res = curl_easy_perform(curl);

    if(res != CURLE_OK)
      fprintf(stderr,"%s\n",curl_easy_strerror(res));

    /* always cleanup */
    curl_easy_cleanup(curl);
  }
  return (int)res;
}