telnet connects without socket being accepted-CodePudding

Here is a minimal example code:

#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <sys/epoll.h>

#include <stdio.h>
#include <stdlib.h>

#define NPROTO 10000
#define LOCALHOST "127.0.0.1"

/* */
int main(int argc, char *argv[])
{
 int                 sfd, epfd;
 struct sockaddr_in  so_address;
 struct epoll_event  ev, evs[1];

 sfd = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
 if(sfd < 0){
  fprintf(stderr, "socket() failed\n");
  exit(1);
 }
 epfd = epoll_create(1);
 if(epfd < 0){
  fprintf(stderr, "socket() failed\n");
  exit(1);
 }

 memset(&so_address, 0, sizeof(so_address));
 so_address.sin_family       = AF_INET;
 so_address.sin_port         = htons(NPROTO);
 if(1 != inet_pton(AF_INET, LOCALHOST, (struct in_addr*)(&so_address.sin_addr.s_addr))){
  fprintf(stderr, "inet_pton() failed\n");
  exit(1);
 }

 if(bind(sfd, (struct sockaddr *)&so_address, sizeof(so_address)) < 0){
  fprintf(stderr, "bind() failed\n");
  exit(1);
 }

 if(listen(sfd, 4) < 0){
  fprintf(stderr, "listen() failed\n");
  exit(1);
 } 

 ev.data.fd = sfd;
 ev.events  = 0;
 epoll_ctl(epfd, EPOLL_CTL_ADD, sfd, &ev);

 epoll_wait(epfd, evs, 1, -1);

 return 0;
}

The code creates socket that listens for the connections and registers the socket with an epoll set. Last lines wait for the activity on a epoll set.

Note that events flags of a struct epoll_event is set to zero. With the above example code, telnet connects to the program. If I change the flags to EPOLLIN, telnet reports "Connection refused".

The question is, why telnet connects whithout an accept being called?

CodePudding user response：

Because the port that telnet connects to is available after the listen.

In all [your] cases, telnet sees a connection established.

When you do ev.events = 0;, the epoll_wait will wait forever.

When you do ev.events = EPOLLIN, the epoll_wait will return. But, since you have nothing after that, main returns and the socket is closed when the server terminates. That is what telnet sees (e.g. Connection closed by foreign host.)

If you do:

printf("running ...\n");
while (1)
    sleep(1);

after the epoll_wait, then telnet will see the same effect, regardless of the value of ev.events