c-服务器端意外的套接字关闭

Question

我正在运行一个基于libevent的示例服务器，但是，有时，根据wireshark捕获，该服务器会在TCP握手后立即发送TCP FIN。服务器没有崩溃。操作系统为ubuntu 18.04，终端的ulimit -n为100000。知道为什么吗？

#include <event2/listener.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
int printed = 0;
static void echo_read_cb(struct bufferevent *bev, void *ctx) {
/* This callback is invoked when there is data to read on bev. */
struct evbuffer *input = bufferevent_get_input(bev);
struct evbuffer *output = bufferevent_get_output(bev);
//evbuffer_add_buffer(output, input);  //if I replace the next line with this, same issue of unexpected server side closure on some sockets
evbuffer_add(bufferevent_get_output(bev), "a", 1);
}
static void echo_event_cb(struct bufferevent *bev, short events, void *ctx) {
if (events & (BEV_EVENT_EOF|BEV_EVENT_ERROR)) {
int fd = bufferevent_getfd(bev);
evutil_closesocket(fd);
bufferevent_free(bev);
return;
}
}
static void
accept_conn_cb(struct evconnlistener *listener,
evutil_socket_t fd, struct sockaddr *address, int socklen,
void *ctx)
{
/* We got a new connection! Set up a bufferevent for it. */
struct event_base *base = evconnlistener_get_base(listener);
struct bufferevent *bev = bufferevent_socket_new(
base, fd, BEV_OPT_CLOSE_ON_FREE);
bufferevent_setcb(bev, echo_read_cb, NULL, echo_event_cb, NULL);
bufferevent_enable(bev, EV_READ|EV_WRITE);
}
static void
accept_error_cb(struct evconnlistener *listener, void *ctx)
{
struct event_base *base = evconnlistener_get_base(listener);
int err = EVUTIL_SOCKET_ERROR();
fprintf(stderr, "Got an error %d (%s) on the listener. "
"Shutting down.n", err, evutil_socket_error_to_string(err));
event_base_loopexit(base, NULL);
}
int
main(int argc, char **argv)
{
struct event_base *base;
struct evconnlistener *listener;
struct sockaddr_in sin;
int port = 9876;
if (argc > 1) {
port = atoi(argv[1]);
}
if (port<=0 || port>65535) {
puts("Invalid port");
return 1;
}
base = event_base_new();
if (!base) {
puts("Couldn't open event base");
return 1;
}
/* Clear the sockaddr before using it, in case there are extra
* platform-specific fields that can mess us up. */
memset(&sin, 0, sizeof(sin));
/* This is an INET address */
sin.sin_family = AF_INET;
/* Listen on 0.0.0.0 */
sin.sin_addr.s_addr = htonl(0);
/* Listen on the given port. */
sin.sin_port = htons(port);
listener = evconnlistener_new_bind(base, accept_conn_cb, NULL,
LEV_OPT_CLOSE_ON_FREE|LEV_OPT_REUSEABLE, 10000,
(struct sockaddr*)&sin, sizeof(sin));
if (!listener) {
perror("Couldn't create listener");
return 1;
}
evconnlistener_set_error_cb(listener, accept_error_cb);
event_base_dispatch(base);
return 0;
}

Answer 1

原来函数echo_event_cb中不需要evutil_closesocket(fd)。拥有它是错误的，因为它会导致双自由。

至于为什么服务器程序没有崩溃，这是一个奇迹：-(