#define MAX_BUF  65536
char wr_buf[MAX_BUF+1];
void test() 
{
    .....
    if (curl) {
 curl_easy_setopt(curl, CURLOPT_COOKIEFILE, "d:/cookie.txt"); // 指定cookie文件
 curl_easy_setopt(curl, CURLOPT_COOKIEJAR, "d:/cookie.txt");
 curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers );
 curl_easy_setopt(curl, CURLOPT_URL, sign_url);   // 指定url
 curl_easy_setopt(curl, CURLOPT_READDATA, (void *)&wr_error);
 curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data );//调用写回调

 /* Allow curl to perform the action */
 res = curl_easy_perform(curl);
 printf( "ret = %d (write_error = %d)\n", res, wr_error );

  /* Emit the page if curl indicates that no errors occurred */
 if ( res == 0 ) 
printf( "%s\n", wr_buf );
 curl_slist_free_all(headers);
 curl_easy_cleanup(curl);
}
    .....
}
/*
 * Write data callback function (called within the context of
 * curl_easy_perform. 
 
 */
size_t write_data( void *buffer, size_t size, size_t nmemb, void *userp ) {
int segsize = size * nmemb;

  /* Check to see if this data exceeds the size of our buffer. If so,
   * set the user-defined context value and return 0 to indicate a
   * problem to curl.
   */
  if ( wr_index + segsize > MAX_BUF ) {
    *(int *)userp = 1;
    return 0;
  }

  /* Copy the data from the curl buffer into our buffer */
  memcpy( (void *)&wr_buf[wr_index], buffer, (size_t)segsize );

  /* Update the write index */
  wr_index += segsize;

  /* Null terminate the buffer */
  wr_buf[wr_index] = 0;

  /* Return the number of bytes received, indicating to curl that all is okay */
  return segsize;
}