socket::accept continual to return EGAIN - linux

Question

I use nonblocking socket to receive new connection. But the code repeatedly fails to accept().
int sockfd = ::socket(family, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, IPPROTO_TCP);
::bind(sockfd, bind_addr, static_cast<socklen_t>(sizeof(struct sockaddr_in6)));
ret = ::listen(sockfd, SOMAXCONN);
while (True) {
::poll(&*pollfds_.begin(), pollfds_.size(), timeoutMs);
struct sockaddr_in6 addr;
bzero(&addr, sizeof addr);
socklen_t addrlen = static_cast<socklen_t>(sizeof *addr);
int connfd = ::accept4(sockfd, sockaddr_cast(addr),
&addrlen, SOCK_NONBLOCK | SOCK_CLOEXEC);
}
errno is EAGAIN.

Answer 1

From the manpage to accept(2):
EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and no connections are present to be accepted. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
This means that the call to accept is made before the client has connected.

Answer 2

Before calling accept, you must call listen and bind.
But as your socket is not blocking, you should wait for client to wait to connect. You can do that with select function:
int sockfd = ::socket(family, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, IPPROTO_TCP);
// addr is for accept call, sin for bind call
struct sockaddr_in6 addr, sin;
bzero(&addr, sizeof addr);
// prepare sin to tell bind to listen on any connection on given port
sin.sin6_family = family;
sin.sin6_addr = in6addr_any;
sin.sin6_port = htons(port); // choose port on which client could connect
sin.sin6_scope_id = 0;
// bind socket to interface
if (::bind(sock, (struct sockaddr*) &sin, sizeof(sin)) < 0)
{
perror("bind");
}
// listen for new connection
if (::listen(sock, SOMAXCONN) < 0)
{
perror("socket");
}
while (1)
{
fd_set conset;
FD_ZERO(&conset);
FD_SET(sockfd, &conset);
struct timeval timeout = {10, 0};
int maxfd = sockfd;
// wait for new client
select(maxfd + 1, &conset, NULL, NULL, &timeout);
if (FD_ISSET(sockfd, &conset))
{
// a new client is waiting
int connfd = ::accept(sockfd, &addr);
if (connfd < 0)
{
perror("accept");
}
else
{
// do thing with new client
}
}
else
{
printf("no new client in last 10 seconds")
}
}