c++ - curl 多接口(interface)不起作用

Question

我尝试运行 curl 多接口(interface)示例:http://curl.haxx.se/libcurl/c/multi-single.html

我正在使用 mingw g++。

首先我得到了一个错误“undefined reference to 'select@20'”。

我在编译器中添加了“-lws2_32”。

现在没有编译错误，但程序打印“select() 返回错误，这很糟糕”。

我做错了什么？

#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. */ 

    rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);

    switch(rc) {
    case -1:
      /* select error */ 
      still_running = 0;
      printf("select() returns error, this is badness\n");
      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;
}

Answer 1

您的 maxfd 返回 -1。

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

If no file descriptors are set by libcurl, max_fd will contain -1 when this function returns. Otherwise it will contain the higher descriptor number libcurl set. When libcurl returns -1 in max_fd, it is because libcurl currently does something that isn't possible for your application to monitor with a socket and unfortunately you can then not know exactly when the current action is completed using select(). When max_fd returns with -1, you need to wait a while and then proceed and call curl_multi_perform anyway. How long to wait? I would suggest 100 milliseconds at least, but you may want to test it out in your own particular conditions to find a suitable value.

所以，请延迟尝试:

#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 badness\n");
      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;
}

希望对您有所帮助。