当我使用c ++构建mjpeg解码器时，响应为null

Question

我想从lilin ip摄像机接收mjpeg流。我使用libevent库来建立http连接。主要功能是buildHttpConnect。示例代码如下：

#include <event2/http.h>
#include <event2/http_struct.h>
#define REQUEST_GET_FLAG                3
struct http_request_get {
  struct evhttp_uri *uri;
  struct event_base *base;
  struct evhttp_connection *cn;
  struct evhttp_request *req;
};
/************************** Ahead Declare ******************************/
void buildHttpConnect(struct event_base *base, const char *ip);
int start_url_request(struct http_request_get *http_req, int req_get_flag);

void http_requset_get_cb(struct evhttp_request *req, void *arg)
{
  struct http_request_get *http_req_get = (struct http_request_get *)arg;

  char str[1024];
  char timebuffer[80];
  gettimebuffer(timebuffer);
  sprintf(str, "%s [%i] %s%s?%s\r\n", timebuffer, req == NULL ? -1 : req->response_code, evhttp_uri_get_host(http_req_get->uri), evhttp_uri_get_path(http_req_get->uri), evhttp_uri_get_query(http_req_get->uri));
  Utility::WriteLog(Utility::DEBUG_MSG, str);
  inst->appendText(str);
  if (req == NULL)return;
  switch (req->response_code)
  {
  case HTTP_OK:
  {
    struct evbuffer* buf = evhttp_request_get_input_buffer(req);
    size_t len = evbuffer_get_length(buf);
    printf("print the head info:");

    char *tmp = (char*)malloc(len + 1);
    memcpy(tmp, evbuffer_pullup(buf, -1), len);
    tmp[len] = '\0';
    printf("print the body:%s", tmp);
    free(tmp);

    event_base_loopexit(http_req_get->base, 0);
    break;
  }
  case HTTP_MOVEPERM:
    printf("%s", "the uri moved permanently");
    break;
  case HTTP_MOVETEMP:
  {
    const char *new_location = evhttp_find_header(req->input_headers, "Location");
    struct evhttp_uri *new_uri = evhttp_uri_parse(new_location);
    evhttp_uri_free(http_req_get->uri);
    http_req_get->uri = new_uri;
    start_url_request(http_req_get, REQUEST_GET_FLAG);
    return;
  }

  default:
    event_base_loopexit(http_req_get->base, 0);
    return;
  }
}

int start_url_request(struct http_request_get *http_req, int req_get_flag)
{
  if (http_req->cn)
    evhttp_connection_free(http_req->cn);

  int port = evhttp_uri_get_port(http_req->uri);
  port = (port == -1 ? 80 : port);
  http_req->cn = evhttp_connection_base_new(http_req->base,
    NULL,
    evhttp_uri_get_host(http_req->uri),
    port);

  /**
  * Request will be released by evhttp connection
  * See info of evhttp_make_request()
  */if (req_get_flag == REQUEST_GET_FLAG) {
    http_req->req = evhttp_request_new(http_requset_get_cb, http_req);
  }
  if (req_get_flag == REQUEST_GET_FLAG) {
    const char *query = evhttp_uri_get_query(http_req->uri);
    const char *path = evhttp_uri_get_path(http_req->uri);
    size_t len = (query ? strlen(query) : 0) + (path ? strlen(path) : 0) + 1;
    char *path_query = NULL;
    if (len > 1) {
      path_query = (char*)calloc(len, sizeof(char));
      sprintf(path_query, "%s?%s", path, query);
    }

    ///** Set the header properties */
    //evhttp_add_header(http_req->req->output_headers, "Host", evhttp_uri_get_host(http_req->uri));

    ////evhttp_add_header(http_req->req->output_headers, "Cache-Control", "no-store, no-cache, must-revalidate, pre-check=0, post-check=0, max-age=0");
    //evhttp_add_header(http_req->req->output_headers, "Connection", "keep-alive");
    evhttp_add_header(http_req->req->output_headers, "Content-Type", "multipart/x-mixed-replace;boundary=--boundarydonotcross");
    //"Expires" : "Mon, 3 Jan 2000 12:34:56 GMT",
    //evhttp_add_header(http_req->req->output_headers, "Pragma", "no-cache");
    evhttp_add_header(http_req->req->output_headers, "Authorization", "Basic YWRtaW46cGFzcw ==");

    int ret = evhttp_make_request(http_req->cn, http_req->req, EVHTTP_REQ_GET,
      path_query ? path_query : "/");
    char str[1024];
    char timebuffer[80];
    gettimebuffer(timebuffer);
    sprintf(str, (ret != 0) ? "%s [X] %s%s\r\n" : "%s [V] %s%s\r\n", timebuffer, evhttp_uri_get_host(http_req->uri), path_query);
    Utility::WriteLog(Utility::DEBUG_MSG, str);
    inst->appendText(str);
  }

  return 0;
}

static inline void print_uri_parts_info(const struct evhttp_uri * http_uri)
{
  printf("scheme:%s", evhttp_uri_get_scheme(http_uri));
  printf("host:%s", evhttp_uri_get_host(http_uri));
  printf("path:%s", evhttp_uri_get_path(http_uri));
  printf("port:%d", evhttp_uri_get_port(http_uri));
  printf("query:%s", evhttp_uri_get_query(http_uri));
  printf("userinfo:%s", evhttp_uri_get_userinfo(http_uri));
  printf("fragment:%s", evhttp_uri_get_fragment(http_uri));
}

http_request_get *start_http_requset(struct event_base* base, const char *url, int req_get_flag)
{
  int len = 0;
  if (req_get_flag == REQUEST_GET_FLAG) {
    len = sizeof(struct http_request_get);
  }
  struct http_request_get *http_req_get = (struct http_request_get *)calloc(1, len);
  http_req_get->uri = evhttp_uri_parse(url);
  print_uri_parts_info(http_req_get->uri);
  http_req_get->base = base;

  start_url_request(http_req_get, req_get_flag);
  return http_req_get;
}

void buildHttpConnect(struct event_base *base, const char *ip)
{
  char * account = new char[256];
  account = "admin:pass";
  //
  struct http_request_get *http_req_get = start_http_requset(base,
    //"http://x.x.x.x/getimage?credentail=YWRtaW46cGFzcw==", 
    "http://x.x.x.x/snap480p?credentail=YWRtaW46cGFzcw==", 
    REQUEST_GET_FLAG);
}

当我将URL“ http://x.x.x.x/snap480p?credentail=YWRtaW46cGFzcw==”传递给函数start_http_requset时，很好。我可以在函数http_requset_get_cb中获得响应代码为HTTP_OK。但是，当我将URL“ http://x.x.x.x/getimage?credentail=YWRtaW46cGFzcw==”传递给函数start_http_requset以获取mjpeg流时，我在函数http_requset_get_cb中获得了所传递变量req的空值。我不知道为什么

有人知道如何解决这个问题吗？谢谢。