卷曲多接口不起作用

你的 maxfd 返回 -1。

http://curl.haxx.se/libcurl/c/curl_multi_fdset.html 摘录 ：

如果 libcurl 未设置任何文件描述符，则max_fd将包含 -1 此函数返回。否则，它将包含更高的描述符 数字库库尔集。当 libcurl 在 max_fd 中返回 -1 时，这是因为 libcurl 目前做了一些对你来说不可能的事情 使用套接字监视的应用程序，不幸的是，您可以 不知道当前操作何时使用 select（） 完成。 当max_fd返回 -1 时，您需要等待一段时间，然后继续 无论如何都要打电话给curl_multi_perform。等多久？我会建议 至少 100 毫秒，但您可能想自己测试一下 找到合适值的特定条件。

因此，请尝试延迟：

#if defined(__WIN32__) || defined(_WIN32) || defined(WIN32) || defined(__WINDOWS__) || defined(__TOS_WIN__)
  #include <windows.h>
  inline void delay( unsigned long ms )
    {
    Sleep( ms );
    }
#else  /* presume POSIX */
  #include <unistd.h>
  inline void delay( unsigned long ms )
    {
    usleep( ms * 1000 );
    }
#endif 
#include <stdio.h>
#include <string.h>
/* somewhat unix-specific */ 
#include <sys/time.h>
#include <unistd.h>
/* curl stuff */ 
#include <curl/curl.h>
/*
 * Simply download a HTTP file.
 */ 
int main(void)
{
  CURL *http_handle;
  CURLM *multi_handle;
  int still_running; /* keep number of running handles */ 
  curl_global_init(CURL_GLOBAL_DEFAULT);
  http_handle = curl_easy_init();
  /* set the options (I left out a few, you'll get the point anyway) */ 
  curl_easy_setopt(http_handle, CURLOPT_URL, "http://www.example.com/");
  /* init a multi stack */ 
  multi_handle = curl_multi_init();
  /* add the individual transfers */ 
  curl_multi_add_handle(multi_handle, http_handle);
  /* we start some action by calling perform right away */ 
  curl_multi_perform(multi_handle, &still_running);
  do {
    struct timeval timeout;
    int rc; /* select() return code */ 
    fd_set fdread;
    fd_set fdwrite;
    fd_set fdexcep;
    int maxfd = -1;
    long curl_timeo = -1;
    FD_ZERO(&fdread);
    FD_ZERO(&fdwrite);
    FD_ZERO(&fdexcep);
    /* set a suitable timeout to play around with */ 
    timeout.tv_sec = 1;
    timeout.tv_usec = 0;
    curl_multi_timeout(multi_handle, &curl_timeo);
    if(curl_timeo >= 0) {
      timeout.tv_sec = curl_timeo / 1000;
      if(timeout.tv_sec > 1)
        timeout.tv_sec = 1;
      else
        timeout.tv_usec = (curl_timeo % 1000) * 1000;
    }
    /* get file descriptors from the transfers */ 
    curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
    /* In a real-world program you OF COURSE check the return code of the
       function calls.  On success, the value of maxfd is guaranteed to be
       greater or equal than -1.  We call select(maxfd + 1, ...), specially in
       case of (maxfd == -1), we call select(0, ...), which is basically equal
       to sleep. */ 
    while(rc==-1){
      delay(1);
      rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);
    }
    switch(rc) {
    case -1:
      /* select error */ 
      still_running = 0;
      printf("select() returns error, this is badnessn");
      break;
    case 0:
    default:
      /* timeout or readable/writable sockets */ 
      curl_multi_perform(multi_handle, &still_running);
      break;
    }
  } while(still_running);
  curl_multi_remove_handle(multi_handle, http_handle);
  curl_easy_cleanup(http_handle);
  curl_multi_cleanup(multi_handle);
  curl_global_cleanup();
  return 0;
}

我希望这会有所帮助。